mental ray stores finally rendered pixel data in frame buffers.
There are up to five main frame buffers pre-defined in mental ray:
for RGBA color, depth, normal vectors,
motion vectors, and
labels. The depth, normal vector, motion
vector, and label frame buffers store the respective values of
the frontmost object at each sample of the image. If multiple
samples are taken for a pixel, the frame buffer value for that
pixel may be either any one sample value, or a blend of all samples.
The number and type of frame buffers to be rendered is controlled by
Any number of additional user frame buffers of any data type but identical
resolution to the main frame buffers may be defined. These buffers will not
be filled by mental ray; their content is controlled by custom shaders only,
like output shaders. However,
mental ray will take care of writing the buffer content to image files.
The framebuffer statement allows to create 'named' frame buffers in mental ray, together with data type and other properties for this specific buffer. The number of frame buffers is not limited by mental ray but only dependent on available system resources. Storing a frame buffer to an output file can be activated per buffer. It is possible to store several frame buffers in a single file with multiple layers, for capable formats like OpenEXR. mental ray will take care of performing all required operations like data conversions and compression. For backwards compatibility, frame buffer creation may also be controlled with output statements.
The primary purpose of output statements is to specify output shaders which operate on the frame buffers. For backwards compatibility, deprecated output statements can also specify files to write.
mental ray is able to temporarily store frame buffers in frame buffer files on disk instead of keeping them in memory during rendering, given that sufficient disk space is available. This technique is also referred to as virtual frame buffers. It allows to use very large frame buffers, or very large numbers of frame buffers, without being limited by available physical memory. This feature can be controlled with configuration settings or on the command line of the standalone mental ray.
By default, mental ray is using a frame buffer cache mechanism, which is storing fragments of the frame buffers in disk files as soon as their are finished. Only a small fraction of each frame buffer is kept in memory, like the currently rendered tile, tiles recently accessed by the display callbacks or output shaders, or when writing final images to files. This feature may save significant amount of memory when rendering very high resolution images, and is especially useful on 32bit systems with limited memory resources. This technique is typically superior to memory mapped files, the other virtual frame buffer option supported by mental ray. In this case, frame buffers need to be stored and restored in full resolution, which is more appropriate when rendering very many but smaller frame buffers. It requires that the frame buffers fully fit into the virtual memory address space of the process, making it more attractive for 64-bit systems. However, if sufficient memory is available then virtual frame buffers might be turned off to gain overall performance.
Note Memory mapping large files can occupy a significant portion of the application's virtual address space. The address space for a single process is limited on 32-bit architectures to 4 gigabytes or less, dependent on reserved space for system frameworks and libraries. Trying to map a very large file or many files might fail since there is not enough room left for the process to address it.
Frame buffer files are stored in the directory specified with configuration or command line options; otherwise in the default temporary directory of the system.
Frame buffer files are removed from disk when rendering has finished.
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