Separating HDR lighting, exposure, tone mapping, and display output

Keep scene brightness available

A bright window and a dim room can require a wider numerical range than the final image. Preserve their scene-linear lighting in an HDR intermediate, commonly floating point, before deciding how they should appear. Prematurely clamping bright values discards distinctions that a later curve cannot recover. Microsoft's toolkit treats the HDR source and tone-mapping operation as separate inputs and choices. [2]

Assign each stage a purpose

Exposure scales scene values. Tone mapping shapes their placement within a chosen display range. Display encoding then produces the signal expected by the output path. ACES explicitly separates rendering from display encoding; its output transform also considers target gamut and luminance. [1]

An HDR render target does not by itself enable HDR presentation. The operating-system path, swap-chain configuration, display capabilities, and output colour interpretation must agree. [3] Document these independently from the artistic curve.

Calculate an illustrative curve

Suppose three scene-linear samples are 0.25, 2, and 16. For demonstration only, use T(x) = x / (1 + x). Their mapped values are approximately 0.200, 0.667, and 0.941. Increasing exposure by one stop multiplies the inputs by 2, producing 0.5, 4, and 32; the same curve yields about 0.333, 0.800, and 0.970.

The example shows why bright differences compress more strongly near the curve's upper end. It is arithmetic, not a recommended production transform or a model of a particular display. Applying a curve independently to RGB can also alter colour relationships, so inspect coloured highlights rather than relying only on a grey ramp.

Validate a stable sequence

Fix exposure before tuning material response. Capture a neutral ramp, bright emissive patches, saturated lights, and a dark interior beside a bright opening. Inspect values before and after mapping, then confirm the final encoding separately.

If automatic exposure is desired, test entry and exit transitions with the camera moving at gameplay speed. Record the selected exposure over time and check whether a small bright object causes distracting adaptation. Include UI and overlays in the final presentation test because their intended brightness may require separate treatment.

Diagnose the right failure

Clipped input, excessive exposure, aggressive highlight compression, and incorrect output encoding can all produce a washed-out image through different mechanisms. Change one stage at a time. Preserve an SDR reference as well as the intended HDR presentation.

Do not claim that an attractive screenshot validates display compatibility. Review the actual target devices and output modes, and retain enough pre-output information to distinguish rendering errors from presentation errors.

Sources

• [1] Output Transforms — Academy of Motion Picture Arts and Sciences, ACES.
• [2] ToneMapPostProcess — Microsoft DirectX Tool Kit.
• [3] DirectX with Advanced Color — Microsoft.