How can you have a scene linear reflectance greater than 100%?

Started 3 months ago | Questions thread
spider-mario Contributing Member • Posts: 958
Re: 100% of what?

alanr0 wrote:


As far as I can tell, video folk appear to use reflectance as a relative measure of image brightness. For wide dynamic range formats, the levels corresponding to nominal reflectances of 18% grey and 100% grey are both considerably less than the peak image brightness.

spider-mario quoted from one set of recommendations - specifically ITU report BT.2408-4.

If I understand correctly, the calibration is for a signal which produces an output luminance of 203 cd/m² from a wide dynamic range display. This is intended to represent 100% diffuse reflectance from a surface with "nominal" illumination in the region of interest. The actual luminance at the subject will depend on the illumination employed, the camera gain and exposure settings, and the intent of the photographer.

That matches my understanding as well.

The same section refers to a display with nominal peak luminance of 1000 cd/m², while the introduction discusses "ideal peak luminance" of 10000 cd/m².

Table 1 on page 6 (8 in the pdf) specifies display luminance values for both HLG and PQ formats.

To give more context: PQ is absolute and display-referred. Signals represent an absolute output luminance on a 10 000 cd/m² display. Thus, a PQ signal of 58% intrinsically means an output luminance of 203 cd/m².

HLG is different: it is relative and scene-referred. A 75% HLG signal represents approximately 26.5% of the maximum scene luminance that can be captured, to which the OOTF (opto-optical transfer function) still needs to be applied to get to display light. For HLG, that OOTF takes the form of a gamma function applied to luminance, and the gamma value happens to be 1.2 for a peak display luminance of 1000 cd/m² in a reference surround luminance of 5 cd/m². It was decided that it would be on such a “reference” HLG display that the output luminance of the reference white would coincide between PQ and HLG (see “Conversion between PQ and HLG” page 24 of that report: “There is currently an industry consensus that this common peak luminance should be 1000 cd/m²”), and indeed, we have 1000 × 0.265^1.2 ≈ 203 cd/m².

On a 500 cd/m² HLG display, the gamma would instead be 1.2 × 1.111^log₂(500 / 1000) ≈ 1.08, and therefore reference white would be output at 500 × 0.265^1.08 ≈ 119 cd/m². On a 500 cd/m² PQ display, it would likely stay closer to 203 cd/m² depending on how the tone mapping is done.

Conversely, on a 2000 cd/m² HLG display, reference white would brighten to ~340 cd/m².

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