Imaging Essentials Input / Output: Raster, Bitmap, Photo and Vector Images Defined

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countway.harvard.edu/imaging

Imaging Essentials

Input / Output:

Raster, Bitmap, Photo

and Vector Images Defined

Resolution, Physical Dimensions, Color

Mode and File Formats Explained

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Research Imaging Solutions RIS@hms.harvard.edu

countway.harvard.edu/imaging

Beth Beighlie

Digital Imaging Coordinator Research Imaging Solutions

Information Technology Department Harvard Medical School

220 Longwood Avenue Goldenson 521a Boston, MA 02115 (617) 432-2323 b2@hms.harvard.edu

Written by Beth Beighlie

Copyright © 2006 by

Research Imaging Solutions

Version 2.1, February 14, 2006 Version 2.2, June 9, 2006

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IMAGES DEFINED... 1

PIXEL-BASED IMAGES: PHOTOS, RASTER AND BITMAP IMAGES... 1

VECTOR OBJECTS... 1

VECTORS THAT ARE RASTERIZED... 2

PHOTOS, RASTERS, BITMAP, VECTORS…DO I CARE? ... 3

Object Type Chart... 3

DETERMINING A TARGET RESOLUTION ... 4

PRACTICAL CONSIDERATIONS... 4

OUTPUT DEVICES: RESOLUTION AND PHYSICAL DIMENSIONS... 5

Resolution Chart ... 6

RESIZING IMAGES ... 9

REDUCING THE RESOLUTION OR PHYSICAL DIMENSION (RESIZING BY RESAMPLING) ... 9

INCREASING RESOLUTION OR PHYSICAL DIMENSION (RESIZING BY ROBBING PETER) ... 10

Increasing Resolution or Physical Dimension: The Details:... 11

To increase physical dimension (which will reduce the resolution): ... 11

To increase resolution (which will reduce the physical dimension): ... 11

Increasing both resolution and physical dimensions: ... 12

RESIZING WITH THE CROP TOOL... 12

COLOR MODES ... 13

Color Mode Chart... 14

PIXEL-BASED FILE FORMATS ... 15

Common Graphic Image File Format Chart ... 16

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Imaging Essentials- Input / Output: Resolution, Physical Dimensions, Color Mode and File Formats Explained

download additional copies of this handout at

http://countway.harvard.edu/imaging/docs/imaging_basics.pdf Page 1

Basic Imaging Concepts

Images Defined

Pixel-Based Images: Photos, Raster and Bitmap Images

For our purposes, we’ll consider these three terms interchangeable. They all refer to images that are made up of a grid of pixels. Put a bunch of individual pixels together and when viewed as a whole, they’ll represent an image. The higher the resolution, that is, the more pixels per inch, the better the representation will be, but only up to the extent of the output device’s ability to display it. For example, if you have a monitor that displays at 72 dpi, and you have two versions of the same image, one at 72 dpi, and one at 600 dpi, you will not be able to differentiate between the two when viewing it on the 72 dpi monitor (except for the amount of time it takes for each file to open).

Photo, raster and bitmap images cannot be reliably resized for greater resolution or larger physical dimension. If you were to select any of these types of images and try to resize it to a larger physical dimension by using one of it’s selection edges, you would seriously diminish the quality of the image. Likewise, if you were to try to increase the resolution of these image types, you would have a significant loss of quality. The most obvious problem would be that the pixels would become more visible or that the image would appear to blur. For this reason, if you need to modify the resolution or physical dimension of a photo, raster or bitmap image, you will need to use an image editing application like Photoshop.

For solutions for increasing the physical dimension or resolution of a photo, raster or bitmap image, see the Resizing Image section of this document.

File formats for photo, raster and bitmap images include .jpg, .tif, .gif, .psd, .bmp, .pict, .jpg2, .jp2, .png, and any scanned images that haven’t been OCR (optical character recognition) processed.

Vector Objects

Vector images or objects are defined by points and curves rather than by pixels.

Because vector objects are mathematical calculations that display and print the image, they are scalable and not resolution dependent- because there are no pixels, there is no resolution. Resize them to any degree and they will always display and print beautifully. Because this is true, if you need to change the size of vector objects, you should not use a pixel-based image editing application like Photoshop. To resize vector objects, you need only select the object (perhaps you will need to ungroup the elements) and use the selection handles to drag and resize.

