1. Advanced D3D Programming Sim Dietrich SDietrich@nvidia.com
Guard Band Clipping
Advanced D3D Programming
Sim Dietrich

2. Guard Band Clipping What is Guard Band Clipping?
Why would you want to use it?
Using Guard Band Clipping
2D Clip Testing and Guard Band
3D Culling and Guard Band
How to Detect it

3. What is Guard Band Clipping?
The ability for hardware to accept screen coordinates outside of the current viewport range.
The basic idea of guard band clipping is that the hardware can accept triangles that are partially or totally off-screen.
Some graphics processors support 2D coordinates within the range [-2048,-2048 to 2047,2047].

4. Guard Band and Viewport
( 1024x768 )
Guard Band
( 4096x4096 )

5. Guard Band and Viewport
This triangle can
be trivially
accepted or
rejected
These triangles
can be trivially
accepted
This triangle must
be clipped

6. 3D Frustum Clipping is Slow
Requires dot products with 6 frustum planes for each vertex
Produces extra vertices
Each value at vertex ( u,v, fog, alpha, diffuse color, specular color ) requires interpolation
More bandwidth required for each new vertex
Breaks vertex cache coherency
Clipping can break up strips and fans

7. Clip Testing is Faster
2D or 3D Clip testing combined with Guard band addresses these issues
Most triangles that normally require clipping could be passed through to HW instead
Keeps more strips and fans intact
Maintain vertex cache coherency
Apps must still clip to near and far clip planes

8. Guard Band Clipping
Only in the rare case of a primitive crossing both the viewport and a Guard Band boundary is 3D frustum clipping necessary
Either clip to the guard band boundary or the viewport boundary
D3D clips triangles to the guard band
Clipping to the viewport is preferred
It’s smaller than the guard band, so less pixels
Clipper will reject triangles outside viewport

9. Using Guard Band Clipping
Direct3D takes advantage of Guard Band clipping automatically if present and Direct3D performs clipping
If the app performs own transforms
Detect the Guard Band extents
If found, use 3D cull testing or 2D clip testing after culling and perspective transform
If apps pass 2D coordinates to D3D, they should perform their own clipping, to keep Direct3D from de-projecting during its clipping

10. 2D Clip Testing
A version of Cohen-Sutherland clip-testing (FOLEY90)
Generate outcodes for the viewport and the guard band’s borders for each vertex
Outcode is a Bitfield : 0 means in, 1 means out
Bitwise AND (&) and Bitwise OR ( | ) the outcodes in parallel to test various cases
If primitive is within viewport accept
If primitive is outside of viewport on at least 1 side, reject
If primitive is in GB, accept
If primitive crosses a GB boundary, clip to viewport

11. 3D Culling and Guard Band
An alternative to 2D clip testing is to use a second, larger frustum extending through the guard band borders for your 3D culling test.
This eliminates the need to project the vertices to 2D before clip testing
It is easily added to the end of your existing 3D culling

12. Top View of Guard Band and View Frustums
Far Clip Plane
Guard Band Frustum
Left Guard
Band Frustum
Plane
Right View
Frustum Plane
Near Clip
Plane
View Frustum
Screen

13. 3D Culling and Guard Bands
Create a 3D view frustum that passes through the guard band borders
Cull boxes or spheres of objects to this frustum and the normal viewing frustum
If the object is within the guard band frustum and crosses an edge of the view frustum, you can trivially accept it instead of clipping it to the view frustum

14. 3D Culling and Guard Bands
For each object, calculate signed distance from the center of bounding sphere to each view frustum plane and compare to radius of sphere
If it’s completely outside view frustum, reject
If it’s completely inside view frustum, accept
If it’s partially in, test against guard band frustum
If completely in, accept it - This is where you save performance in avoiding clipping
If it crosses a guard band frustum plane, clip it to view frustum as normal

15. Top View of Guard Band and View Frustums, and Bounding Spheres
Far Clip Plane
Guard Band Frustum B
Clipped A
Accepted C
Rejected
Near Clip
Plane
Screen
View Frustum

16. Trivially Accepting Polygons
If you trivially accept a set of polygons, render them with
D3DDP_DONOTCLIP | D3DDP_DONOTUPDATEEXTENTS
If you omit this, Direct3D will clip test to guard band for you
If you must clip a set of polygons that straddle both the view and guard band frustums, clip them to the view frustum

17. Detecting a Guard Band
Call the DIRECT3DDEVICE3 interface GetCaps method to get the guard band extents
hr = device->GetCaps( &aD3DHWDevDesc, &aD3DSWDevDesc );
aD3DHWDevDesc.dvGuardBandLeft;
aD3DHWDevDesc.dvGuardBandRight;
aD3DHWDevDesc.dvGuardBandTop;
aD3DHWDevDesc.dvGuardBandBottom;
These extents will all be set to 0 to indicate no guard band support

18. ? ? ? ? ? ? ? Questions Sim Dietrich SDietrich@nvidia.com