12812452e8f90ead6e587b39ca87a425294dd625
2
Commits
| Author | SHA1 | Message | Date | |
|---|---|---|---|---|
 dtourolle
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4b6c110816 |
Count the sensor's own numbers, so a cull can see headroom the render hides
FR-CULL-3's remaining two bullets. What existed was a *display* histogram tagged FR-DSP-7: it binds AdjustPass's Rgba8Unorm output, recovers an 8-bit code value, and counts clipping as `r == 255`. Its own documentation says a clipped bin means "a highlight that is actually gone rather than one the transform might still recover", which is the opposite of what a culling decision needs. FR-CULL-3 asks for the histogram of the sensor data, on the explicit grounds that a rendered image "systematically lies about what is recoverable in the raw", and a readout that measures the render cannot answer that however it is presented. So this is a second instrument beside the first rather than a setting on it. Both are true; they are true about different things; the panel offers both behind a chip row and the words travel with the numbers, because a raw saturation figure drawn under a heading saying Highlights would be mislabelled exactly where the difference matters. **What is reduced over, and what it cost to decide.** ARCH §5.5 specified the pre-demosaic CFA samples. This reduces over the demosaiced scene-linear texture instead, and §5.5 is amended to record the choice rather than let the specification and the code disagree in silence. The texture is camera-native — unbalanced, unmatrixed, uncurved — and normalised by the sensor's own black and white levels, so 1.0 is saturation by construction and the distribution below it is the headroom question with no calibration to carry. Retaining the CFA samples would mean keeping the packed u32 buffer Demosaicer::run currently drops: 48 MB at 24 MP, 120 MB at 60 MP, resident per open photograph whether or not anyone looks at the histogram, on a platform §6.2 exists because memory is scarce on. Three things it therefore cannot say, written into the module docs and into §5.5 rather than left to be discovered: it counts pixels not photosites, so a saturated site drags its interpolated neighbours up and per-channel clipping is smeared by about a demosaic kernel; it cannot see above white, because demosaic.wgsl clamps each photosite at 1.0 for its own good reasons (a Canon 6D reads to 16383 against a declared 15070) so "at saturation" and "a stop past it" share a bin; and it is measured after the CFA pattern is gone, so it can name which colour clipped in the reconstructed image but not which photosite went first. The axis is stops below saturation, 16 bins per stop over 256 bins — the same bin count the display reduction uses, so the fold into drawable columns is shared and a divergence between the two plots would have to be deliberate. A linear axis spends half its width on the top stop, which is why nobody has ever drawn a useful linear raw histogram. The fourth series is the brightest channel rather than luma: these values are unbalanced, so any weighted sum of them is a number about nothing, and the brightest channel is the one that saturates first and so the one the headroom question is actually about. It is a property of the file and not of the render, which has two consequences. It is computed once per photograph and cached — nothing downstream of the demosaic can move a count in it — so a cull does not pay the display histogram's per-frame cost three thousand times. And it describes the whole frame rather than the visible region, deliberately opposite to DevelopSession::histogram: a crop changes what is on screen and changes nothing about what the sensor recorded. Tags are on the reduction, the type, its constructor and the presentation arithmetic, each of which has a test that fails if the behaviour goes. The Slint panel and the push from lib.rs keep their reasoning as prose: nothing asserts them, and a tag would claim coverage the assertions are not making. |
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 dtourolleandClaude Opus 5
|
0233df4bf2 |
See what the highlights are doing: a live histogram (FR-DSP-7)
