This assumes you have a good understanding of how to manipulate imageData and maskData. If you don’t, take a look at tip imag003 - Understanding ImageData, MaskData and AlphaData before continuing.

The following is an example of making an image 50% transparent using the alphaData property of an image. It is similar to setting the ink of an image to blend and then setting the blendLevel, but is useful to show how to manipulate the alphaData of an image.

on mouseUp
  put the imageData of image 1 into iData
  put the width of image 1 into tW
  put the height of image 1 into tH
  put empty into aData
 
  repeat with i = 1 to tH  -- iterate each row
    repeat with j = 1 to tW   -- iterate each column
      -- Set every pixel to be 50% transparent (1/2 of 255 = 128)
      put binaryEncode("C",128) after aData
    end repeat
  end repeat

  set the alphaData of image 1 to aData
  set the imageData of image 1 to iData
end mouseUp

This can be shortened further because you’re using every pixel of the image, so you don’t need to know the height and width of the image. Here’s the shorter version (thanks, Sarah!):

on mouseUp
  put the imageData of image 1 into iData
  put empty into aData
 
  repeat for each char X in the alphaData of image 1  -- See Note below
    -- Set every pixel to be 50% transparent (1/2 of 255 = 128)
    put binaryEncode("C",128) after aData
  end repeat

  set the alphaData of image 1 to aData
  set the imageData of image 1 to iData
end mouseUp

Note: Using this approach provides a way to iterate over the pixels using a repeat for each looping construct (which is much faster than a repeat with loop). Both the maskData of image and the alphaData of image will return a number of characters (a "length") that is the total number of pixels in the image (for a refresher on this, see imag003 - Understanding ImageData, MaskData and AlphaData.) So, since I could use either the maskData of image or the alphaData of image, I decided to use the alphaData of image to avoid confusion. Hope that’s clear...

Have fun!

Posted 7/2/2002 by Ken Ray