How to save an image as Tiff or PNG with an alpha channel or alpha mask in iPhone SDK?

Question

I have an image with something inside in a white backround. I want to save that image in a format that allows alpha channel or using an alpha mask in a way that the white pixels became transparents. Any light out there?

Answer 1

I don't know of any libraries where this is super easy. But, there's a lot of relevant sample code in the GLImageProcessing example here. (I haven't run the following)

UIImage *some_image = [UIImage imageNamed:@"somethin'.tiff"];
CGImageRef cg_image = some_image.CGImage;
CFDataRef data = CGDataProviderCopyData(CGImageGetDataProvider(cg_image));
size_t bpp = CGImageGetBitsPerPixel(CGImage);

uint32_t *stuff = (uint32_t *)CFDataGetBytePtr(data);
int w = CGImageGetWidth(CGImage);
int h = CGImageGetHeight(CGImage);
int N = w * h;
for (int i = 0; i < N; i++ ) {
    // do your stuff, test for white, set the alpha mask
    stuff[i] = stuff[i] & ((uint32_t)0xFFFFFFFF | alpha_mask);
}

You could instead use this function

UIKIT_EXTERN NSData *UIImagePNGRepresentation(UIImage *image);                               

and write the data to disk. I hope this helps. Post the solution if you find it...