rt — raytrace an mged model, giving a shaded picture


rt [options...] model.g objects...


rt operates on the indicated objects in the input model.g and produces a color-shaded image as the output. By default, the output is placed on the current framebuffer as it is computed. The environment variable FB_FILE specifies the current framebuffer, see brlcad(1). Alternatively, the output can be stored in a pixel file (pix(5) format) by specifying -o output.pix in the option list. The orientation of the rays to be fired may be specified by the -a and -e options, in which case the model will be autosized, and the grid will be centered on the centroid of the model, with ray spacing chosen to span the entire set of objects. Alternatively, with the -M option, a transformation matrix may be provided on standard input which maps model-space to view-space. In this case, the grid ranges from -1.0 = X,Y = +1.0 in view space, with the size of the grid (number of rays fired) specified with -s. The -M option is most useful when rt is being invoked from a shell script created by an mged(1) saveview command.

The following options are recognized:

-. #

selects the factor used in NUgrid algorithm. Default is 1.5 .

-, #

selects which space partitioning algorithm to use

-a #

selects auto-sizing, and gives the view azimuth in degrees. Used with -e and conflicts with -M .

-b "# #"

is sometimes useful for debugging. A single ray is fired at the pixel coordinates specified. This is best when accompanied by debugging options. Note that the standard getopt(3) routine only allows options to have one parameter, so the X and Y pixel coordinates need to be combined into one string parameter by enclosing them in double quotes. This option must follow any viewing parameters.

-c "script_command"

supplies, on the command line, any command that can appear in a -M command script. For example, -c "set" will print the values of all settable variables.

-d #

sets flag for reporting of pixel distances: 1 for on, 0 (default) for off. (Each pixel is represented by 3 bytes of color information followed by 8 bytes for a double, in IEEE format, which gives the distance to the ray's first hit. If the ray doesn't hit, a zero distance is reported by default which causes rt(1) to write only the color information. See pix(5) and pixd(5).)

-e #

selects auto-sizing, and gives the view elevation in degrees. Used with -a and conflicts with -M .

-f #

sets the expected playback rate in frames-per-second; the default is 30. Several time-evolving shaders such as fire and grass need to know the time at which each frame and pixel occur to drive their evolution.

-g #

sets the grid cell width, in millimeters. Ordinarily, the cell width is computed by dividing the view size by the number of pixels of width, so this option need not be used. To obtain exactly 1 inch ray spacing, specify -g25.4 . If the grid cell width is specified and the grid cell height (which would be set using -G) is not specified, the grid cell height defaults to be the same as the width.

-m density,r,g,b

enables an exponential shading of the void between the eye and the first hit location. This allows a cheap simulation of atmospheric haze with lower performance impact than explicit modeling of the air volume. The haze is computed using the form:

f = e^(-distance * density)

g = 1.0 - f

color = color * f + hazeRGB * g

The color values are optional. If not specified, they will have the passable values 0.8 .9 .99 .

Example usage: -m '2.5e-8,.75,.95,.99'


enables incremental mode processing. (First, a single ray is fired. Then, the screen is subdivided into four parts by evenly subdividing in the vertical and horizontal directions. The previously fired ray was the lower left corner; three more rays are fired. This process recurses until full resolution has been reached. Assumes square image. Also assumes fast path between CPU and display hardware.)

-j xmin,ymin,xmax,ymax

enables processing of just a sub-rectangle, located within the rectangle (0,0) (width-1,height-1), and whose screen coordinates (lower left and upper right corners) are provided in the argument. Ray-tracing will be limited to just those pixels within the sub-rectangle, and only those pixels will be transmitted to the framebuffer. The corner pixels (xmin,ymin) and (xmax,ymax) are included in the sub-rectangle. This option exists primarily for mged(1) to allow sweeping out a rectangular portion of the view for ray-tracing.

-k xdir,ydir,zdir,dist

enables the use of a cutting plane. The xdir, ydir, zdir, and dist define a plane (equivalent to a halfspace primitive with the same parameters). The vector (xdir, ydir, zdir) is normal to the plane and points away from the space to be subtracted. The dist parameter specifies the distance from the origin to the plane in the normal direction. The resulting image approximates subtracting the halfspace from the objects being raytraced. Note that this is not exactly the same as actually subtracting the halfspace, but it is faster. If explicit lights are used, objects that are cut away by the cutting plane will still cast shadows.

