Rectangles

Nobody has claimed this yet.

Currently every draw object is associated with a full Bézier path. That'll be particularly wasteful when we have more draw objects that are images or glyphs drawn from a texture atlas. I'm implementing axis-aligned rectangles as a special case of geometry, and finding it useful to write the design down for references.

As usual, a change like this touches many stages of the pipeline. In addition to the changes listed below, there are places that make the assumption that the draw object index and path index are the same, and that will need to change (that's already something that

## Encoding

The design is for axis-aligned rectangles. Therefore the "append fragment with transform" function needs to handle one special case: the transform is not axis-aligned, and the appended fragment contains rectangles. In that case, the rectangles need to be expanded to full paths.

(in the piet render context, this logic is simpler, as the transform is known at the time the draw command is issued, but we're not planning on retaining this)

Alternative considered: use parallelogram as the primitive rather than axis-aligned rectangle, so it's closed under general affine transform.

The encoding is as follows: the draw object tag gets an extra bit incidating rect or path. If it's a rect, the drawdata stream gets 4 f32's representing the coordinates.

(to be determined: many choices for bit encoding; it's already got a bunch of bit magic for making size calculations faster)

Alternative considered: separate stream for rectangle coordinate data. That would require another scalar in DrawMonoid.

## Draw scan

No major change here except to incorporate size into scene_offset. (easiest way to do that is to have just one bit for rect, mask and shift to add it to size). Also obviously don't increment path_ix when rect bit is set.

Currently draw_leaf fetches the bounding box, but it looks like that's unnecessary and can be cleaned up.

Note that rectangles are not represented in the pathtag or path data stream, so the pathtag scan and coarse path rendering are not affected.

## Binning

Binning is the main place the bbox is processed, as it's the place in the pipeline where the clip bbox is available.

The output rect is stored into draw_bbox_alloc. This will serve as the dimensions of the rectangle as well for downstream processing.

Discussion question: I'm tempted to have draw leaf compute the bbox (otherwise it would be output in path_coarse). If it's the same format, that format will need to change, as it's currently quantized to integers (so it can be operated on by atomics). If the format is changed (say, 8 bits of fraction), then binning can be completely unaffected.

## Coarse raster

When the rect bit is set, don't reference the path data structure, rather do some special-case processing.

The interior of the rect is a solid, but tiles on the edge need to issue a Cmd_Rect, see below for more details on that.

## Fine raster

One new command: Cmd_Rect. Rendering is by multiplication of half-planes antialiased by clamp (this should be about as fast as non-AA rendering).

Also note: the most natural encoding for rect coords is 4 f32's, but those could be more compact. Using f16's (tile-relative) would also be natural, but even u8 (with 1/16 pixel quantization) is doable.

Start by tracing the encoding, draw scan, binning, coarse raster, and fine raster stages described in the issue, including append fragment with transform and draw_leaf. Review the proposed rectangle tag, drawdata coordinates, scene_offset, draw_bbox_alloc, and Cmd_Rect design, then resolve the open encoding and bounding-box questions before implementation; done means rectangles work through the full pipeline without path entries.

Written by the indexing model from the issue text.