# Splitting an image into tiles

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Hi!

Let's say I have an image and I want to loop through the pixels. I know how to do this in scan-line form:

for (x = 0; x < w; x++)
{
for (y = 0; y < h; y++)
{
currentPixel = image[y*w + x];
}
}


How would I do the same thing if I was splitting my image into square tiles?

Thanks :)

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Something like this? Pseudocode, of course, four nested loops is unsightly and can probably be refactored depending on your code, e.g. foreach tile process tile.

for (tileY = 0; tileY < h / tileH; ++tileY)
for (tileX = 0; tileX < w / tileW; ++tileX)
for (y = tileY * tileH; y < (tileY + 1) * tileH; ++y)
for (x = tileX * tileW; x < (tileX + 1) * tileW; ++x)
{
// do stuff on pixel (x, y)
}


Obviously the image needs to split properly into tiles of the specified dimension (tileW, tileH), otherwise you'll need to handle the leftover pixels somehow.

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So, first of all, fix your loops -- having y inside x is terrible for your cache performance.

The general answer to tiling the image is to wrap your y-x pixel loop in another y-x tile loop. Divide the height and width of the whole image by the height and width of your tiles, respectively, to get the number of iterations in your y-x tile loops. You'll need to take special care of images that don't divide evenly into perfect tiles.

Edited by Ravyne