Intel 815 Manuel D’Utilisation

238  

 

 

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.