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My iPhone App has custom buttons that display png-images.
I like to replace the white color in those images by a transparent color. Is there a tool on the Mac that allows me to do that? (or is there some other way to indicate which color is transparent in xcode?)
The Preview application has a mode on its select tool called “Instant Alpha” which eases selection of backgrounds. Once you have your background selected, you can simply press the “Delete” key and that area of the image will be removed. Save it as a PNG with the “Alpha” box checked and you’re done.
Most other image editing tools, particularly those supporting multiple layers in the image, will also properly support PNG alpha channels. These will likely be easier than using Preview, but they are all third-party and many cost money so you will need to make that decision.
Please note however that if you are starting from an image with a solid background to which some things have a faded edge that blends in to the background, it is unlikely that you’ll be able to remove the background entirely to your satisfaction without cutting in to the actual image.
You may have seen GIF images in the past which were designed to blend in on a white background and had some bright pixels at the edges which stood out and look ugly on darker backgrounds. An image properly saved as PNG or any other format with an alpha channel from the beginning will not have this problem.