Question

What is the fastest way to trim an image (NSImage or CGImageRef) so that all transparent areas around the image are removed?

I’d imagine I could look at each pixel starting from the top and determine its alpha value, but maybe there’s another (faster) way.

Thanks, Mark.

PS: My usecase:

I’m using QLThumbnailImageCreate to generate thumbnails of files. The problem is that all images returned by that call are of equal width and height.

For example, if I create a thumbnail for a photo of the dimensions 300×500, the resulting preview would be of the same width and height with transparent areas added at the top and bottom to preserve the aspect ratio.

Answer 1

It’s actually really fast to iterate over the pixels in an image if you just get a pointer to the raw pixel data. Even on iOS devices this is pretty quick.

Using a CGImageRef you’d get the pixel data like this:

CFDataRef imageData = CGDataProviderCopyData(CGImageGetDataProvider(anImage)); 
const UInt32 *pixels = (const UInt32*)CFDataGetBytePtr(imageData);

and you can just iterate through it:

for (int j = 0; j < (anImageHeight * anImageWidth); j++)
{
    if (pixels[j] & 0xff000000)
    {
    //this is not a transparent pixel
    }
}