replace color to transparent

[nicolas1]

Hello,

There is a 2 color image (text + background) PNG and it is necessary to replace background color (white) to transparent. How is it possible to do in magick core c api?

Thanks

[el_supremo]

It's easier to do this with MagickWand. See the third and fourth examples in my MagickWand Examples in C.
A floodfill may not do what's required if there are "islands" of background colour which must also be replaced. In that case you would use MagickTransparentPaintImage.

Pete

[el_supremo]

I've added a fifth example to the examples which shows how to use MagickTransparentPaintImage to target all white pixels.

Pete

[anthony]

As it is a gradient of two colors. this is easy.

first normalize or contrast-stretch the image to black and white. Then negate if needed to make opaque = white, black = transparent. now just run -background {color} -alpha shape the background color will be the shapes color and everything else will be transparent, or semi-transparent.

See http://www.imagemagick.org/Usage/basics/#alpha

ASIDE the above is equivelent to using CopyOpacity and re-seting the colors.

This is a LOT better than just making pure or near-pure white perfectly transparent! Though does not work well for GIF image saves.

You may also like to look at +level-colors if you just want to assign a new colors to black and white and have the gradient work out right.

[nicolas1]

del