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 was identified in the Tip below relating to the order of the color bytes in imageData.

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.

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