D862 - A render scale draws resident targets larger than the guest's

Status: measured Date: 2026-10-14

Amends: D835 - a render resolution setting was rejected there until write-back could resample.

config.toml's [performance] section takes a fourth setting, render-scale, and a title's own render_scale replaces it for its runs, as the other three are (D835). It is how many times the guest's size, across and down, a resident colour target and its depth target are drawn at: one to four, one the guest's own.

A scaled target's guest-facing extent stays the guest's. Everything the guest sees passes through an image of that extent, resampled on the device: its pixels are blitted down on the way to guest memory - averaged where the format filters, the nearest texel for an integer or 32-bit float target - and up, each guest pixel over its square, on the way in. The viewport, scissor and line width a pipeline is built with are scaled with it. A depth target, which only a depth texture reads, is drawn and sampled at the scaled extent.

The display keeps its own copy of a flipped frame at the drawn extent: LazyCopies takes a show snapshot beside the one written to guest memory, and the window shows what it is given, scaled to fit.

Why: a title drawing at its console size is soft on a larger display, and a player with the host to spare can draw it sharper. Neverball's menu at 1920x1080 drawn at twice that ran 6.5 -> 4.2 frames a second in one run each, four times the pixels; the frame written back is the guest's extent and matches one drawn at scale one.

Known limits:

• A pixel shader reading its own screen position sees the drawn one, not the guest's.
• A target sampled as a texture is read from guest memory, so a frame built from earlier passes (post-processing) sharpens only in its last pass.
• A flip shown from guest memory rather than from the device copy (a tiled scanout, D719) is shown at the guest's extent.

Rejected:

• Resampling on the host: a 2x 1920x1080 write-back is 33 MB to read and average a flip.
• Scaling the window's picture alone: it already does, and that only enlarges the guest's pixels.