VT8601 Via, VT8601 Datasheet - Page 128

• Manufacturer Part Number: VT8601
• Description: Slot-1 / Socket-370 PCI North Bridge
• Manufacturer: Via

Bresenham Edge Parameters
Bresenham Edge parameters describe an edge of a primitive or
a line.
DoubleWord 0 – Start Coordinates
DoubleWord 1 – Drawing Direction / Bresenham Constant
DoubleWord 2 – Error Term / Strip Length
Revision 1.3 September 8, 1999
31-16 Start YS1
28-16 Bresenham (or Modified) Constant
15-13 Reserved
31-29 Reserved
28-16 Initial Error Term
15-12 Reserved
15-0 Start XS1
12-0 Bresenham (or Modified) Constant
11-0 Strip Length
31
30
29
7HFKQRORJLHV ,QF
Starting coordinate of the line in the Y direction
(signed 12.4 number). The fractional part must be 0.
This parameter is ignored in minor edges.
Starting coordinate of the line in the X direction
(signed 12.4 number). The fractional part must be 0.
YS Drawing Direction
XS Drawing Direction
Swap
Strip length of modified Bresenham line.
:H &
:H &R R QQHFW
0
1
0
1
0
1
QQHFW
Positive
Negative
Positive
Negative
Normal (X / Y not swapped)
X / Y swapped
................................................... ignored
........................... must be written as zero
........................... must be written as zero
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DDA Edge Parameters
DDA Edge parameters describe an edge of a primitive or a
line.
DoubleWord 0 – Start Coordinates
DoubleWord 1 – Drawing Direction / Edge Slope
When a DDA edge is used as a polygon boundary, the
fractional bits should round up to the next integer.
Interpolation values should be adjusted accordingly. DDA
edge walking shares the same logic as Bresenham edge
walking by using an error advance method. In DDA walking,
fractional bits should be rounded up to the next integer.
Rounding up is performed by changing drawing convention
according to whether the fractional parts are 0 as follows:
Because the error advance method is used for DDA walking,
the fractional part is always one step ahead of the coordinate.
For the starting point of a line, the fractional part is assumed
to be 0.
31-16 Start YS1
28-26 Reserved
15-0 Start XS1
25-0 Edge Slope
31
30
29
Left fractional is 0: Left inclusive.
Left fractional is not 0: Left exclusive.
Right fractional is 0: Right exclusive.
Right fractional is not 0: Right inclusive.
Starting coordinate of the line in the Y direction
(signed 12.4 number). The fractional part must be 0.
This parameter is ignored in minor edges.
Starting coordinate of the line in the X direction
(signed 12.4 number). The fractional part must be 0.
YS Drawing Direction
XS Drawing Direction
Swap
12.14 signed number
0
1
0
1
0
1
Positive
Negative
Positive
Negative
Normal (X / Y not swapped)
X / Y swapped
................................................... ignored
VT8601 Apollo ProMedia
3D Graphics Engine Registers