playwright/tests/image_tools/fixtures/should-fail/julia-ssim-trap/README.md

# Julia SSIM trap

[SSIM](https://en.wikipedia.org/wiki/Structural_similarity) is a metric used to compare image similarity.

While original SSIM is computed against the luma channel (i.e. in a gray-scale),
the Julia language [computes a weighted combination of per-channel SSIM's](https://github.com/JuliaImages/ImageQualityIndexes.jl/blob/e014cee9bef7023a1047b6eb0cbe49fbf28f2fed/src/ssim.jl#L39-L41).

This sample is a white image and a gray image that are reported equal by Julia SSIM.
It also traps all the suggestions for color-weighted SSIM given here:
https://dsp.stackexchange.com/questions/75187/how-to-apply-the-ssim-measure-on-rgb-images
feat: implement a new image comparison function (#19166) This patch implements a new image comparison function, codenamed "ssim-cie94". The goal of the new comparison function is to cancel out browser non-determenistic rendering. To use the new comparison function: ```ts await expect(page).toHaveScreenshot({ comparator: 'ssim-cie94', }); ``` As of Nov 30, 2022, we identified the following sources of non-determenistic rendering for Chromium: - Anti-aliasing for certain shapes might be different due to the way skia rasterizes certain shapes. - Color blending might be different on `x86` and `aarch64` architectures. The new function employs a few heuristics to fight these differences. Consider two non-equal image pixels `(r1, g1, b1)` and `(r2, g2, b2)`: 1. If the [CIE94] metric is less then 1.0, then we consider these pixels EQUAL. (The value `1.0` is the [just-noticeable difference] for [CIE94].). Otherwise, proceed to next step. 1. If all the 8 neighbors of the first pixel match its color, or if the 8 neighbors of the second pixel match its color, then these pixels are DIFFERENT. (In case of anti-aliasing, some of the direct neighbors have to be blended up or down.) Otherwise, proceed to next step. 1. If SSIM in some locality around the different pixels is more than 0.99, then consider this pixels to be EQUAL. Otherwise, mark them as DIFFERENT. (Local SSIM for anti-aliased pixels turns out to be very close to 1.0). [CIE94]: https://en.wikipedia.org/wiki/Color_difference#CIE94 [just-noticeable difference]: https://en.wikipedia.org/wiki/Just-noticeable_difference 2022-12-03 00:22:05 +01:00			`# Julia SSIM trap`

			`[SSIM](https://en.wikipedia.org/wiki/Structural_similarity) is a metric used to compare image similarity.`

			`While original SSIM is computed against the luma channel (i.e. in a gray-scale),`
			`the Julia language [computes a weighted combination of per-channel SSIM's](https://github.com/JuliaImages/ImageQualityIndexes.jl/blob/e014cee9bef7023a1047b6eb0cbe49fbf28f2fed/src/ssim.jl#L39-L41).`

			`This sample is a white image and a gray image that are reported equal by Julia SSIM.`
			`It also traps all the suggestions for color-weighted SSIM given here:`
			`https://dsp.stackexchange.com/questions/75187/how-to-apply-the-ssim-measure-on-rgb-images`