Premultiplied Alpha Targas in batch process

[magick]

Try

• convert input.tga +matte -resize 50% output.tga

to remove the alpha channel.

[magick]

It seems your alpha channel is inverted. Try displaying the image with Gimp it produces the same results as ImageMagick. You could try -channel A -negate to correct the problem.

[magick]

We're using ImageMagick 6.3.0 and this command seems to produce what you're looking for:

• convert \( Asimov.tga +matte -resize 50% \) \( Asimov.tga -fx a -resize 50% \) -compose CopyOpacity -composite image.tga

[magick]

ImageMagick resizing uses unassociated alpha when the matte channel is active. In your case your alpha channel is not really alpha values so we inactivate the alpha channel with the +matte option and deal with the alpha channel separately.

[anthony]

An alturnative that may be easier is to use the new multi-image -separate option. This will separate an image into a list of gray scale images on per channel. This makes it useful to disconnect any color-alpha relationship that may be involved. Remember more options with IM are applied to ALL images in a image sequence.

Code: Select all

  convert Asimov.tga -channel RGBA -separate -resize 50% -combine image.tga

[anthony]

Hmmm after trying this myself I found that using -separate on your image produces four images, but each still contained the alpha channel!!!!! I think that may have been an unexpected feature, and in this case unwanted.

Adding a +matte after the -separate fixed the problem.

Code: Select all

  convert Asimov.tga -channel RGBA -separate +matte -resize 50% -combine image.tga

I'll update the IM Examples page about -separate to make note of this.

[anthony]

Should be much faster on larger images. -fx has to interpriate the mathematical argument it is given once for every pixel and color channel involved. -separate just does the job directly.