Making a Single Color Transparent Using MaskData
Mac OS 9 Mac OS X Windows Linux

UPDATE 7/29/02: The MetaCard 2.4.3 engine (used by MetaCard 2.4.3 and Revolution 1.5A7 or later) fixed a bug that relating to the order of the color bytes in imageData.

The Tip on this page is based on the MetaCard 2.4.3 engine.

If you have a version of MetaCard/Revolution that uses an earlier engine than 2.4.3, you can view the original Tip for assistance.

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 creating a single-color transparency of an image much like what is used in GIF images. I have chosen to use pure red (RGB: 255,0,0) as the mask color.

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 mData
  repeat with i = 1 to tH  -- iterate each row
    repeat with j = 1 to tW   -- iterate each column
      -- Get a pointer to the specific end byte of a pixel
      -- Remember there are 4 bytes per pixel
      put ((i-1)*tW*4)+(j*4) into tByte
      -- Go backwards from the end byte to get your R, G and B
      put charToNum(char (tByte-2) of iData) into tR
      put charToNum(char (tByte-1) of iData) into tG
      put charToNum(char (tByte) of iData) into tB
      if (tR = 255) and (tG = 0) and (tB = 0) then
        -- pixel is red, mask it
        put binaryEncode("C",0) after mData
        put binaryEncode("C",255) after mData
      end if
    end repeat
  end repeat

  set the maskData of image 1 to mData
  set the imageData of image 1 to iData
end mouseUp
Hope you find this as useful as I did. :-)

Posted 7/2/2002 by Ken Ray