Renderinimas Layeriais ?

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Multipass Rendering

Sometimes, very large scenes are not rendered in one pass but in multiple passes, because they are too large for the machine, or because different passes are rendered on different machines to increase parallelism, or because some passes are expected to change while others stay the same, so only the modified passes need to be rerendered. (This last one is very common in entertainment content production.)

It is generally not sufficient to render multiple images and then composite them into one. Pass compositing is not simple alpha blending but a complex user-defined process that involves at least the Z depth. Although mental ray supports saving the Z buffer to files, there is only one Z value per pixel.

mental ray supports multipass rendering by saving rendering results not as image files but as sample files. Every primary ray is a sample, and is stored with all available frame buffer values. If the scene contains motion, an averaged result over all temporal samples is stored. The file format used for sample files is unpublished and may change with new versions of mental ray, it is not useful as a data interchange format for other programs, and is not intended for long-term storage. Merging is done by a user-defined function (if absent, a simple Z switcher is used) that has access to all frame buffer results of all passes, one sample at a time. Multipass rendering is controlled with a pass statement script in the camera definition that is similar to output statements.

Multipass rendering is a new feature in mental ray 3.1.2 that allows rendering multiple scenes with the same frame number, resolution, oversampling parameters, and list of type of frame buffers, but different contents. This is useful if a scene is extremely complex, or consists of many distinct elements, so that rendering it all at once is impractical either because of the size, or because certain elements are expected to change and it would be too expensive to re-render the entire scene for each small change. Multipass rendering allows splitting such a scene into sub-scenes (called passes or layers), rendering each sub-scene, and putting the results together in a separate step called merging.

Scenes are typically broken into sub-scenes by clustering geometry, for example one for each complex character or creature, or props such as vehicles, or backgrounds. Elements not present in a sub-scene are either omitted completely or replaced by very simple stand-ins that do not appear in the rendered pass image, for example simple invisible and nontraceable objects that cast shadows. This allows objects of one pass to cast shadows on objects in another pass. (In this particular case it is useful to enable the shadow BSP tree with the option statement bsp shadow on to manage the shadow stand-ins more efficiently.)