The lineart model often outputs a lot of almost-black noise. SD1.5 ControlNets seem to be OK with this, but SDXL ControlNets are not - they need a cleaner map. 12 was experimentally determined to be a good threshold, eliminating all the noise while keeping the actual edges. Other approaches to thresholding may be better, for example stretching the contrast or removing noise.
I tried:
- Simple thresholding (as implemented here) - works fine.
- Adaptive thresholding - doesn't work, because the thresholding is done in the context of small blocks, while we want thresholding in the context of the whole image.
- Gamma adjustment - alters the white values too much. Hard to tuen.
- Contrast stretching, with and without pre-simple-thresholding - this allows us to treshold out the noise, then stretch everything above the threshold down to almost-zero. So you have a smoother gradient of lightness near zero. It works but it also stretches contrast near white down a bit, which is probably undesired.
In the end, simple thresholding works fine and is very simple.
The HTML Canvas context has an `imageSmoothingEnabled` property which defaults to `true`. This causes the browser canvas API to, well, apply image smoothing - everything gets antialiased when drawn.
This is, of course, problematic when our goal is to be pixel-perfect. When the same image is drawn multiple times, we get progressive image degradation.
In `CanvasEntityObjectRenderer.cloneObjectGroup()`, where we use Konva's `Node.cache()` method to create a canvas from the entity's objects. Here, we were not setting `imageSmoothingEnabled` to false. This method is used very often by the compositor and we end up feeding back antialiased versions of the image data back into the canvas or generation backend.
Disabling smoothing here appears to fix the issue. I've also disabled image smoothing everywhere else we interact with a canvas rendering context.
