The polynomial fit isn't perfect and we end up with alpha values of 1 instead of 0 when applying the mask. This in turn causes issues on canvas where outputs aren't 100% transparent and individual layer bbox calculations are incorrect.
Previously we used erode/dilate and a Gaussian blur to expand and fade the edges of Canvas masks. The implementation a number of problems:
- Erode/dilate kernel sizes were not calculated correctly, and extra iterations were run to compensate. The result is the blur size, which should have been pixels, was very inaccurate and unreliable.
- What we want is to add a "soft bleed" - like a drop shadow with no offset - starting from the edge of the mask, extending out by however many pixels. But Gaussian blur does not do this. The blurred area starts _inside_ the mask and extends outside it. So it kinda blurs inwards and outwards. We compensated for this by expanding the mask.
- Using a Gaussian blur can cause banding artifacts. Gaussian blur doesn't have a "size" or "radius" parameter in the sense that you think it should. It's a convolution matrix and there are _no non-zero values in the result_. This means that, far away from the mask, once compositing completes, we have some values that are very close to zero but not quite zero. These values are quantized by HTML Canvas, resulting in banding artifacts where you'd expect the blur to have faded to 0% alpha. At least, that is my understanding of why the banding artifacts occur.
The new node uses a better strategy to expand the mask and add the fade out effect:
- Calculate the distance from each white pixel to the nearest black pixel.
- Normalize this distance by dividing by the fade size in px, then clip the values to 0 - 1. The result represents the distance of each white pixel to its nearest black pixel as a percentage of the fade size. At this point, it is a linear distribution.
- Create a polynomial to describe the fade's intensity so that we can have a smooth transition from the masked region (black) to unmasked (white). There are some magic numbers here, deterined experimentally.
- Evaluate the polynomial over the normalized distances, so we now have a matrix representing the fade intensity for every pixel
- Convert this matrix back to uint8 and apply it to the mask
This works soooo much better than the previous method. Not only does it fix the banding issues, but when we enable "output only generated regions", we get a much smaller image. Will add images to the PR to clarify.
