Pulling the Jpeg with DBImage

Pictures inside a database

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The DBImage - take one

The first thing I do when trying to do something new with Delphi is to "ask"

Delphi Help for help. This is what the Help system replies: TDBImage (Data Controls page on the component palette) represents a graphic image from a BLOB (binary large object) field of the current record of a

dataset. Use TDBImage to represent the value of graphic fields. TDBImage allows a form to display graphical data from a dataset. The DBImage is nothing more than a TImage

component with some data aware properties. The two most important

ones are: DataSource and Field. The DataSource property links the image component to a dataset. We have a DataSoure

component named DataSource1 on our form that represent a dataset. The Field property indicates the field (in a table) that holds the image.

All clear, put a DBImage on form and leave the DBImage1 name. To actually link a DBImage with a BLOB field in a Table

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we simply need to do the following assignment (using the Object Inspector):

DBImage1.DataSource = DataSource1 DBImage1.Field = Picture

This should do the trick of displaying the JPEG image stored in the Picture field of the Applications table.

To see whether this assignment will work the only thing we have to do is to set the Active property of the ADOTable1 component to True. We can do this at design time with the Object Inspector.

Once you set it you'll get the following:

Now what? Why does it say "Bitmap image is not valid." We have a JPEG picture not the BMP - is this the problem? Let's go back to the Help.

After a few clicks through the Help the conclusion is: to get the JPG inside a database we need to use the TJpegImage object. To display the picture we need the simple, non-data aware, version of the Image component. Even more we'll need to use streams to load a picture from a BLOB object. The Help states that we should use TADOBlobStream to access or modify the value of a BLOB or memo field in an ADO dataset.

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Pictures inside a database

Page 3: Streaming the Jpeg - the wrong way.

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Pulling the Jpeg - take two!

Since we can do nothing with the DBImage - remove it from the form and place an ordinary TImage component (Additional palette) on it.

Name it ADOImage. Unfortunately the Image component does not have any data-aware properties, so all the code needed to show a picture from a table inside it will require a separate procedure. The easiest thing to do is to put a Button on a form and place all the code inside it's OnClick event. Name the button 'btnShowImage'.

To use the ADOBLOBStream the Help suggests to create an instance of TADOBlobStream, use the methods of the stream to read from a

graphic field in a dataset, and then free the BLOB stream. Somewhere

"in the middle" we'll need to use LoadFromStream to load a Jpeg image from a TADOBlobStream object. The Image's component Picture.Graphic property will be used to actually store and display the picture.

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Field object, what are those?

At this moment I'll assume that only a small amount of knowledge on Field objects will be enough for you to keep with this chapter. In Delphi database development one of the primary objects is the TField object. Field components are non-visual objects that represent fields of the dataset at run (and design) time. The TADOTable (and other TDataSet descendant) gives access to the Fields Editor at design time. The Fields Editor enables you to select the fields that you want to include in the dataset. More important, it creates a persistent lists of the field components used by the dataset in your application. To invoke the Fields Editor double click the TADOTable component. By default, the list of fields is empty. Click Add to open a dialog box listing the fields in the Applications table. By default, all fields are selected. Select OK.

When Delphi gives (default) names to Fields the following notation is used: Table name + Field name. This means that our picture field has the name: ADOTable1Picture.

The TADOBlobStream Create method creates an instance of

TADOBlobStream for reading from or writing to a specific BLOB field object, which is in our case the ADOTable1Picture field.

We will place the code in the OnClick event for a btnShowImage button.

The code should read the picture from the Picture field of the currently selected row. This is how the code should look like:

uses jpeg;

bS := TADOBlobStream.Create

(AdoTable1Picture, bmRead);

try

Pic:=TJpegImage.Create;

try

Pic.LoadFromStream(bS);

ADOImage.Picture.Graphic:=Pic;

finally

Ok, let's run the project now. Of course set the ADOTable1.Active

property to True. The form is displayed and after clicking on a button this is what we got:

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Hm, what now? The code in the procedure is 100% correct but the image doesn't get displayed! Remember the "Never give up, never surrender"?

Let's go down to byte level to see what's happening!

Next page > Seeking the start of Jpeg in the BLOB - the correct way >

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