The Tip on this page is based on the MetaCard 2.4.2 engine and earlier. If you have a version of MetaCard/Revolution that uses the MetaCard 2.4.3 engine or higher, you can view the revised Tip for assistance.

After some trial and error, there is a rhyme and a reason to MetaCard/Revolution’s imageData, maskData and alphaData properties. Keep in mind that it appears that BOTH MetaCard’s AND Revolution’s documentation on this is a bit off, so hopefully either (a) I will be proven wrong, or (b) their documentation will be fixed. — Editor.

The Basics

IMPORTANT! This is the key to working with imageData in MetaCard/Revolution. If you don’t get the byte order correctly as BGRO (in Windows) and ORGB (Macintosh), you will not get anywhere with your image manipulation code. Also note that this is an acknowledged bug in MetaCard/Revolution, and has been fixed in the MetaCard 2.4.3 engine (used by MetaCard 2.4.3 and Revolution 1.5).

Example So Far

Using the basic information above, a three pixel (1 row by 3 pixels) image where the first pixel is pure Red, the second pure Green and the third pure Blue ( ) would have the following (remember the odd byte order):

	Red Pixel				Green Pixel				Blue Pixel
Pixel	1				2				3
Byte	1	2	3	4	5	6	7	8	9	10	11	12
Bit	Blue	Green	Red	Other	Blue	Green	Red	Other	Blue	Green	Red	Other
Bit Value	00000000	00000000	11111111	00000000	00000000	11111111	00000000	00000000	11111111	00000000	00000000	00000000
Byte Value	0	0	255	0	0	255	0	0	255	0	0	0

Retreiving Image Data

Setting Image Data
Keep in mind that you don’t set specific parts of imageData, maskData or alphaData, you set these properties as a whole. That is, let’s say you want to make the second row of pixels in an image to be transparent. You choose to set the maskData of these pixels to 0, and you’ll need to use the binaryEncode function in order to set the binary data (remember that all of this image information is stored in binary). Here’s a simple example that will do that:

on mouseUp
  put the imageData of img 1 into iData
  put empty into mData
  put the height of img 1 into tH
  put the width of img 1 into tW
  put binaryEncode("C",0) into transparentPixel
  put binaryEncode("C",255) into opaquePixel
  repeat with i = 1 to tH   -- iterate through each row
    repeat with j = 1 to tW   -- iterate through each column
      if i = 2 then
        put transparentPixel after mData
      else
        put opaquePixel after mData
      end if
    end repeat
  end repeat
  set the maskData of img 1 to mData
  set the imageData of img 1 to iData
end mouseUp

IMPORTANT! You need to set the imageData of the image after you have applied a mask or an alpha change. It doesn’t seem to work if you do it the other way around.

There are two basic uses of binaryEncode when setting imageData, maskData, or alphaData. In both of these instances, you are setting the equivalent of one pixel of the image.

The first is used to set imageData:

binaryEncode("CCCC",0,redByte,greenByte,blueByte)

binaryEncode("C",{maskOrAlphaByte})

To show you how this works, below is a script that creates a 20 x 20 pure blue image, that is set to 50% transparency:

on mouseUp
  create image
  put it into tID
  set the width of tID to 20
  set the height of tID to 20
  put empty into iData
  put empty into aData
  put binaryEncode("CCCC",0,0,0,255) into bluePixel
  put binaryEncode("C",128) into halfTransPixel
  repeat with i = 1 to 400
    put bluePixel after iData
    put halfTransPixel after aData
  end repeat
  set the alphaData of tID to aData
  set the imageData of tID to iData
end mouseUp

One note of caution, however, is that any changes you make to the image data of an image is permanent; if you screw up, you will have to reimport the original image to get it back to the way it was. One way around this is to store in a custom property of the image a copy of the original imageData prior to manipulating it; something like:

set the origData of image 1 to (the imageData of image 1)

set the imageData of image 1 to (the origData of image 1)

Posted 7/2/2002 by Ken Ray
Updated 7/3/2002 by Ken Ray