This project includes a simple app with some sample images to demonstrate how this works. Using the mh_combineWithAlphaImage:backgroundColor: method from the UIImage category in this project, you can combine the JPEG source image with a second image that contains only an alpha channel, to create an image that does have transparency. Because of the JPEG compression the resulting image loses a little bit of clarity but if you tweak the compression settings you can usually get away with it. You can combine these two images at runtime to make the image transparent again. This is a grayscale image with black representing fully transparent, white fully opaque, and gray everything in between. The first image is the regular JPEG with as much compression as you can get away with, the second image is the alpha channel. A value of 255 means this pixel is fully opaque, 0 is fully transparent, and anything in between will mix the pixel's RGB values with the underlying color.īy saving a JPEG image not as one but as two image files you can still have transparent images. For every pixel not only red, green and blue values are stored but also an "alpha" value that determines how transparent that pixel is. The transparency in a PNG file comes from the so-called "alpha channel". But with a little trick we can create a transparent image from a JPEG anyway. The downside is that JPEG images cannot have transparency.
The advantage of JPEG is that it compresses much better than PNG, especially for photos. When you distribute images with your app you usually pick the PNG or JPEG format.
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