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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]] or [[GPU Commands]].

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]] or [[GPU Commands]].

= Geometry Pipeline =

= Geometry Pipeline =

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.

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 [[GPU:Internal Registers#GPUREG_ATTRIBBUFFERS_FORMAT_HIGH|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 use a fixed attribute, set the bit corresponding to the attribute in [[GPU/Internal Registers#GPUREG_ATTRIBBUFFERS_FORMAT_HIGH|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 [[GPU:Internal Registers#GPUREG_FIXEDATTRIB_INDEX|GPUREG_FIXEDATTRIB_INDEX]] followed by writes with packed a float24 4-tuple to the 3 [[GPU:Internal Registers#GPUREG_FIXEDATTRIB_DATA|GPUREG_FIXEDATTRIB_DATA]] registers. The value is always specified as a float 4-component vector, the configured type is ignored.

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

== Immediate-Mode Vertex Submission ==

== 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.

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 [[GPU:Internal Registers#GPUREG_ATTRIBBUFFERS_FORMAT_HIGH|GPUREG_ATTRIBBUFFERS_FORMAT_HIGH]]. (All other settings in related registers seem to be ignored.) Then setup the GPU 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 [[GPU:Internal Registers#GPUREG_FIXEDATTRIB_INDEX|GPUREG_FIXEDATTRIB_INDEX]] and then follow by repeatedly writing vertex data to [[GPU:Internal Registers#GPUREG_FIXEDATTRIB_DATA|GPUREG_FIXEDATTRIB_DATA]].

To use this feature, configure the number of attributes per vertex in [[GPU/Internal Registers#GPUREG_ATTRIBBUFFERS_FORMAT_HIGH|GPUREG_ATTRIBBUFFERS_FORMAT_HIGH]]. (All other settings in related registers seem to be ignored.) Then setup the GPU 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 [[GPU/Internal Registers#GPUREG_FIXEDATTRIB_INDEX|GPUREG_FIXEDATTRIB_INDEX]] and then follow by repeatedly writing vertex data to [[GPU/Internal Registers#GPUREG_FIXEDATTRIB_DATA|GPUREG_FIXEDATTRIB_DATA]].

Each set of writes to the 3 data registers specifies one attribute and all attributes (as configured in GPUREG_ATTRIBBUFFERS_FORMAT_HIGH) 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.

Each set of writes to the 3 data registers specifies one attribute and all attributes (as configured in GPUREG_ATTRIBBUFFERS_FORMAT_HIGH) 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, [[GPU:Internal Registers#GPUREG_RESTART_PRIMITIVE|GPUREG_RESTART_PRIMITIVE]] should be used to end the previous strip before (or while) drawing.

When drawing using triangle strips or fans, [[GPU/Internal Registers#GPUREG_RESTART_PRIMITIVE|GPUREG_RESTART_PRIMITIVE]] should be used to end the previous strip before (or while) drawing.