Intel 815 manual Gfxrenderstatezbiasalphafuncref, 3DState24NP Non-pipelined 14h

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

R

13.17.GFXRENDERSTATE_Z_BIAS_ALPHA_FUNC_REF

DWord	Bit		Description
0	31:29	Client : 03h – Render Processor
	28:24	3DState24NP (Non-pipelined) : 14h
	23	Reserved : 0
	22	Z Bias State Mask : 1 = Update ; 0 = Do Not Update
	21:14	Z Bias : This is a signed value that is added to the source Z value prior to the Z
		compare function. This 2’s complement fixed point value has 7 integer bits and 0
		fractional bits and is sign extended before being added to the destination Z value.
		Valid data values range from -128 to 127. (S7) The default value is 0. The Z value
		written back to Z-buffer included this Z bias value.
	13	Alpha Function State Mask : 1 = Update; 0 = Do Not Update
	12:9	Alpha Function : Valid values are :
		00h	= Reserved
		01h	= Never ( never pass)
		02h	= Less (pass if the source Alpha is less than the Alpha reference)
		03h	= Equal (pass if the source Alpha is equal to the source Alpha)
		04h	= Lequal (pass if the source Alpha is less than or equal to the source Alpha)
		05h	= Greater (pass if the source Alpha is greater than the source Alpha)
		06h	= Notequal (pass if the source Alpha is note equal to the source Alpha)
		07h	= Gequal (pass if the source Alpha is greater than or equal to the source Alpha)
		08h	= Always (always pass)
	8	Alpha Reference State Mask : 1 = Update; 0 = Do Not Update
	7:3	Alpha Reference value for color calculation equation(s) : This is bits 7:3 of an
		alpha reference value. This value specifies a reference alpha value which pixels are
		tested against when alpha-testing is enabled. The default value is 0. (unsigned int)
	2:0	Reserved : 000

The Z Bias state variable specifies a value with which to offset the source z value before performing the z compare test (refer to as backend Z bias). This is used for coplanar polygon priority. If two polygons are to be rendered which are coplanar, due to the inherent precision differences induced by unique x, y and z values, there is no guarantee which polygon will be closer or farther. By using the Z bias state variable, it is possible to offset the source z value (compare value) before comparing with the destination z value.

The Z bias value is added at the LSBs of a 16 bit z representation. Using the Z bias state variable affects the source z value and the result of the z-bias addition is written to the z buffer.

Also, Z biasing is supported by adding to the source z coefficients (Co) after setup and then interpolation is performed (refer to as frontend Z bias). This is done through the vertex instruction packet by enabling z-bais in the control word and specifying a z-bias value with the vertex data. The result of the addition will be used in z function comparison. The result is written back to the Z buffer.

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