RIVA系列GPU的Guard Band Clipping技术详解与优化

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"Guard Band Clipping" 是一种硬件特性，特别应用于早期的NVIDIA图形处理器，如RIVA 128、RIVA 128ZX、RIVATNT、RIVATNT2和GeForce 256。这种技术旨在显著降低在3D应用程序中对视口内多边形进行裁剪的成本。传统上，图形处理器在处理屏幕坐标时会受到视口范围的限制，例如RIVA系列支持的屏幕坐标范围是-2048到2047，无论屏幕分辨率或视口设置如何。然而，通过启用guard band clipping，硬件可以接受超出常规视口范围的屏幕坐标，比如在GeForce 256上支持横纵超过1亿的值。 guard band的概念是硬件允许部分或完全超出屏幕边界（即guard band区域）的三角形被接受，而无需进行昂贵的实时裁剪操作。这主要通过简化坐标检查和减少复杂的参数化原形裁剪来实现，从而提升性能。由于不需要在每一帧都对可能出界的几何体进行精确判断，这在计算密集型的3D图形渲染中节省了大量的CPU和GPU资源。在利用guard band clipping时，开发人员需要注意以下几点： 1. **理解硬件支持的guard band大小**：不同的GPU有不同的guard band范围，确保应用程序适配这些限制。 2. **优化几何体管理**：将几何体设计成能合理利用guard band，避免不必要的裁剪，提高渲染效率。 3. **正确设置视口**：虽然可以使用较大的guard band，但仍需保持合理的视口设置，以免性能优势被边缘绘制效果抵消。 4. **性能测试与监控**：评估启用guard band clipping后对性能的实际影响，确保其对整体游戏或应用性能的提升是积极的。 guard band clipping是一种利用硬件特性来增强3D渲染性能的技术，通过减少不必要的坐标检查，提高了图形处理的效率，尤其是在那些对实时性能有较高要求的应用中。开发者应根据其GPU的具体特性灵活运用这一特性，以优化他们的3D应用程序。

Guard Band Clipping in Direct3D

Sim Dietrich

NVIDIA Corporation

Sdietrich@nvidia.com

Guard band clipping is a hardware feature implemented in the RIVA 128, RIVA 128ZX,

RIVA TNT, RIVA TNT2 and GeForce 256 graphics processors. It greatly reduces the

cost of clipping polygons to the viewport. This paper will define guard band clipping,

and discuss how to take advantage of it in 3D applications.

What Is Guard Band Clipping?

Hardware is said to support guard band clipping when it can accept screen coordinates

outside of the current viewport range. For instance, the RIVA line of graphics processors

support 2D screen coordinates from in the range -2048,-2048 to 2047,2047 inclusive,

regardless of the screen resolution or viewport setting. This allows for more trivial

accepting and rejecting of polygons, and less parametric primitive clipping, thus

providing a performance benefit. The GeForce 256 supports a guard band of

-100,000,000 to +100,000,000 both horizontally and vertically.

The basic idea of guard band clipping is that hardware with a guard band region can

accept triangles that are partially or totally off-screen, thus avoiding expensive clipping

work.

Typically, applications render into a viewport of 640x480, 800x600, 1024x768,

1280x1024, or 1600x1200. Primitives that lie partially or totally off-screen are typically

clipped by the CPU to the screen or viewport boundary, which is represented by a 3D

view frustum. This is a slow process, because each edge of each triangle that crosses the

viewport boundary must have an intersection point calculated, and each parameter of the

vertex ( x,y,z diffuse r,g,b, specular r,g,b, alpha, fog, u and v ) must be interpolated

accordingly. Eliminating or reducing these calculations can lead to increased

performance.

Guard band clipping reduces how often the CPU must perform these clipping calculations

by accepting triangles that are partly off-screen. It allows the CPU to perform simpler,

faster culling and clip tests in the majority of cases. Only rarely do triangles have to pass

through the more expensive view frustum clipping code.

On the following page is a diagram of the guard band and the viewport.

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RIVA系列GPU的Guard Band Clipping技术详解与优化

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clipping_ber_my.rar_OFDM PAPR clipping_ber clipping_ofdm be

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clipping.asv

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