The twisted logic of alpha "post-blending" has to do with a node-sandwich issue specific to HSL blend modes. The Right Thing to do here is to make alpha pre-blending possible with an optional flag (using the existing syntax, the trailing "p" in foo[hsl-saturation@20p]), and making it clear in node-sandwich (maybe in the forthcoming UI?) that HSL blend modes will probably want pre-blending enabled by default.

This should composite the source layer onto the destination layer with destination-in (intersection) before then compositing the destination onto the source.

This opens the door up to either:

1. Having to do threaded pixel processing (using Web Workers in the browser, something else in Node), which means that compositing would happen asynchronously; or
2. Creating a WebGL-based API for compositing with fragment shaders, which would work browser-side only.

More here soon.

These would be passed along directly to ctx.drawImage()