-l #

selects lighting model. Default is 0. Model 0 is a full lighting model with the ability to implement Phong shading, transparent and reflective objects, shadow penumbras, texture maps, etc. In addition to ambient light, a small amount of light is supplied from eye position. All objects in the active model space with a material property string of "light" represent additional light sources (up to 16 are presently permitted), and shadow computations will be initiated automatically. (This mechanism is subject to further change.) Model 1 is a diffuse-lighting model only, and is intended for debugging. Model 2 displays the surface normals as colors, and is useful for examining curvature and surface orientation. Model 3 is a three-light diffuse-lighting model, and is intended for debugging. Model 4 is a curvature debugging display, showing the inverse radius of curvature. Model 5 is a curvature debugging display, showing the principal direction vector. Model 6 is for debugging the UV parameterization used for surface texturing. It is exactly the same as applying the texture 'testmap' to every region. The U parameter is displayed in the red channel and the V parameter is displayed in the blue channel. Model 7 is for using photon mapping to simulate global illumination and caustics in a given scene. Options to photon mapping are passed through the -l parameter in the following format:

-l7,Num_Photons,Caustics_Percent,Irradiance_Rays,Angular_Tolerance, Random_Seed,Importance_Mapping_Toggle,Irradiance_Hypersampling_Toggle, Visual_Irradiance_Toggle,Scale_Lumens,Load_Save_File

Example: -l7,16384,0,10,60.0,0,0,0,0,1.0,

Note: If just -l7 is used, then the default values above will be applied. Light sources must also be explicitly defined with lumens defined for each source, i.e., default light sources will not work.

-n #

defines the height of the image as a number of scanlines.

-o output.pix

specifies the name of a file to store the pix(5) format output. The pix-fb(1) utility can be used later to display ".pix" files. Note that using -o disables parallel processing.

-p #

sets the perspective, in degrees, with 0 < = # < 180


requests that overlapping regions be reported (on by default); opposite of -R .

-s #

specifies the number of rays to fire in the X and Y directions on a square grid. The default size is 512 (i.e., 512 x 512).


causes reversal of the order of grid traversal (default is not to do that)

-u units

specifies units, with "model" being used for the units of the local model

-v #

sets the verbosity bit vector flags for rt . Use -v 0 for no output or -v fff for all available output. See include/brlcad/rtprivate.h for various bit definitions.

-w #

defines the width of each scanline in pixels.

-x #

sets the librt(3) debug flags to the given hexadecimal bit vector. See raytrace.h for the bit definitions. (Notice that -! # sets the libbu(3) debug flags to the given hexadecimal bit vector. See bu.h for the bit definitions.)

-A #

sets the ambient light intensity, as a fraction of the total light in the scene in the range of 0.0 to 1.0 .


turns on the "benchmark" flag. This causes all intentional random effects such as ray dither, color dither, etc., to be turned off to allow benchmark timing comparison and benchmark result comparison.

-C #/#/#

sets the background color to the RGB value #/#/#, where each # is in the range of 0 to 255. All non-background colors will be dithered away from this value.

-D #

This is the "desired frame" flag, specifying the frame number to start with. Used with animation scripts; specify the desired ending (kill-after) frame with -K # .

-E #

sets the distance from the eye point to the center of the model RPP; default is the square root of 2. Only useful with auto-sizing, and conflicts with -M .

-F framebuffer

indicates that the output should be sent to the indicated framebuffer. See libfb(3) for more details on the specification of a framebuffer.

-G #

sets the grid cell height, in millimeters. If the grid cell height is specified and the grid cell width (which would be set using -g) is not specified, the grid cell width defaults to be the same as the height.

-H #

This is the "hypersample" flag, specifying number of extra rays to fire for each pixel to obtain more accurate results without needing to store the extra pixels. Automatically invokes -J 1. The final colors of all rays are averaged together. Better results can be obtained by simply increasing the resolution, and decimating the results with a filter such as pixhalve(1).


turns on interactive mode. If this flag is off and your image is "large" (> = 256 x 256), it will lower the priority of the rt process.

-J #

This is the "jitter" flag, which is a bit vector. If one (-J 1), it randomizes the point from which each ray is fired by +/- one half of the pixel size. Useful if doing your own post-filtering for antialiasing, or to eliminate systematic errors. This has been useful in obtaining more accurate results in a volume-calculating raytrace as well. If two (-J 2), it shifts the origin of the output frame once for each frame. The shift is along a circular path in the plane of the frame, with a period of ten seconds, and a radius of +/- 1/4 pixel width. If three (-J 3), it performs both operations.

