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

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

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].

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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