GPU/Programming Guide

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This page is intended to contain more higher-level explanation of concepts and features provided by the 3DS GPU. For more detailed register-level information check GPU/Internal Registers.

Geometry Pipeline

Fixed Vertex Attributes

If a certain vertex attribute is constant for the duration of a draw call, instead of specifying a vertex array with repeated contents or changing the shader to use a uniform, fixed vertex attributes can be used. They let you specify a fixed value, which will be assumed by the attribute for all vertices of the batch.

To use a fixed attribute, set the bit corresponding to the attribute in GPUREG_ATTRIBBUFFERS_FORMAT_HIGH and ensure that no vertex arrays are configured for the attribute. (Any configured arrays will override the fixed value, regardless of the bit setting.) Even if a vertex array isn't being used for the attribute it still needs to be counted in the number of active attributes specified in the same register.

To specify the actual value of the fixed attribute, write the attribute index to GPUREG_FIXEDATTRIB_INDEX followed by writes with packed a float24 4-tuple to the 3 GPUREG_FIXEDATTRIB_DATA registers. The value is always specified as a float 4-component vector, the configured type is ignored.

Immediate-Mode Vertex Submission

Instead of using vertex arrays to supply vertex data, drawing can be done by directly writing vertex data to a register. This allows vertex data to be inlined directly in the command buffer. Since this is restricted to 4-component float data, it is more useful for small draws like UI elements or debug displays, to avoid using an unreasonable amount of memory and processing time appending the vertices to the command buffer.

To use this feature, configure the number of attributes per vertex in GPUREG_VSH_NUM_ATTR. (All settings in the registers related to the vertex loader are ignored.) Then setup the GPU and shaders the same as if doing a regular draw call with GPUREG_DRAWARRAYS or GPUREG_DRAWELEMENTS, but instead of writing to either register, write the value 0xF to GPUREG_FIXEDATTRIB_INDEX and then follow by repeatedly writing vertex data to GPUREG_FIXEDATTRIB_DATA.

Each set of writes to the 3 data registers specifies one attribute and all attributes (as configured in GPUREG_VSH_NUM_ATTR) need to be written, in order, to specify a vertex. Drawing happens automatically as vertices are specified. After finishing specifying vertices, follow with the same writes used after a draw arrays/elements.

When drawing using triangle strips or fans, GPUREG_RESTART_PRIMITIVE should be used to end the previous strip before (or while) drawing.

Drawing elements

The 3DS GPU is capable of drawing vertex + index arrays, triggered by GPUREG_DRAWELEMENTS. A set of commands commonly used by the standard GL implementation to accomplish this is as follows:

Command Index Register Description
0 GPUREG_GEOSTAGE_CONFIG Set whether drawing triangle elements
1-2 GPUREG_GEOSTAGE_CONFIG2 Set whether drawing triangle elements
3 GPUREG_PRIMITIVE_CONFIG Set primitive mode
4 GPUREG_PRIMITIVE_CONFIG Set number of output map registers
6 GPUREG_GEOSTAGE_CONFIG2 Set function indicator to 0
8 GPUREG_NUMVERTICES Set vertex count
9 GPUREG_START_DRAW_FUNC0 Set mode to drawing
11 GPUREG_START_DRAW_FUNC0 Set mode to configuration
12 GPUREG_VTX_FUNC Trigger post-vertex cache clear
14 GPUREG_GEOSTAGE_CONFIG Clear drawing triangle elements
15 GPUREG_GEOSTAGE_CONFIG2 Clear drawing triangle elements
16 GPUREG_PRIMITIVE_CONFIG Clear primitive mode
17 GPUREG_VSH_ENTRYPOINT Clear entry point