Hardware vs. Software Rendering

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Ideally, software rendering algorithms should be translatable directly to hardware. However, this is not possible because hardware and software rendering use two very different approaches:

Software is completely unconstrained, except by performance considerations, and can perform any algorithm whatever. The subpixel loop method is chosen because it allows rendering nonlocal effects that require considering different portions of the scene in order to compute a single pixel or subpixel. For example, reflections require access to both the reflecting and the reflected object. A more complex example is global illumination, which considers the indirect light from all surrounding objects to compute the brightness of the subpixel being rendered. Hardware can do none of this because it only ever knows one triangle at a time, and has no notion of other objects.

Hardware rendering uses a limited range of workarounds to address some of these limitations. These workarounds typically involve pre-rendering objects into "maps", which are rectangular pixel rectangles encoding properties of other objects and are stored in the graphics hardware in the form of texture images. While graphics hardware cannot deal with multiple objects, it can deal very efficiently with texture images. Examples for such mapping techniques are:

Workarounds like this have one difficulty: they require a lot of manual preparation and fine-tuning to work; for example, deciding on a good environment map. The computer cannot do this because while computing a correct map is possible, it would be very expensive and easily defeat the expected time savings.

The driving force behind graphics hardware design is gaming, which involves rendering as many triangles per second as possible to a video screen. Also, procedural shading is still quite primitive and requires workarounds as described above. This creates some limitations of hardware rendering:

None of this is mentioned in the literature and sales material for graphics hardware boards. In general, it is unrealistic to expect to attain advertised realtime frame rates, often over 100 frames per second, for general-purpose rendering without spending the years of custom development that game designers generally require.

Yet, despite all the limitations of hardware rendering, the things it can do are done at extremely high speed, sometimes more than a thousand times faster than software. Harnessing this speed is the object of the hardware shading support in mental ray 3.3. Although the limitations make it impossible to simply switch from software to hardware rendering, much of the advantage can be realized with an appropriate combination of hardware rendering and software rendering, combining hardware speed with software flexibility. This document describes how mental ray does this.

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