Hardware/GX/Transform Unit

The transform unit (XF) is a component of the Wii's GX subsystem. It is responsible for transforming the incoming vertices to screen coordinates.

XF registers

XF viewport registers

These registers define the viewport coordinates.

GX_XF_VIEWPORT_X0 (0x101A)
	310
Access	R/W

• This register defines the X0 coordinate of the viewport.

GX_XF_VIEWPORT_Y0 (0x101B)
	310
Access	R/W

• This register defines the Y0 coordinate of the viewport.

GX_XF_VIEWPORT_Z (0x101C)
	310
Access	R/W

• This register defines the depth (far plane - near plane) of the viewport.

GX_XF_VIEWPORT_X1 (0x101D)
	310
Access	R/W

• This register defines the X1 coordinate of the viewport.

GX_XF_VIEWPORT_Y1 (0x101E)
	310
Access	R/W

• This register defines the Y1 coordinate of the viewport.

GX_XF_VIEWPORT_FP (0x101F)
	310
Access	R/W

• This register defines the far plane of the viewport.

The viewport coordinates can be calculated as shown below:

• GX_XF_VIEWPORT_X0 = viewport_width / 2
• GX_XF_VIEWPORT_Y0 = (-viewport_height) / 2
• GX_XF_VIEWPORT_X1 = (viewport_x + (viewport_width / 2)) + 342
• GX_XF_VIEWPORT_Y1 = (viewport_y + (viewport_height / 2)) + 342
• GX_XF_VIEWPORT_FP = viewport_farplane * 16777215
• GX_XF_VIEWPORT_Z = (viewport_farplane - viewport_nearplane) * 16777215

Note: These formulas are taken from the libogc function GX_SetViewportJitter().

GX_XF_VIEWPORT_FP and GX_XF_VIEWPORT_Z are 24-bit fixed-point values ranging between 0 and 16777215. These numbers respectively represent 0.0 and 1.0 in floating-point.

All the other viewport registers are 32-bit floating-point values.

XF projection registers

These registers define the projection matrix. Actually, there are 7 registers, so they don't really define a matrix, but rather parts of it.

GX_XF_PROJ_0 (0x1020)
	310
Access	R/W

GX_XF_PROJ_1 (0x1021)
	310
Access	R/W

GX_XF_PROJ_2 (0x1022)
	310
Access	R/W

GX_XF_PROJ_3 (0x1023)
	310
Access	R/W

GX_XF_PROJ_4 (0x1024)
	310
Access	R/W

GX_XF_PROJ_5 (0x1025)
	310
Access	R/W

• These registers define 6 projection values.

GX_XF_PROJ_MODE (0x1026)
	310
Access	R/W

• This register defines how the projection values will be translated.

Depending on GX_XF_PROJ_MODE, you must assign different parts of your 4x4 projection matrix to the projection values:

• GX_XF_PROJ_MODE = 0 (perspective):
  • GX_XF_PROJ_0 = matrix[0][0]
  • GX_XF_PROJ_1 = matrix[0][2]
  • GX_XF_PROJ_2 = matrix[1][1]
  • GX_XF_PROJ_3 = matrix[1][2]
  • GX_XF_PROJ_4 = matrix[2][2]
  • GX_XF_PROJ_5 = matrix[2][3]
• GX_XF_PROJ_MODE = 1 (orthographic):
  • GX_XF_PROJ_0 = matrix[0][0]
  • GX_XF_PROJ_1 = matrix[0][3]
  • GX_XF_PROJ_2 = matrix[1][1]
  • GX_XF_PROJ_3 = matrix[1][3]
  • GX_XF_PROJ_4 = matrix[2][2]
  • GX_XF_PROJ_5 = matrix[2][3]

Note: these assignments are taken from the libogc function GX_LoadProjectionMtx().