What's an efficient way to tell if a bitmap is entirely black?

Question

I\'m wondering if there\'s a super-efficient way of confirming that an Image object references an entirely black image, so every pixel within the bitmap is ARGB(255, 0, 0, 0).

Answer 1

A reasonably reliable method would be to check the file size of the image. That is, if the images that are not all black have a relatively normal distribution of colors.

If you know the file type, you know some basic things about the average compression ratios. And you can determine the dimensions of the file pretty readily without cycling through the whole file.

An all black image, of any dimension, using a compressed file format, is going to have a very small file size compared to an image of identical dimensions with a fairly normal distribution of colors.

This method would take a little bit of time to test and build up a knowledge base of just what the file size of an all-black image should be compared to a non-all-black image, but it would be very fast.

If you have many instances where the non-all-black images are pretty close to all-black, obviously then this method would not work.