Auxiliary Bitplane Control

The framebuffer always contains a color bitplane and a depth bitplane. It may also have a stencil bitplane or accumulation bitplane. In addition, if it is a render-to-texture framebuffer, it may have auxiliary bitplanes. These allow more or less arbitrary user-defined data to be stored along with each pixel in the framebuffer, alongside its color and depth value.

The main use case for this is to pass along additional information to a postprocessing shader, which operates on the pixels of the rendered image. For example, the cartoon inking shader needs to have the normal vector of every rendered pixel in order to determine where the edges of the objects are.

When per-pixel lighting is enabled via the shader generator, the shader generator can be asked to produce extra data into the auxiliary bitplanes. This is done by setting an AuxBitplaneAttrib:

np.set_attrib(AuxBitplaneAttrib::make(bits));

Where bits is a set of bits indicating what should be written into the auxiliary bitplanes.

Although the framebuffer’s alpha channel is not technically an auxiliary bitplane, it can be thought of as such, since it is not generally used to store any data of value, since alpha blending has already occurred before the values are written to the framebuffer.

When the shader generator is not enabled, this attrib has no effect.

Values That Can be Requested

The following is a list of bits that can be passed to AuxBitplaneAttrib.make():

AuxBitplaneAttrib::ABO_glow

Copy the glow map (aka self-illumination map) into the alpha channel of the framebuffer. Usually this is a prelude to running a bloom filter over the scene.

AuxBitplaneAttrib::ABO_aux_normal

Store the camera-space normal of the polygon surface in the RGB channels of the first auxiliary bitplane. This is often used to help detect edges in a cartoon inking filter.

AuxBitplaneAttrib::ABO_aux_glow

Copy the glow map (aka self-illumination map) into the alpha channel of the first auxiliary bitplane.