Vector objects include text elements as well as any shape, illustration, cartoon or line art that you draw with the drawing tools in Microsoft Office products (PowerPoint, Word, Excel), Adobe Illustrator or Deneba Canvas. There are a many proprietary scientific applications that generate vector objects

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Vectors that are Rasterized

While vector-based file formats (.pdf, .eps, .ps, .ai, .fh, .swf and .ppt.) can maintain based images as pixel-based images and vectors as vectors, pixel-pixel-based file formats (.jpg, .tif, .gif, .psd, .bmp, .pict, .jpg2, .jp2, .png, and any scanned images that haven’t been OCR [optical character recognition] processed) can only contain pixels and as a result, will rasterize any vectors that are within.

To rasterize an image is to make it a pixel-based image. If you save a file that contains vector objects in any photo, bitmap, raster or pixel-based file format (.jpg, .tif, .gif, .psd, .bmp), you will lose all the benefits of the vector object: it will rasterize the image and it’s objects and you will no longer be able to resize it without a pixel-based image editing application like Photoshop. All text, illustrations and lines will become pixels, and have that ‘jaggy’ look.

If you save images that contain objects that were at any point a pixel-based file format (.jpg, .tif, .gif, .psd, .bmp) as a vector file format (.ps, .ppt, .eps; .ai) you will not regain those vector attributes, nor the ability to scale the image without using an image editing application like Photoshop.

If you scan images that contain illustrations, cartoons or text, the process of scanning them makes them raster images, not vector images. You cannot reliably resize it. When scanning images that contain illustrations, cartoons, text and line drawings, you should scan at a higher resolution than you would scan photo-only images (See Chart A).

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Photos, Rasters, Bitmap, Vectors…Do I care?

Yes, you do. Given the choice, keep your vector objects as vectors. Don’t save them as any raster file format unless someone specifically requests it

If submission guidelines request figures to be submitted as .tif, .jpg, .ps, eps, or .pdf and if your figure has any vectors (text, arrows, shapes drawn with Drawing Toolbars) you should choose .ps, eps, or .pdf for best results.

The best way to do this is to use the right application for creating your figures. PowerPoint files are an excellent solution for handling both photo images and vector objects in the same file. If you correctly prepare your pixel-based images in Photoshop (with the correct resolution [see Chart A], physical dimension [don’t resize images once inserted into PowerPoint] and file format [.jpg or .tiff]) and insert them into PowerPoint you can annotate (add text, arrows, highlighting circles, etc.) in a way that will guarantee a good print with sharp looking text and curved lines.

Don’t use Photoshop to annotate. Photoshop is a raster-based application and will ultimately rasterize text and annotation so it looks pixilated. Use Photoshop to edit your images, then insert them into PowerPoint to annotate.

Chart A

Object Type Chart

Often

Often • •..tiftif

• •.jpg.jpg • •.gif.gif

Occasionally

Occasionally •

•..epseps

•

•Scanned IllustrationsScanned Illustrations •

•Illustrations that haveIllustrations that have

been

been ‘‘rasterizedrasterized’’

•

•All .All .tiftif,,.jpg.jpg& .gif files& .gif files

•

•Pixel basedPixel based •

•Described in dpiDescribed in dpi •

•aka aka bitmapbitmap

•

•Cannot grab objectCannot grab object&& stretch to resize larger

stretch to resize larger

Rasters

Rasters thatthat were vectors

were vectors

• •..epseps

• •..psps

• •..pdfpdf

•

•..pptppt, ., .aiai, ., .cnvcnv •

•IllustrationsIllustrations •

•Text objectsText objects •

•Drawing tool objectsDrawing tool objects •

•Never .Never .tiftif, .jpg or, .jpg or.gif file.gif file type

type •

•No pixels / no resolutionNo pixels / no resolution •

•Mathematical algorithmMathematical algorithm •

•OK to grab object &OK to grab object &

Vector

Vectorobjectsobjects

• •..tiftif

• •.jpg.jpg

• •.gif.gif

•

•PhotographsPhotographs •

•All images fromAll images from

Scanners Scanners Microscopes Microscopes cameras cameras •

•AllAll..tiftif,,.jpg.jpg& .gif files& .gif files

•

•Pixel basedPixel based •

•Described in terms ofDescribed in terms of

resolution (dpi)

resolution (dpi) •

•aka aka bitmapbitmap

•

•Cannot grab objectCannot grab object&& stretch to resize larger

Raster objects

Usual file types

Examples Examples Characteristics Characteristics Object type Object type

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Determining a Target Resolution

