Improve performance of grayscale calculation steps #2
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Since the pixel rewrite loop will run literally thousands of times on just a single image, it makes sense to optimize it as much as possible. Reducing to a single iterative loop instead of two nested loops. Also reduce the math run inside of each loop to calculate the pixel position in the array.
The standard "Luma" formula is closer to how humans perceive colors and grayscale. While an average is technically correct, it doesn't match what the human eye perceives. See http://www.tannerhelland.com/3643/grayscale-image-algorithm-vb6/ for more explanation.
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The grayscale calculation involving nested loops that check properties of the image on every loop are inefficient. On top of that, the extra calculation to determine the pixel position based on the interation of the nested loops adds more overhead
This can be collapsed into a single iterative loop, without an inner calculation. Considering this loop runs literally thousands of times on even a small image, simplification makes for a significant performance improvement.