Light Ramps

In standard OpenGL and DirectX lighting, the following calculations are performed:

  • the lighting value is calculated

  • it is clamped to the range 0-1

  • it is combined with the textures

  • it is clamped to the range 0-1 again

  • it is written to the frame buffer

This process contains two clamps. The LightRampAttrib is a means to specify that you wish to replace these two clamping operators with something a little smarter. This is particularly relevant for two major graphics algorithms: HDR tone mapping, and cartoon shading.

It must be emphasized that light ramps have no effect unless per-pixel lighting is enabled via the shader generator.

HDR Tone Mapping

Note

For a more advanced way to apply High Dynamic Range rendering, see the HDR postprocessing filter described in Common Image Filters.

In HDR tone mapping, the first clamp is removed entirely, and the second one is replaced with the tone mapping operator. The tone mapping operator maps brightness values in the range 0-infinity to new brightness values in the range 0-1, however, it does so without clamping. To turn on HDR tone mapping, use one of the following:

np.set_attrib(LightRampAttrib::make_hdr0());
np.set_attrib(LightRampAttrib::make_hdr1());
np.set_attrib(LightRampAttrib::make_hdr2());

The HDR2 tone mapping operator is a familiar operator that is used in many systems. It has the downside that it tends to reduce contrast a lot:

  • FINAL_RGB = (RGB) / (RGB + 1)

The HDR1 tone mapping operator is similar, but it allocates more of the contrast range to brightnesses in the range 0-1, and less to brightnesses in the range 1-infinity. This yields a higher-contrast scene, but with more washout:

  • FINAL_RGB = (RGB^2 + RGB) / (RGB^2 + RGB + 1)

The HDR0 tone mapping operator allocates even more of the available contrast range to brightnesses in the range 0-1. This is even more contrasty, but with even more washout:

  • FINAL_RGB = (RGB^3 + RGB^2 + RGB) / (RGB^3 + RGB^2 + RGB + 1)

Cartoon Shading (Quantized Lighting)

In cartoon shading, the first clamp is removed entirely, and the second one is replaced with a quantization function. This replaces a continuous gradient of brightness values with a discrete set of light levels. This quantization function only applies to directional lights, not ambient ones.

To enable quantized lighting, use one of these:

np.set_attrib(LightRampAttrib::make_single_threshold(t0, l0));
np.set_attrib(LightRampAttrib::make_double_threshold(t0, l0, t1, l1));

In a single-threshold system, the brightness of the diffuse lighting contribution is compared to the threshold t0. If the threshold is not met, the diffuse light contribution is eliminated. If it is met, the pixel’s brightness is normalized to the specified level l0.

In a double-threshold system, the brightness of the diffuse lighting contribution is compared to the thresholds t0 and t1. If neither is attained, the diffuse light contribution is eliminated. If it is met, the pixel’s brightness is normalized to either l0 or l1, depending on which threshold was passed.

Future Light Ramps

We are interested in knowing if there are any other light ramps you would like to see. If so, please notify us on the forums.