The first consideration is to determine is which device or devices you are going to use for output. If you are submitting a grant, a paper to a journal or preparing images for publication, refer to their submission guidelines to determine what resolution and physical dimension is required. Each output device has different resolution and physical dimension requirements. To determine these numberes look at the Output Devices section of this document and see the Resolution Chart. If your use requires several output needs, scan for the highest output resolution then Save As… different versions for each different output device. Also, remember to consider the physical dimensions of the image that you are going to output.

Ideally you want to prepare an image in Photoshop that is the correct resolution, physical dimension, file format and color mode for your needs. You want to avoid resizing or editing the image in any application but Photoshop.

Practical Considerations

In light of the information presented in this guide, higher resolution is generally better for both quantization and presentation of scientific data than lower resolution. This leads to large files that are harder to work with and take up more storage space on your computer. You must balance your scientific needs, in terms of file formats and size, with practical considerations in order to make your work go smoothly and without performance and compatibility problems. Here is a list of some considerations:

Mobility and Speed

The larger a file, the longer it will take to load and the more memory on your computer it will consume, especially on older, slower computers. Some file formats are designed for speed of use. Images using compressed file formats load into memory faster because they are smaller and decompression by software is faster than reading a lot of raw data.

Storage Limitations

Large files take up more space on your local drive or network file servers. Storage space is always a valuable commodity on computers. Large files also take longer to transfer over the network.

Portability and Compatibility

Different file formats are handled differently (or not at all), by different programs on different computer platforms. Always consider the types of computer equipment and programs you will be using with the image when selecting format.

Output Compatibility

Some output devices require specific (and even proprietary) file formats for use. File format can also affect how a file appears in final output. Printer driver settings can also affect the output of graphical images. Many of these parameters can be set from within the program that you are using to output the image.

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http://countway.harvard.edu/imaging/docs/imaging_basics.pdf Page 5 Here’s what happens when you prepare a raster image for insertion into a PowerPoint presentation. You can see that depending on how you output your file, you will want to have different resolution/pixel/physical dimensions. This will then affect the file size.

resolution physical dimension pixel dimension file size

72 dpi (i.e.: LCD projection) 10 x 7.5 720 x 540 1.1 M

200 dpi (i.e.: laser printers) 10 x 7.5 2000 x 1500 8.6

300 dpi (i.e.: photo quality printers) 10 x 7.5 3000 x 2250 19.3 M

Output Devices: Resolution and Physical Dimensions

With regards to output, higher resolution doesn’t mean better looking

The quality of your image is only as good as your output device. For example, LCD projectors output at roughly 72-96 dpi. The quality of the image that is projected will not improve no matter how many pixels per inch you throw at the machine, it will only project at 72-96 dpi. In fact, if you do try to project a high-resolution image, you run the risk of hogging so much memory that you (at worst) hang the application or (at best) cause the projected image to draw really, really slowly on the screen.

NOTE: If you need to resize the resolution or physical dimension of your image, see the information on Resizing Images in this document.

Super High-Quality Photographic Printers

These very high quality printers will output a glossy photo-like image for journal submission. Some will output overhead acetates as well. Because of the high cost of printing with these devices, it is recommended that they be used conservatively. Don’t use these printers for proofing, use them instead for final output.

The Fujix Pictographic printer

Physical Dimension of printable area: 8” x 10.5” or 8” x 5”

300-400 dpi for photo images, bitmap images and raster images (with no scanned text or line art within). 600-1200 dpi for scanned or rasterized line art, illustrations, vector objects and cartoons.

