TST: Calculate RMS and diff image in C++ #29102
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The current implementation is not slow, but uses a lot of memory per image. In `compare_images`, we have: - one actual and one expected image as uint8 (2×image) - both converted to int16 (though original is thrown away) (4×) which adds up to 4× the image allocated in this function. Then it calls `calculate_rms`, which has: - a difference between them as int16 (2×) - the difference cast to 64-bit float (8×) - the square of the difference as 64-bit float (though possibly the original difference was thrown away) (8×) which at its peak has 16× the image allocated in parallel. If the RMS is over the desired tolerance, then `save_diff_image` is called, which: - loads the actual and expected images _again_ as uint8 (2× image) - converts both to 64-bit float (throwing away the original) (16×) - calculates the difference (8×) - calculates the absolute value (8×) - multiples that by 10 (in-place, so no allocation) - clips to 0-255 (8×) - casts to uint8 (1×) which at peak uses 32× the image. So at their peak, `compare_images`→`calculate_rms` will have 20× the image allocated, and then `compare_images`→`save_diff_image` will have 36× the image allocated. This is generally not a problem, but on resource-constrained places like WASM, it can sometimes run out of memory just in `calculate_rms`. This implementation in C++ always allocates the diff image, even when not needed, but doesn't have all the temporaries, so it's a maximum of 3× the image size (plus a few scalar temporaries).
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PR summary
The current implementation is not slow, but uses a lot of memory per image.
In
compare_images, we have:which adds up to 4× the image allocated in this function.
Then it calls
calculate_rms, which has:which at its peak has 16× the image allocated in parallel.
If the RMS is over the desired tolerance, then
save_diff_imageis called, which:which at peak uses 32× the image.
So at their peak,
compare_images→calculate_rmswill have 20× the image allocated, and thencompare_images→save_diff_imagewill have 36× the image allocated. This is generally not a problem, but on resource-constrained places like WASM, it can sometimes run out of memory just incalculate_rms.This implementation in C++ always allocates the diff image, even when not needed, but doesn't have all the temporaries, so it's a maximum of 3× the image size (plus a few scalar temporaries).
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