Intel 815 manual Position Mask, Bias, Primitive Rendering Instruction Format

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

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13.1.3.Position Mask

In Variable vertex format, a position mask is sent to indicate the present of X, Y, Z, and 1/W parameters. If 1/W (RHW) is declared not present in the vertex packet, hardware forces 1/w equal to 1. In the case Z is declared not present, the Z value of the last triangle from the last vertex instruction packet will be used for the triangles in this packet.

13.1.4.Bias

A Z bias value can be included in the vertex instruction packet as a per polygon basis. This value is a floating point number that ranges from -1 to 1. The Z bias value is added to the Z value and then clamped to between 0 to 1 before primitive setup calculation. In triangle or line primitive type, Z bias of the last vertex (3rd in triangle, 2nd in line) will be used to add to all the vertices of the primitive and the Z bias value on the other vertices will be ignored. In triangle strip and fan, the Z bias value on the 3rd vertex will be used for the first triangle and the Z bias value on subsequent vertex which defined a triangle will be used for that triangle.

13.1.5.Primitive Rendering Instruction Format

DWord	Bits		Description
0	31:29	Client : 03h – Rendering Engine
	28:24	Rendering Primitive : 1Fh
	23	Reserved: 00h
	22:18	Primitive Type:
		0h	= Triangle or Triangle List
		1h	= Triangle Strip
		2h	= Triangle Strip with Reverse Winding Order
		3h	= Triangle Fan
		4h	= Polygon(Triangle Fan with Common Flat Shaded Vtx)
		5h	= Line or Line List
		6h	= Line Strip
		7h	= Rectangle or Rectangle List (must be axis aligned)
		8h–1Fh = Reserved
	17:0	DWord Count
1	N/A	Primitive[0] Vertex[0]: GFXVERTEX
	N/A	Primitive[0] Vertex[1]: GFXVERTEX
	:		:
Variable	N/A	Primitive[n] Vertex[2]: GFXVERTEX
		Last DWORD of last vertex of last primitive

Refer to GFXVERTEX section for details on defining primitive vertex.

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