The Fujix Printer is a photographic process printer. The print quality is very high. The target resolution is at least 300 ppi (NOTE: If using a Fujix 3000, the ideal resolution for high-quality prints is 320 ppi. If using a Fujix 4000, the ideal resolution for high-quality prints is 400 ppi). If your image is highly complex (such as an electron micrograph) you may want to bring your resolution up to 400ppi to retain sharp edges and lines. Please keep in mind that this will significantly increase your file size.

Photo-quality inkjet printers with glossy paper

Physical Dimension of printable area: dependant on manufacturers specs

180 – 320 dpi for photo images, bitmap images and raster images (with no scanned text or line art within).

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Chart B*

Resolution Chart

The following is a guide for the output resolution of different devices. It is advisable to acquire images at a higher resolution (i.e.: 400 dpi for photo, bitmap or raster images and 600-1200 dpi for line art, illustrations, cartoons and text) and save that as your master image and make lower resolution iterations according to how you’ll be outputting it. Sorry, one ‘size’ doesn’t fit all- you will need different sized images for different output needs. Be careful! Don’t overwrite your master images with your ‘dumbed down’ versions!

Art Work Type

Output Device

Photo Images

Bitmap Images

Raster Images

(with no text or vector

objects within)

Raster images

that contain

Line Art

Illustrations

Vector Objects

Cartoons

Text

PowerPoint On-screen Presentation LCD Data Projector Computer Monitor/Display Web Site Physical Dimension of PowerPoint On-Screen Presentation: 7.5” x 10” otherwise physical dimension of projector or monitor = pixel dimension of device

100 dpi 200 dpi

Laser Printer (LaserJets & LaserWriters)

Printable Area varies

200 dpi 300-600 dpi

Photo- Quality Inkjet Printable Area varies

150 dpi (plain paper) 180 or 240 or 320 dpi

(photo paper)

300-600 dpi

Photo-Quality Printer (i.e.: Fujix, dye sublimation printers) Printable Area = 8” x 10.5” or 8” x 5”

300-400 dpi (usually 320 dpi)

600-1200 dpi

Poster Printer Printable Area =determined by service bureau

125-200 dpi start at 125dpi

300 dpi

If inserting into PowerPoint

100 dpi

If inserting into PowerPoint 225 dpi

Slide Maker Film Recorder

Printable Area = 4096 x 2731 pixels

or with PowerPoint 7.5” x 11.25” If imaging directly from Canvas or Photoshop

the pixel dimensions should be 4096 x 2731

(if you make images smaller than these pixel dimensions, the film recorder will ‘stretch’ the image to fit the slide and it will look terrible.)

This chart is to be used as an approximate guide only- there are more precise numbers that can be applied for specific uses. If you’d like to know these specific calculations, use these numbers as starting points and experiment. The goal should be to have a relatively high resolution master image from which you prepare a file that is the correct resolution, physical dimension and file format for a particular output device. This is especially important when prepping images for PowerPoint presentations: you do not want to use PowerPoint to resize your images once you have inserted them.

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High-Quality Plain Paper Printers

These high quality printers are great for creating drafts for proofing image edits, outputting multiple copies for in-house handouts or review and making images that don’t require the more costly photo-quality images. It is recommended that whenever possible that you print color images on the color printer and send any text only, black and white or grayscale pages to black and white printers.

Laser Printers

(LaserWriter, Phaser 340 - 360, etc.)

Physical Dimension of printable area: as low as 8” x 9”

200 dpi for photo images, bitmap images and raster images (with no scanned text or line art within). 300-600 dpi for scanned or rasterized line art, illustrations, vector objects and cartoons.

The resolution on these printers is not as high as it is with the devices listed above. If you won’t ever need a high-quality print from an image you are working with, than a resolution of 125-150 dpi tends to be sufficient. If your image file has text or vector objects within, you may find better results with higher resolutions of up to 600 dpi. Run a test print to see if you really do need to have such a large image file.

Presentation Output and Display

LCD Data Projection

Physical Dimension of PowerPoint On-Screen Presentation: 7.5” x 10” Otherwise, whatever pixel dimension of projector is (varies)

100 dpi for photo images, bitmap images and raster images (with no scanned text or line art within). 200 dpi for scanned or rasterized line art, illustrations, vector objects and cartoons.

