The probe's comment said a 192px render "averages a small neighbourhood
into each of its pixels". It does not: the composed shader fetches the
source at one position per output pixel - nearest for an unrotated
frame, four photosites blended otherwise - so the probe was a point
sample of a noisy sensor, and two painted-white air conditioners on the
same wall answered +37 and -50.
The tap is now narrowed to the patch of the canvas around the click, a
couple of percent of its width and square on screen, and rendered at
64x64 with interpolation forced on, which puts a sample on every sensor
pixel under it at any ordinary zoom. The samples are averaged, with the
void and clipped ones left out rather than allowed to pull the mean, and
fewer than half surviving is refused. compose_camera_probe takes the
patch; the merge's compose_camera_linear keeps its nearest sampling. The
readback shrinks from six megabytes to sixty-four kilobytes.
A frame of alternating warm and cool columns, averaging neutral, moves
the controls by at most two units; a point sample swung them to sixty.
dtourolle
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