[Apocalypse]

I am trying to write an app that will generate alpha values for 24-bit TIFF images and save them to 32-bit RGBA TIFF images. So far I have been able to get just about everything working perfectly but I am noticing some odd behavior when adding noon-100% alpha values. For some reason, everything works perfectly when I add the alpha channel as 100% opacity for every pixel but if I put any less than that it draws the image with the specified opacity but with no RGB colour components (ie, it is a black, but semi-transparent, image).

Does anyone know why this is happening or of a way around this problem?

Thanks,
Jeff.

[lindberg]

If you're playing with the bits in the image rep... dunno, sounds like the color values are off. Maybe you're using the wrong scale, or a float is getting converted to an integer without a cast (resulting in 0, though there should usually be a warning somewhere around such code).

If you're using postscript drawing to draw the pixel, which composite operation are you using?

[Apocalypse]

An Apple Engineer on the cocoa-dev mailing list helped me out with this so I will post a summary of what he said for anyone interested.

Apparently cocoa pre-multiplies the RGB values based on the given alpha value. To work within this arrangement, the data in the RGB channels of TIFF data must be multiplied by a factor of (alpha/255) before they will appear correctly. This also explains why I was having the earlier problem of black images appearing at less than 255 alpha (the values would wrap-around to 0, ie black, and then have the alpha function applied).

Hope that is of interest and of use,
Jeff.