When creating your slides in PowerPoint, if you plan to use a computer and a LCD projector to display your presentation, don’t forget to set your slide size in the Page Setup Dialog Box as On-Screen Presentation. Images prepared for data projectors are often displayed using PowerPoint. You can safely use images (with no text or vector objects within) as low a resolution as 72 dpi for insertion into

PowerPoint presentations that will be shown using a LCD data projector. Don’t go higher than 100 dpi for photo-only images. Inserting images that are higher than 100 dpi that will make the rate at which the screen displays inserted images to slow to an unacceptable and excruciatingly slow speed: put big, unnecessarily high resolution images in your presentation to ensure your audience will nap as your images slowly draw on the screen. If you have time and access to a data projector, run a test to see if you really do need to have such a large image file. Also, you must test your On-Screen presentation on the computer and projector that you will be using if you are to expect your presentation to go off without a hitch. Download the Brown Bag Seminar Series handouts for more tips on successful PowerPoint Presentations at http://countway.harvard.edu/imaging/handouts

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Slide Makers

(Lasergraphics LFR Mark II)

35mm Slides from PowerPoint:

Physical Dimension of printable area: 7.5” x 11.25”

100 dpi for photo images, bitmap images and raster images (with no scanned text or line art within). 225 dpi for scanned or rasterized line art, illustrations, vector objects and cartoons.

When creating your slides in PowerPoint, don’t forget to set your slide size in the Page Setup Dialog Box as 35mm Slide. If you are working on an image that will be inserted to a PowerPoint presentation and then imaged on the Slide Maker, you can use an image with a resolution of 100 dpi (with not text or vector objects within), as long as you don’t resize it larger once in PowerPoint. To have an image inserted into PowerPoint fill the entire slide, size the Photoshop image you plan to insert for 7.5” x 11.25”. If your image file has text or vector objects within, you may find better results with higher resolutions of 225 dpi.

35mm Slides from Photoshop:

Pixel Dimension = 4096 x 2731

Photoshop images imaged directly to slide film (without first being inserted into a PowerPoint document) will be resized by the film recorder to fill the slide. Smaller physical dimensions (or pixel dimensions less than 4096 x 2731) will be ‘stretched’ to fill the slide, resulting in pixilated images of poor quality. It is for this reason that pixel dimension, not resolution is the important number to target.

Poster Printer

Physical Dimension of printable area: check with service bureau

For correct resolution requirements, check with service bureau, but usually it’s in the neighborhood of 125-200 dpi for photo images, bitmap images and raster images (with no scanned text or line art within). At least 300 dpi for scanned or rasterized line art, illustrations, vector objects and cartoons.

If you have created your poster with PowerPoint and will be printing your poster at 100% of it’s size, the above resolutions will work well. If you are printing at 200% of it’s size, you must double the resolution of any inserted pixel-based images.

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Resizing Images

Reducing the Resolution or Physical Dimension (Resizing by Resampling)

A. Open the Image Size dialog box from the Menu Bar: Image->Image Size...

B. In the Document Size section of the dialog box, choose the units that you are most comfortable with (pixels, inches, cm, points, etc).

C. Make sure the ‘Constrain Proportions’ and the ‘Resample Image’ boxes that are at the bottom of the dialog box are checked.

D. In the Document Size section, reduce the document’s Width or Height. You need only change one dimension. Since the proportions are constrained the other dimension will be automatically calculated. E. Also in the Document Size area, reduce the Resolution according to the ultimate output (refer to Output

Devices section of this document or see the Imaging Essentials document which can be downloaded at http://countway.harvard.edu/imaging/locations).

F. Click ‘OK’

NOTE: When reducing the image size or resolution there is no loss in quality.

Increasing the image size or resolution will diminish the quality of the image if you don’t do it correctly. If you do need to increase the dimensions of the image, it is recommended that you rescan the image at the higher resolution and/or dimension, or resize the image according to the following suggestions:

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Increasing Resolution or Physical Dimension (Resizing by Robbing Peter)

Increasing the resolution or physical dimension of a pixel-based image is generally discouraged. If you must increase the resolution or physical size of an image and cannot rescan or reacquire it to meet your needs, then here’s a possible solution: You can force Photoshop to do the calculation that allows you to increase either the physical dimension or the resolution.

