Graphics Programming
TVP4020
Programmers Reference Manual
68
5.4
Scissor/Stipple Unit
Two scissor tests are provided in P
ERMEDIA
, the User Scissor test and
the Screen Scissor test. The user scissor checks each fragment against
a user supplied scissor region; the screen scissor checks that the
fragment lies within the screen. The stipple test checks each fragment
against an 8x8 pattern.
5.4.1
User Scissor Test
The user scissor test, tests each fragment as follows:
XMin <= X < XMax
YMin <= Y < YMax
Where X and Y are the coordinates for the fragments, and XMin, XMax,
YMin and YMax define the user supplied scissor region. If a fragment
fails the test it is discarded. The test may be screen or window relative.
This test applies to normal pixels and block fill operations.
5.4.2
Screen Scissor Tests
This test ensures that a pixel lies within the screen boundaries. For
each fragment the XY origin stored in the WindowOrigin register is
added to the fragment coordinates and this is tested against the screen
boundaries stored in the ScreenSize register. Since the X and Y
coordinates are held as 2's complement numbers, the window origin can
be moved off the edges of the screen.
The following test is made:
0 <= (X + WX) < SW
0 <= (Y + WY) < SH
Where:
X = Fragment X coordinate
WX = Window origin X coordinate
Y = Fragment Y coordinate
WY = Window origin Y coordinate
SW = Screen Width
SH = Screen Height
The diagram below shows a simple scenario of a screen with a single
window which has a user defined scissor region. The shaded area
shows the region where fragments pass the user and screen scissor
tests and so can progress in the pipeline. Fragments outside this region
are culled from the pipeline. This test applies to normal pixels and block
fill operations.
Summary of Contents for TVP4020 PERMEDIA 2
Page 1: ...Texas Instruments TVP4020 PERMEDIA 2 Programmer s Reference Manual Issue 4 ...
Page 284: ...TVP4020 Programmers Reference Manual A Gouraud Shaded Triangle 275 ...
Page 292: ...TVP4020 Programmers Reference Manual Register Tables 283 ...
Page 314: ...TVP4020 Programmers Reference Manual Index 305 ...
Page 315: ...Index TVP4020 Programmers Reference Manual 306 Index ...
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