Exposure, blacks and whites were set by eye. Nothing said a highlight had blown — the canvas shows white where a channel is at 250 and white where it is at 255, and the difference is the whole question. **Counted on the GPU, not on the readback.** There is a full frame sitting in CPU memory on every canvas update right now — `AdjustPass::read_output`, the bridge spike S1 removes — and walking it would have been thirty lines and no shader. FR-DSP-7 states the mechanism and not just the feature: "these derive from a GPU-side reduction into a small buffer. Per-frame CPU readback of image data is prohibited." A histogram founded on the bridge would be correct today and deleted by S1, and would meanwhile be the reason the bridge could not go. What crosses the bus here is 4104 bytes whatever the image size. The reduction tallies into workgroup memory first and merges once per workgroup. A photograph is not noise: a clear sky puts tens of thousands of adjacent pixels in one bin, and contending for that single global atomic serialises the dispatch. **On the settled frame only.** `render_now` already knows whether a gesture is still moving — `draft` is the flag `redraw` derives from `was_coalesced` — so the dispatch and its transfer happen once when the slider stops rather than on each of the forty frames a drag emits. Nothing is lost: a histogram flickering past under a finger is not a reading anyone takes. FR-DSP-7 requires exactly this, that it not extend the FR-DSP-3 frame budget. Luma is weighted in 8.8 fixed point — 54, 183, 19, summing to 256 exactly — rather than in floats. Not thrift: it makes the shader's arithmetic reproducible bit for bit, which is what lets the test below be an `assert_eq` against a CPU count rather than a tolerance. ARCH §6.13's line about integer state, applied where it happens to also be free. **What the numbers were checked against.** A flat frame must put all 4096 pixels in one bin and one only. A 256-wide ramp must occupy every level with exactly the same count, which is what catches an off-by-one in the quantisation — a `floor` where a rounding was needed shifts the whole photograph one bin left and looks like nothing at all. And a 101x37 frame of seeded pseudo-random pixels — deliberately not a multiple of the 16x16 workgroup, so the edge tiles run off the image — is compared slot for slot against a second, obvious CPU implementation. Exact equality, no tolerance. The CPU version is a deliberate reimplementation rather than shared code: the bugs worth catching here are ones shared code would commit identically on both sides. Above that, the presentation arithmetic is unit-tested headless, because it is where a wrong answer is invisible. A histogram of the wrong shape looks exactly as plausible as one of the right shape. So: 64 columns because it divides 256 and an uneven fold draws an even ramp as a comb; the peak excludes the end columns, or a night scene scaled against its own black spike is a flat line with no information in it; heights are clamped into the plot; and "0%" is kept distinct from "<0.1%" and from "—", since an indicator reading "clipped" over a figure reading "none" is a panel contradicting itself. Clipping counts a *pixel* with any channel at an extreme, not a channel. Any, because a blown red has no gradation left in it however much green and blue still hold — and it is the saturated highlight, the sunset and the red jersey, that clips first and recovers worst. Per pixel, because counting channels can report 200% of a frame clipped, and a percentage above 100 is a readout nobody trusts again. Two affordances for it, which NFR-A11Y-3 asks for: a bar standing at the end of the plot the tones are piling against, and a figure saying how much. Either alone reads. The panel sits directly under the capture metadata and above every control, because it is what the controls are judged against. It is hand-built rather than generated, and ARCH §4.3a is untroubled: a histogram is not an operation — no parameters, changes nothing, answers a question rather than asking one — and nothing in it reads a parameter out of a descriptor. Three plot colours and a neutral luma trace join the palette. That is the swatch's exception rather than a second one: a per-channel histogram has to say which channel, and no achromatic treatment distinguishes red from blue, so the hue is data exactly as the image beside it is. Held well back from full strength for the reason the theme preamble gives. The bounded, non-parking map wait moves out of `AdjustPass` into `readback::await_mapping`, shared with the histogram's transfer. Thirty lines of load-bearing reasoning about frozen interfaces and lost devices, and two copies of it would have drifted. The histogram describes the frame on the canvas, so it is in the output colour space FR-DSP-7 asks for, and when zoomed it describes the visible region — a photographer inspecting a highlight at 4x is asking about that highlight. A device that cannot build the reduction loses the histogram and keeps the photograph. Still to do for FR-DSP-7: the pixel colour readout under the cursor. 324 tests pass, clippy and fmt clean. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> |