SSIM based downscaling algorithm

[HybridDog]

I've found a new downscaling algorithm:
https://graphics.ethz.ch/~cengizo/Files/Sig15PerceptualDownscaling.pdf

[sergeevabc]

@jsummers ImageWorsener project is amazing, a hidden, underrated gem. Please, proceed with development.

[zvezdochiot]

@sergeevabc say:

Please, proceed with development.

There is hardly anything to continue. The library has acquired a stable finished look. The only thing that can be added to it is modifiers, for example RIS: https://github.com/ImageProcessing-ElectronicPublications/photoquick-examples/tree/main/main/resize

[sergeevabc]

There is hardly anything to continue.

technical.txt written by the author has a “to do” section and this issue tracker has opened issues.

[HybridDog]

At first the algorithm squares each pixel value of an image I; let the result be the image I2. Then it downscales the images I and I2 by an integer factor with Pixel Mixing to get the images L and L2. After that it uses L and L2 to create the resulting image; see the paper for details.

When I downscale the rings image by two with my implementation of the algorithm, it preserves all rings and does not produce moire patterns:

(view it in 1:1 resolution)

With a higher scaling factor, the pixel mixing artefacts appear:

I have never tested what happens if I use another downscaling algorithm than pixel mixing (or box filter) to calculate L and L2.

With the ellipse image, the sharpening is visible:

[zvezdochiot]

Hi all!

no RIS	RIS mult = 1	RIS mult = -1