Question

I have an 8bit Image (stored in an Array) containing black(0) and white(255) pixels. Say I want to change all Black pixels in the image to grey(say 120) pixels. What is the fastest way I can change Black to Grey.

I thought of two approaches-

1. Start checking every single pixel in the image. As soon as a black pixel is found change it to grey. Continue till end of image. (Slower but easier)

2. Start checking pixels. When a black pixel is found maintain a counter to track it. Continue incrementing the counter till the next white pixel. Then goto the counter and use a fast function like memset to change a group of black pixels to grey. (Not sure but I think this may be faster)

I have a huge 1GB image therefore approach 1 is pretty slow. Is there a better(faster) way to change/edit pixels?

Answer 1

Probably quicker to do it a word at a time (using word aligned accesses).

You can just bitwise OR with 0x78787878 (assuming 32 bits). This will not affect white pixels but will set black pixels to the required value.