To do either of the following, you must have an excess of either resolution or physical dimension, because you will be appropriating those excesses to the other dimensions.

Increasing Resolution or Physical Dimension (Resizing without Resampling): The Overview:

Increasing the Width, Height or Resolution with the Resample Image box unchecked will result in the calculated decrease of the other two dimensions.

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http://countway.harvard.edu/imaging/docs/imaging_basics.pdf Page 11 Increasing Resolution or Physical Dimension: The Details:

To increase physical dimension (which will reduce the resolution):

For example, this would be appropriate for changing small-dimensioned, high-resolution images that need preparation for LCD projector presentations.

A. Image->Image Size…;

B. In the Document Size section of the dialog box, choose the units that you are most comfortable with (pixels, inches, cm, points, etc);

C. Check the Constrain Proportions Box; D. Uncheck the Resample Image Box;

E. Decrease the resolution to the target resolution. (Photoshop will do the calculation that will increase the physical dimensions proportionally. The file size will remain the same.);

F. Put the checkmark back in the Resample Image Box and enter your target physical dimension into one of the Width/Height boxes (the number you enter must be smaller than the numbers that currently exist in these fields);

G. Click OK

To increase resolution (which will reduce the physical dimension):

For example, this would be appropriate for changing large-dimensioned, low-resolution images that need preparation for higher resolution output demands like laser, photo-quality printers or journal submissions.

A. Image->Image Size…;

B. In the Document Size section of the dialog box, choose the units that you are most comfortable with (pixels, inches, cm, points, etc);

C. Check the Constrain Proportions Box; D. Uncheck the Resample Image Box;

E. Increase the resolution to the target resolution. (Photoshop will do the calculation that will decrease the physical dimensions proportionally. The file size will remain the same.);

F. Put the checkmark back in the Resample Image Box and enter your target physical dimension into one of the Width/Height boxes (the number you enter must be smaller than the numbers that currently exist in these fields);

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http://countway.harvard.edu/imaging/docs/imaging_basics.pdf Page 12 Increasing both resolution and physical dimensions:

You will find moderate success in increasing the resolution and/or physical dimensions by this method. Ideally, you should try to reacquire your image to meet your needs, but if that is not possible try this:

If you have an image that is both small-dimensioned and low-resolution, and therefore it lacks the dimensions to “Rob Peter” in the steps previously described, you can try increasing the resolution / physical dimension in 10% increments by resizing by resampling:

A. Open the Image Size dialog box from the Menu Bar: Image->Image Size...

B. In the Document Size section of the dialog box, choose the units that you are most comfortable with (pixels, inches, cm, points, etc).

C. Make sure the ‘Constrain Proportions’ and the ‘Resample Image’ boxes that are at the bottom of the dialog box are checked.

D. In the Document Size section, increase the document’s physical dimension or resolution no more that 10% larger than the original size.

E. Click ‘OK’

F. Repeat steps A-E until target resolution and physical dimension nearly reached.

G. The last time you repeat steps A-E, enter the exact dimensions and resolution you are targeting.

Resizing with the Crop Tool

To resize with the crop tool, you must be reducing the resolution and / or the physical dimension of your image. If you need to increase either the physical dimension or resolution of your image, you must have an excess of either resolution or physical dimension, because you will be appropriating the excesses to the other dimensions. If you need to increase both the resolution and physical dimension of your image, follow the instructions outlined in the previous step.

You can resize a selected portion of an image by using the Option Bar with the Crop Tool. Insert the desired physical dimensions and/or resolution in the Option Bar- these numbers will remain for every image you open so you can crop uniformly for creating composite images. You do not need to fill in each field, you can leave any two fields in this Option Bar blank.

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Color Modes

It is what it is:

Got color? Make it RGB…;

Not color? Make it Grayscale…;

Most of our printers do not use the four CMYK halftone screens from an image setter like commercial printing presses require. We never create CMYK images unless we are doing prepress for an offset ink press. There are unnecessary losses in RGB/CMYK conversion, particularly in the bright colors. Our laser and inkjet printers are designed to expect normal RGB images. Their printer driver expects to convert RGB to CMYK ink, and uses dithering (stochastic with error diffusion) to produce the required color combinations from the three ink colors.

