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Output Shaders

Output shaders are user-written functions that can be linked at runtime that have access to every pixel in all available frame buffers after rendering. They can be used to perform operations like post-filtering or compositing.

Files to write are specified with data type, file format and file name. If the data type is omitted a default data type is used that is assumed to be the "best" type for the given image format. This default type is marked " " in the table below. The data type implies the frame buffer type. There are special file formats for depth, vector, and label files, in addition to a variety of standard color file formats. By listing the appropriate number and type of output statements, it is possible to write multiple files. For example, both a filtered file and the unfiltered version can be written to separate files by listing three output statements: one to write the unfiltered image, one that runs an output shader that does the filtering, and finally another one to write the filtered image. Output statements are executed in sequence.

The following file formats are supported:

format type compress description
pic rgba RLE Softimage color
Zpic z - Softimage depth map (write only)
alias rgb RLE Alias color
rgb rgba RLE Silicon Graphics 8-bit RGBA color
rgb RLE Silicon Graphics 8-bit RGB color
rgba_16 RLE Silicon Graphics 16-bit RGBA color
rgb_16 RLE Silicon Graphics 16-bit RGB color
jpg rgb JPEG JFIF picture
png rgb RLE Portable Network Graphics 8-bit RGB color
rgba RLE Portable Network Graphics 8-bit RGBA color
exr a OpenEXR 8-bit scalar
rgb OpenEXR 8-bit RGB color
rgba OpenEXR 8-bit RGBA color
a_h OpenEXR 16-bit scalar
rgb_h OpenEXR 16-bit RGB color
rgba_h OpenEXR 16-bit RGBA color
a_fp OpenEXR 32-bit scalar
rgb_fp OpenEXR 32-bit RGB color
rgba_fp OpenEXR 32-bit RGBA color
z OpenEXR depth map
n OpenEXR normal-vector map
m OpenEXR motion-vector map
tif rgba RLE 8-bit RGBA TIFF
rgba_16 RLE 16-bit RGBA TIFF
rgba_fp RLE floating-point RGBA TIFF
rgb RLE 8-bit RGB TIFF
rgb_16 RLE 16-bit RGB TIFF
rgb_fp RLE floating-point RGB TIFF
tifu rgba - 8-bit RGBA TIFF
rgba_16 - 16-bit RGBA TIFF
rgba_fp - floating-point RGBA TIFF
rgb - 8-bit RGB TIFF
rgb_16 - 16-bit RGB TIFF
rgb_fp - floating-point RGB TIFF
iff rgb RLE Alias Maya 8-bit RGB image
rgba RLE Alias Maya 8-bit RGBA image
rgb_16 RLE Alias Maya 16-bit RGB image
rgba_16 RLE Alias Maya 16-bit RGBA image
rgba_fp RLE Alias Maya float RGBA image
a RLE Alias Maya 8-bit alpha
a_16 RLE Alias Maya 16-bit alpha
a_fp RLE Alias Maya floating-point alpha
z RLE Alias Maya depth map
rla rgba RLE 8-bit or 16-bit Utah/Wavefront color, type A
rlb rgba RLE Utah/Wavefront color, type B
picture rgb RLE Dassault Systèmes CATIA PICTURE
hdr rgbe - Radiance high dynamic range color, 8-bit RGB color
ppm rgb - Portable pixmap, 8-bit P6 binary
tga rgba - Targa color
bmp rgb RLE MS Windows and OS/2 color
qntpal rgb YUV Abekas/Quantel, PAL (720576)
qntntsc rgb YUV Abekas/Quantel, NTSC (720486)
ct rgba - mental images 8-bit color (3)
rgba_16 - mental images 16-bit color (6)
rgba_fp - mental images floating-point color (11)
st a - mental images 8-bit alpha (4)
a_16 - mental images 16-bit alpha (7)
a_fp - mental images floating-point alpha (15)
vt vta - mental images alpha basis vectors (5)
wt vts - mental images intensity basis vectors (5)
zt z - mental images depth map (8)
nt n - mental images normal-vector map (9)
mt m - mental images motion-vector map (12)
tt tag - mental images normal-vector map (10)
bit bit - mental images mask bitmap (13)
cth rgbe - mental images HDR color, 8-bit RGB color (14)
map any - mental images memory map
null - - null, deleted on close, write only

OpenEXR [6] in mental ray 3.4 supports the compression modes RLE (default), PIZ (wavelet-based), ZIP, and PXR24 [7]. The first three are lossless; PXR24 is lossy if the written data is stored in full floating-point precision and lossless otherwise. mental ray 3.3 and earlier always uses ZIP compression.

