421c61a517
Continues the previous iteration, where the game was measured calling VdGetCurrentDisplayGamma at video init. The remaining link -- does it then WRITE the ramp -- is a GPU register operation (XE_GPU_REG_DC_LUT_RW_INDEX in CommandProcessor::WriteRegister), unlogged and invisible to kernel logging. Two facts from the source close it without instrumenting. 1. The swap-path gamma stage is a PURE LUT. apply_gamma_table.xesli is the whole transform: index by input*255, fetch from a 256-entry ramp buffer, output. No sRGB encode, no second transfer function. 2. The table DEFAULTS TO IDENTITY. CommandProcessor::Initialize fills it with value = i * 0x3FF / 0xFF, and its own comment says the linear default is "what games set when starting with the sRGB (return value 1) VdGetCurrentDisplayGamma". An unwritten ramp is a no-op. So the only transform is a LUT, the LUT is identity unless written, the game queries the display gamma at init, and the capture differs from our composite by gamma 1.34-1.49 -- which identity cannot produce. The guest wrote a non-identity ramp. Labelled an inference, with its weak joint named: it assumes our composite reproduces the PRE-RAMP framebuffer, which it does not exactly. What carries it is the shape -- a systematic ~1.4 fitted on flat patches across three screens is not a compositor bug. The obvious alternative, a fixed sRGB stage in the presenter, fits neither direction: an encode (^0.45) brightens and we measured darkening; a decode (^2.2) darkens far more than 1.4. Direct observation remains available and cheap, and needs the emulator only to boot: a GPU trace records gamma ramps as their own command type, or one log line at the DC_LUT register write would settle it outright. Not done. METHOD: a default value is evidence; and name the weak joint of an inference in the same breath as the conclusion.