RGB Images

Ideal for color photographs

These are images made with a red/green/blue process. They have a 16 million-color palette. Keep in mind that this is a fairly memory intensive mode to work in - a full-page color image can take up to about 40MB! If you are working with a line drawing, it is best to use the Indexed color mode instead (see below).

Grayscale Images

Ideal for monochrome images.

They have 256 shades of gray. A black and white or continuous tone image should be stored in Grayscale image mode. Since the file sizes tend to be one-third the size of RGB, grayscale images are a fairly economical way to store images.

Indexed Color Images

Often the choice for images on websites.

The colors in these images come from a color look-up table.

In order to conserve file size, a color line drawing can be stored in Indexed Color mode (this has a much smaller color palette [approximately 256 colors - can vary] than RGB). If you need to do any image editing to your image, however, you will need to work with it in RGB mode and then change it to Indexed Color upon completion. Do not change an image file back and forth between Color Modes: work in RGB, and after all editing is completed, save it in the Indexed Color Mode as one of your last tasks.

CMYK

Publishers choice.

This is a good option for the user that will be having their images published on paper by a printer (the business, not the machine). If you do use an outside printer/publisher to put your image on paper, call them to see what resolution, color mode & file format they want. Printing using a method called ‘four color process printing’ is best set up using the CMYK mode. It is best not to switch images back and forth between modes since they store color information differently and information is changed and may be lost by repeatedly switching modes. Gamut is defined as the range of colors that can be displayed or printed by a given color mode; RGB and CMYK have different gamuts and if you change between modes you may see error messages stating that your image has colors in its palette that are out of gamut. This usually means that a color (or colors) displayed on your monitor cannot be reproduced when the image is printed.

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Color Mode Types

Generally speaking, if you plan to print to our in-house printers, RGB color mode is the best choice. If you are preparing images to go to an outside publisher or print house, call them to determine the resolution, color mode and file format they want you to provide.

Chart C

Color Mode Chart

Art

Work

Type

Output

Color

Line Art,

Illustrations,

Vector Objects

and Cartoons

Color

Photo Images,

Bitmap Images

and Raster

Images

Black & White

Line Art,

Illustrations,

Vector Objects

and Cartoons

Black & White

Photo Images,

Bitmap Images

and Raster

Images

Internet/

Web/

Computer

Monitor/

LCD

Projector

Indexed Color

Laser

Printers

Inkjet

LFR Mark

Film

Recorder

(Slide

maker)

Fujix

Pictrograph

RGB

or Indexed*

*You can economize on file size by using

Indexed Color for Color Line Art, Illustrations, Vector Objects and Cartoons

Grayscale

Publisher/

Printer

Probably CMYK

(call them to ask what color mode

& file format)

Probably Grayscale

(call them to ask what color mode

& file format)

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Pixel-Based File Formats

When it comes to editing pixel-based images, it is best to edit in the editing software’s proprietary format: i.e.: .psd for Photoshop editing or .cnv for Canvas editing.

JPG

.jpg is a lossy compression file format that can greatly reduce your file sizes.

The pros: .jpg files are the most versatile file format: it is cross-platform (read by Macs and PCs) and can be opened by many common applications. This format also has variable compression that can be set when saving the file.

The cons: Because it is a lossy compression process, the more a file is compressed, more information is lost and has to be filled in by best-guess algorithms when reopening the file. When saving an image in this format, the amount of compression should be experimented with to determine how much the file can be compressed and still appear satisfactorily. JPG images can be compressed up to 20 fold without unacceptable degradation to the image, but after that, you run the risk of not being able to open the image due to lack of data.

TIFF

.tif is a lossless compression file format that moderately reduces file sizes.

The pros: .tif files are nearly as versatile as .jpg files. These files are also cross-platform and readable by many common applications. Because the compression algorithm is lossless, the quality of the image is somewhat better.

The cons: Because the compression algorithm is lossless, the size of the image file is significantly larger than .jpg files.

Pro or con?: .tif files can support layers- while this is good for future editing of images, .tif files with layers included are problematic for insertion into Office applications like PowerPoint and Word.