-K #

sets the final (kill-after) frame number. Used with animation scripts in conjunction with -D # .


causes reading of animation (model2view) matrix or animation script from standard input. Conflicts with -a and -e options.

-N #

sets the NMG_debug flags to the given hexadecimalbit vector. See h/nmg.h for the bit definitions.

-O output.dpix

specifies the name of a file to store the double-precision floating-point version of the image. dpix-pix(1) can be used to later convert the file to pix(5) format output. (Deferred implementation)

-P #

specifies the maximum number of processors (in a multi-processor system) to be used for this execution. The default is system dependent. On "dedicated" multi-processors, such as workstations and super-minis, the default is usually set for the maximum number of processors, while on shared multi-processors, such as SuperComputers, usually just one processor is used by default.

-Q x,y

selects pixel ray for query with debugging. The option is used to specify a single pixel for which the debug flags should be turned on. The computation of all other pixels will be performed without debugging. This is similar to the -b option, except that it does not limit rendering to the single ray. The entire image is computed. This option is useful in debugging problems which arise from specific computation sequences (such as the order of random numbers previously drawn).


requests that overlapping regions not be reported; opposite of -r .


turns on stereo viewing. The left-eye image is drawn in red, and the right-eye image is drawn in blue.

-T # or -T #,# or -T #/#

sets the calculation tolerances used by librt(3). If only one number is given, it specifies the distance tolerance. Any two entities closer than this distance are considered to be the same. The default distance tolerance is 0.005mm. If the second number is given, it specifies the value of the dot product below which two lines are to be considered perpendicular. This number should be near zero, but in any case should be less than 1. The default perpendicular tolerance is 1e-6. If both calculation tolerances are provided, they shall be separated by either a slash ("/") or a comma (","), with no white space between them. For example, the default tolerances would be specified as -T0.005/1e-6 on the command line.

-U #

sets the Boolean variable use_air to the given value. The default is 0, which ignores regions that have non-zero aircode values.

-V #

sets the view aspect. This is the ratio of width over height and can be specified as either a fraction or a colon-separated ratio. For example, the NTSC aspect ratio can be specified by either -V1.33 or -V4:3 .


inverts the foreground and background colors, changing the default from white lines on a black background to black lines on a white background

-X #

turns on the rt(1) program debugging flags. See raytrace.h for the meaning of these bits.

-! #

turns on the libbu(3) library debugging flags. See bu.h for the meaning of these bits. In particular, specifying -!1 will cause bu_bomb() to dump core, rather than logging the error and exiting. This is useful with dbx(1) or gdb(1) for debugging. (Notice that -x # sets the libbt(3) debug flags to the given hexadecimal bit vector. See raytrace.h for the bit definitions.)

-+ #

specifies (setting argument to "t") that output is NOT binary (default is that it is). -+ is otherwise not implemented.

The rt program is a simple front-end to librt(3) which can be invoked directly, or via the rt command in mged(1).


You can obtain progress information from a running rt process by sending it a SIGUSR1 or SIGINFO signal. The raytracer will report progress via details about the ray sample being processed at the time of the signal. The progress is reported to stderr in a form similar to: frame 0, xy=1516,2466 on cpu 0, samp=0


For information on the animation script language accepted by rt, please refer to the manual pages for tabinterp(1), tabsub(1), and anim_script(1). Also please consult the HTML documentation for a complete animation tutorial.


When using rt to create an animation sequence destined for NTSC video, it is useful to give all these options at one time: -V4:3 -c "set gamma=2.2" -w1440 -n972 -J1 and then post-process the output using pixhalve(1) for final display. Performing the gamma correction inside rt instead of post-processing the pix(5) file with bwmod(1) avoids Mach-bands in dark regions of the image, and retains the maximum amount of image detail.


brlcad(1), mged(1), lgt(1), pix-fb(1), rtray(1), rtpp(1), librt(3), ray(5V), pix(5)


Numerous error conditions are possible, usually due to errors in the geometry database. Descriptive messages are printed on stderr (file descriptor 2).




This software is Copyright (c) 1984-2016 United States Government as represented by the U.S. Army Research Laboratory.


Most deficiencies observed while using the rt program are a consequence of problems in librt(3).


Reports of bugs or problems should be submitted via electronic mail to <>, or via the "" script.