Efficiently split very large image in many tiny tiles

[TanukiSharp]

Hello, and first, great work on this library.

Here is my problematic, I have a very large image loaded in memory, the file can be between 700 MB to 1 GB, and I need to split it in tiles of, let's say, 256 x 256 pixels. Tile size is customizable and doesn't have to be a power of 2.

So far, from what I understood, I have to clone the image and crop it.

Let's assume (very rough numbers but good enough for the example) the loaded image in memory takes 1 GB and one tile take 30 KB, then on a machine with only 1.1 GB of memory, cloning and cropping wouldn't work. I don't want to throw money at that to get more memory because it still feels like wasting, and it's probably going to be very slow.

Another approach could be to load, mutate / crop to tile, save, then load the original again and mutate / crop again, and so on, but this still feels inelegant.

I thought about creating a new image of 256 x 256 pixels for the tile and draw to it with the DrawImage processor, but it seems I can't decide which area of the source image to draw to the target tile image. I tried to use ResizeMode.Crop on resize, but so far I could only get a "resize effects" and never a "crop with no scaling".

I searched StackOverflow but didn't find anything that looked like my problem.

Any suggestion is good to hear, thanks for your help.

[TanukiSharp]

For those who might be interested, I found a very straightforward and satisfying way to do it:

private static Image<Rgba32> Extract(Image<Rgba32> sourceImage, Rectangle sourceArea)
{
    Image<Rgba32> targetImage = new(sourceArea.Width, sourceArea.Height);

    int height = sourceArea.Height;

    for (int i = 0; i < height; i++)
    {
        Span<Rgba32> sourceRow = sourceImage.GetPixelRowSpan(sourceArea.Y + i);
        Span<Rgba32> targetRow = targetImage.GetPixelRowSpan(i);

        sourceRow.Slice(sourceArea.X, sourceArea.Width).CopyTo(targetRow);
    }

    return targetImage;
}

I could parallelize the for loop, but for the moment it's already fast enough for my use case.

[JimBobSquarePants]

Seeing this makes me very happy. It’s great to see someone being inventive with our general APIs!

[jjxtra]

Is GetPixelRowSpan gone?

[antonfirsov]

In 2.0+ replaced them with ProcessPixelRows:

https://docs.sixlabors.com/articles/imagesharp/pixelbuffers.html#efficient-pixel-manipulation

[jjxtra]

Thanks, this is perfect