This lossless compression format is a good place to start. It is the most versatile file format & it is easily digested by most of the applications that we use for graphics. GIF & JPG are also acceptable, but they’re not as good as TIFF. TIFF is good for raster (photo) images.

GIF

GIF is a lossless image format, which has a different compression scheme that maintains the appearance of borders between colors and lines better than the JPG format. GIF is the better of the two formats for graphic illustrations, cartoons and line (chart and drawing) images. GIF files support transparency and animation. The transparency feature allows background in the image to be transparent instead of white or other background color and allows the image to be placed over other images or content with the other content still appearing through the GIF image. Animation allows multiple GIF images to be place in a single file and automatically viewed in rapid succession.

Graphic Images for Web Pages

Rapid opening graphic image file formats and small file size are critical performance parameters for use of images in web pages. These parameters work together to provide web pages that load and open quickly. Keeping the total size of all images and content on web pages to a minimum is quite a challenge and are important in making a quality web page. JPG and GIF file formats are the most common formats used for web pages. Both formats are compressed and optimized for rapid opening within web and graphics browsers. These formats use different compression schemes and have different limitations in the way they handle image data, however. The format selected should be chosen dependent on the content of the image to be presented.

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Chart D

Common Graphic Image File Format Chart

Name Ext Description

Recommended for images containing only photos, bitmap, raster and/or pixel-based objects

Joint Photographic

Experts Group .jpg

• Widely used: cross-platform and can be opened by most graphics programs

• Preferred for photos used on web & in email • Lossy compression algorithm

• Smaller file size and lesser image quality than .tif

• Best choice for inserting into Word, PowerPoint and Acrobat • Single page document

Tag Image File Format

.tif .tiff

• Widely used: cross-platform and can be opened by most graphics programs

• Lossless compression algorithm

• Larger file size & better image quality than .jpg

• Good choice for inserting into Word, PowerPoint and Acrobat • Supports layers

• Single page document Graphics

Interchange

Format .gif

• Widely used bitmap format with compression • Limited to 256 colors

• Can be animated and transparent

• Preferred for illustrations and graphics used on web Recommended for images containing vector objects

Portable Document

Format .pdf

• Widely used: can be opened with free Reader download • Preferred for retaining formatting of shared documents • Preferred for distribution on web & in email

• Supports bitmap and vector objects in same document • Supports multiple page documents

Encapsulated PostScript & PostScript

.eps .ps

• Printer language

• Supports bitmap and vector objects in same document

Flash File Format .swf • Supports animation, audio and interactivity

• Supports bitmap and vector objects in same document

PowerPoint .ppt • Supports bitmap and vector objects in same document

• Supports multiple page documents Proprietary formats

Photoshop .psd •

Lossless bitmap format • Supports layers

• Single page documents Illustrator .ai

• Editable postscript format

• Supports bitmap, vector, and text data • Supports layers

• Single page documents

Canvas .cnv • Lossless format supports bitmap and vector data

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Reference Materials

Site-specific Imaging Equipment Instructions http://countway.harvard.edu/imaging/locations

If you are part of Genetics, Microbiology, Neurobiology, Pathology or the West Quad departments, you can scroll through the above web page for documents on how to use your department’s imaging devices. Other users may find documents that correspond to equipment that they have in their own areas.

General Poster Printer Instructions

http://countway.harvard.edu/imaging/docs/poster_recipe.pdf Class Notes and Handouts

http://countway.harvard.edu/imaging/handouts http://countway.harvard.edu/imaging/guides.shtml

Additional copies of this and other handouts can be downloaded Countway Library’s Research Imaging web site. Imaging Essentials

http://countway.harvard.edu/imaging/docs/imaging_basics.pdf

This document answers questions about resolution, color mode and file format. Describes how to determine correct scanning resolution and dimension. Outlines various local output devices and their resolution and dimension specifications.

Excellent Websites for graphics applications troubleshooting and instructions:

PowerPoint FAQ http://www.rdpslides.com/pptfaq/

PowerPoint Help http://www.echosvoice.com/

Planet Photoshop http://www.planetphotoshop.com

Scan Tips http://www.scantips.com/

Planet PDF http://www.planetpdf.com/

Mikes SketchPad (Fantastic!) http://www.sketchpad.net/