RGB: stands for red-green-blue
The RGB model is a model used for describing colors generated additively - as is the case on a monitor screen. RGB images are also referred to as true-color image and can contain up to 16 777 216 (=256 x 256 x 256) different colors via the additive mix- ing of the three primary colors red, green and blue. If all color components are at ze- ro, the image will be black; mixing 100% of the three primary color produces white.
HSI HSI is another model for describing color values. HSI is an abbreviation for Hue, Sat- uration and Intensity. HSI values of a color originate from RGB values via coordinate transformation. The HSI model is easier to understand than the RGB model because its parameters are more in line with one’s own intuitive description of a color. The HSI color space
can be shown mathe- matically by a double clicking. The central ax- is is the intensity, the distance from the axis is the saturation and the angle corresponds to the hue. The color wheel is a two dimensional projection of the HSI color space. The color changes with the angle. The satura- tion increases towards the outer limits.
Processing images
Adjusting image color - Background Information
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Gamma correction Gamma correction increases or reduces image contrast selectively dependent on in- tensity values:
where I(R) is the intensity value of the resulting image; I(O) the intensity value of the original image.
A γ< 1 gamma value darkens the image as a whole. Contrast of bright image areas is increased. A γ> 1 gamma value brightens the image as a whole. Contrast of dark image areas is increased. A γ< 1 gamma value does not change the image in any way.
Define RGB Adjustment
The Oper menu has a submenu called Adjust Colors containing commands for cor- recting false color and intensity within images. The different commands function the same way in principle: For example, the Oper > Adjust Color > Define RGB Adjust- ment dialog box provides slide controls and a field for each of the three color compo- nents (red, green, blue). Slide controls are for fast, less precise adjustment of color values. Once you have approximate values, use the fields to fine-tune the settings - entries can be made manually directly into the field, or click on the arrow keys. DEFAULT SETTING: when you open the dialog box the parameters of the most re- cently-conducted color adjustment appear as the default setting.
The table below shows what parameters can be adjusted. The range of available val- ues is defined by the lowest and greatest value. The middle value means the image remains unchanged.
Intensity Adjustment: The gamma, brightness and contrast parame- ters have been adjusted in the above image ar- eas. left: Gamma > 1 means the image is brightened and contrast in dark im- age areas is increased. center: Brightness (+200) means that in- tensity is increased. Color values remain constant. right: Contrast (+1000) mainly raises color satu- ration. Color values re- main unchanged.
I(E) = I( )A 1⁄γ
command parameter value range Intensity Adjustment Gamma 0.1 1 10
Brightness -1000 0 1000 Contrast -1000 0 1000 HSI Adjustment Intensity -1000 0 1000 Hue -360 0 360 Saturation -1000 0 1000 RGB Adjustment Red -255 0 255 Green -255 0 255 Blue -255 0 255
Processing images Adjusting image color - Step-by-step
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HSI Adjustment: The parameter intensity, hue and saturation have changed in the top part of the image. left: Intensity (+200) in- creases brightness. center: A mid value (ca. ±200) results in a maxi- mum change in color. A maximum value of ±360 results in the original im- age. Negative and posi- tive values cause the same kind of color change. right: Saturation (+1000) is maximum.
Step-by-step
Adjusting colors (RGB)
1) Select the Oper > Adjust Colors > Define RGB Adjustment... command to adjust the red, green and blue components of the active image.
2) Select the Preview check box to be able to watch alterations as they occur with- in the image. Click the Window button to define an arbitrary rectangular area within the image for the preview.
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The preview window is only relevant while you define color adjustment. When the defined color adjustment is conducted, colors are adjusted for the entire image, or if a frame has been set, to the area within the frame.•
The first time you start up the default setting for the preview is for the whole image.3) Defining color-adjustment parameters:
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Move one of the slide controls to the left to reduce that color within the im- age.•
Move a slide controls to the right to increase that color within the image.•
A value of 0 means that a color component’s original value remains unal- tered.4) Click the Reset button to return to the original image as it was before you made any adjustments. All parameters are reset to 0.
5) Click the File... button to save a set of parameters. A parameter set you have saved can be reloaded anytime and used for color adjustment on any image.
6) Click the Execute button to execute color adjustment using the current param- eters.
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The dialog box will be closed and the current preview image will be written into the destination image buffer.•
WARNING: If you have selected a certain image area using the Image > Set Frame command, color adjustment will only be applied to the image area selected via the frame.Processing images
Filtering gray-value images - Background Information