Each of these file formats implies a particular default data type (the first entry in column "Supported data types"); for example, "jpg" implies 8-bit RGB, and "zt" implies Z. The default data type may be overridden by explicitly specifying another data type, such as a 16-bit type, in the output statement, as long as it is supported and appears in the above table. mental ray will adjust its frame buffer list to compute the requested types. For example, the standard RGBA frame buffer stores 8 bits per component by default, but if any output statement references a 16-bit type, the RGBA frame buffer also switches to 16 bits.

The null file format is useful to create a stub file during rendering, that can be used to attach the imf_disp viewer, without leaving the file on disk when rendering finishes.

The rgb_h and rgba_h image types are supported only for OpenEXR output statements. They cause the data to be written to the OpenEXR file in half-float format. mental ray's frame buffer is stored in full floating point format. This allows the full dynamic range without the storage overhead of storing full floating-point values in the OpenEXR file. mental ray 3.3 does not support half-float frame buffers.

The RGBE high dynamic range color data type can store RGB data whose components may exceed the value of 1.0, which is the normal limit for standard RGB data. There are two specialized file formats (HDR and CTH) for RGBE, but mental ray will also allow storing RGBE data in any 8-bit RGBA format, such as SGI RGB. This will result in images that cannot be displayed by standard viewing programs because they would interpret the data as RGBA, but it has become common practice to use such formats to transport RGBE data to other tools that understand RGBE.

The available data types are:

type comp bpc contents
rgba 4 8 RGBA color
rgba_16 4 16 RGBA color (16 bits per component)
rgba_fp 4 32 RGBA color (floating-point)
rgb 3 8 RGB color
rgb_16 3 16 RGB color (16 bits per component)
rgb_fp 3 32 RGB color (floating-point)
rgbe 4 8 high dynamic range RGB color
a 1 8 Alpha channel
a_16 1 16 Alpha channel (16 bits per component)
a_fp 1 32 Alpha channel (floating-point)
s 1 8 synonymous with a
s_16 1 16 synonymous with a_16
s_fp 1 32 synonymous with a_fp
z 1 32 depth channel
n 3 32 normal vectors
m 3 32 motion vectors
tag 1 32 label channel
vta 2 16 UV vector texture
vts 2 16 synonymous with vta
bit 1 1 bitmask channel
coverage 1 32 coverage of most important object

The difference between "vta" and "vts", and between n and m, is significant only when automatic conversions are done. The file contents are identical except for the magic number in the file header.

The floating-point RGBA data type "rgba_fp" allows color and alpha values outside the normal range , and no dithering is applied even if explicitly enabled. In contrast, any conversion to the 8-bit or 16-bit formats will clamp values outside this interval. Note that dithering reduces the effectivity of RLE compression.

All mental images file formats contain a header followed by simple uncompressed image data, pixel by pixel beginning in the lower left corner. Each pixel consists of one to four 8-bit, 16-bit, or 32-bit component values, in RGBA, XYZ, or UV order. The header consists of a magic number byte identifying the format, a null byte, width and height as unsigned shorts, and two unused null bytes reserved for future use. All shorts, integers, and floats are big-endian (most significant byte first).

mental ray can combine samples within a pixel in different ways. The combination of existing samples can also pad the frame buffers to "bridge" unsampled pixels. Interpolation of colors, depths, normals, and motion vectors means that they are averaged, while interpolation of the labels means that the maximum label is used (taking the average label is not a good idea). Interpolation of depths only takes the average of non-infinite depths, and interpolation of normals and motion vectors only takes the average of vectors different from the null vector. Interpolation is turned on by writing a "+" in the beginning of the output type and turned off by writing a "-" there. For example, to interpolate the depth samples, write "+z" in the output statement:

type meaning
-rgba last color
+rgba average color
-z lowest depth
+z average depth, excluding infinite depths
-n last normal
+n average normal, excluding null vectors
-m last motion vector
+m average motion vector, excluding null vectors
-tag last label
+tag maximum label

If interpolation is turned off for a frame buffer, the last sample value (color, normal, motion vector, or label) within each pixel is stored, and pixels without samples get a copy from one of the neighbor pixels. Interpolation off for depth images is an exception: rather than using the last sample depth, the min depth is used - this can be useful for compositing. Interpolation is on by default for color frame buffers (including alpha and intensity frame buffers) and off by default for depth, normal, motion vector, and label frame buffers.

[6] The distribution of OpenEXR requires the following copyright notice:

Copyright (c) 2002, Industrial Light & Magic, a division of Lucas Digital Ltd. LLC

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.


[7] The distribution of the PXR24 compressor requires the following copyright notice:

Copyright (c) 2004, Pixar Animation Studios

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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