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QRT Users Manual INTRODUCTION QRT (Quick Ray Tracer) is an easy to use image generation system that uses a ray tracing algorithm for image rendering. Versions exist for the Amiga personal computer, the Data General MV10000, the HP 9000 series, and the IBM AT. QRT overcomes many of the problems of other ray tracing packages, and is designed to operate nicely in a multi-tasking environment. COMPARISON TO OTHER SYSTEMS QRT was developed on the Amiga personal computer, so it will be compared to other Amiga ray tracers. There are, to my knowledge, five other Amiga ray tracers, each with its own strengths and weaknesses. I will describe each system briefly, and compare it to QRT. All the Amiga ray tracers can operate in HAM (4096 color) mode. RT: RT was the first ray tracer written for the Amiga, by Eric Graham. It will model a universe made of only spheres, a sky, and a checkered or solid ground. It is relatively fast, but not generally useful for realistic modeling because of the sphere limitation. The input language is cryptic, although some error checking is done. The system will only generate low resolution images. SILVER: I have never seen SILVER, so I cannot say much about this system. SCULPT-4D: This package incorporates an interactive editor for creating objects, and is capable of quickly generating a preliminary image of the scene by using hidden surface techniques. However, every primitive is made of polygons, and some primitives such as spheres require hundreds of polygons for a smooth texture, so the ray tracing is very slow. Also, the package takes a large amount of memory to run, and is prone to system crashes. Its chief feature is the ability to create arbitrary shaped objects using a series of triangles. Mirrored, dull, or shiny objects are supported. CLIGHT: This ray tracer also has an interactive editor, but produces very poor quality images. It is not capable of any patterning or reflection characteristics. QRT Ray Tracer Page 1 User Manual DBW: This is possibly the most complete ray tracer for the Amiga. It will support objects with arbitrary degrees of reflection and gloss, depth of field effects, some texturing, wavy surfaces, fractals, transparent surfaces, diffuse propagation of light from object to object, and 5 primitive types (sphere, triangle, parallelogram, fractal, and ring). The input language, however, is so cryptic as to be nearly incomprehensible, and if there is any error in the input file, it will crash the system. It is also painfully slow; some images take 16 to 24 hours to complete. QRT is meant to be a compromise between the fast, simple ray tracers and the slow powerful systems. It compares favorably in speed to RT, and in power to Sculpt-3d or DBW. It has a very friendly input language with extensive error checking. Here are some features of QRT: o Multiple primitive types, including user defined quadratic surfaces o Arbitrary levels of diffuse reflection, spectral reflection, transmission, ambient lighting, and gloss o User defined pattern information for objects o Bounding boxes for groups of objects o Shadows o Multiple light sources with different characteristics o Arbitrary Phong spectral reflection coefficients o Color dithering to increase the apparent number of colors o Easy to use, free format input language with error checking. Parameters are by keyword and may appear in any order. o Supports medium resolution (128k dots/screen) Each of these features of QRT will be discussed is greater detail in this document. QRT Ray Tracer Page 2 User Manual THE QRT WORLD QRT constructs an image of the world by performing certain manipulations on an internal representation of a group of objects. You must provide a description of these objects to QRT through the QRT input language. QRT writes a machine independent bitmap file which can be read by a computer specific post processor. The post processor either displays the image or converts it into the proper form for display by system programs. (See the Machine Dependent Information section of this manual for details). QRT is capable of dealing with 5 types of primitive objects: spheres, parallelograms, triangles, rings (annulus), and quadratic surfaces. The latter can be used to model cones, oblong spheroids, and other interesting surfaces. Each of these objects can have an arbitrary orientation in space, and arbitrary surface characteristics. QRT Ray Tracer Page 3 User Manual QRT INPUT LANGUAGE The QRT input language is free format, in that commands may be placed anywhere on a line, and newlines may be placed at any point. This makes it easy to use indentation. Comments are also supported by surrounding text with curly braces '{' and '}'. Comments may span lines. Once a file with QRT commands is built (using any available text editor), qrt can be made to process these commands: QRT <commands.qrt where "commands.qrt" is the file containing the input. QRT also produces some statistics, so this output can also be redirected. Since QRT runs for a long time, you will probably want to run it as a background task with a low priority. So, the full set of commands to run QRT would be: SetTaskPriority -5 run QRT <commands.qrt >qrt.out SetTaskPriority 0 This set of commands is specific to the Commodore Amiga; see the Machine Dependent Information section of the manual for other operating systems. QRT will write a very large file, which may be anywhere from 400K to 4 megabytes depending on the specific computer display resolution. (See the Machine Dependent Information section of this manual for the exact file size). There should be at least enough room on your disk to accommodate this file. I recommend sending the output to a RAM disk, so that the physical disk does not get used for 60 minutes while QRT computes the image. A typical QRT command to create a sphere using the default surface attributes would, be: SPHERE ( loc = (5,10,100) { this is a white sphere } radius = 12.5 ) For comparison, the sphere command for DBW looks like this: s 0 0 0 1 0 0 0 .2 .2 .2 .8 .8 .8 5 10 100 12.5 Or, in RT, the command would be: <1,1,1> 2 (5,10,100):1; QRT Ray Tracer Page 4 User Manual The QRT input should be much easier to understand. If QRT sees an error in its input file, it will stop and print the line number of the offending line, along with a descriptive error message (ILLEGAL PARAMETER, etc). The QRT input language is not case sensitive. A full description of the language and a shorter, context free grammar can be found elsewhere in this manual. QRT Ray Tracer Page 5 User Manual THE QRT ILLUMINATION MODEL Before meaningful images can be created, the QRT illumination model must be understood. The light that reaches the observer from each object is composed of several components: o Diffuse light - The "color" of the object o Ambient light - The color of the light that falls on the surface of the object if no lamps are shining on this spot. Note that this specifies the color of light, not the color of the object itself. o Reflected light - If the object acts as a mirror, some light is reflected. o Specular highlights - The "bright spots" of a shiny object viewed in a light. o Transmitted light - The object may transmit some of the light that strikes the back of the object (glass surfaces) All of these light characteristics can be specified for any object. If none are specified, the current defaults are assumed. These defaults can be changed (see DEFAULT in Language Reference Guide). Most of these light characteristics deal with a percentage of light. For example, a light may reflect 80% of the red light that strikes it, 20% of the green, and 40% of the blue. This is the "color" of the object (diffuse light), and would be specified in QRT by the following syntax: diff = (.80, .20, .40) In QRT, 1.00 is 100%. The language is also free-format, so the above is equivalent to: diff = (.80, .20, .40 ) The commas between parameters are optional, but make the input easier to read. Semicolons can also be used. Ambient light is specified in a similar manner: QRT Ray Tracer Page 6 User Manual amb = (.20, .20, .20) The ambient light values should be fairly small. Most of the light hitting the object comes from lamps, but some parts of the object may be in the shadow of another object. If no ambient light is specified, these shadows will appear totally black, which looks unrealistic. Ambient light will give the affect of a small amount of light hitting areas in a shadow, producing a more realistic looking image. Reflection is specified using the MIRROR attribute: mirror = (.90, .90, .90) If a true mirrored surface is desired, the mirror values should be fairly high. If one of the values has a higher value than the others, the mirror will appear red, green, or blue. Transmission is specified as follows: trans = (.80, .80, .80) density = (.022, .022, .022) index = 1.33 Note the addition of another parameter, the index of refraction. An explanation of the index of refraction for an object is beyond the scope of this document - see a book on elementary optics for details. The density factor is a light attenuation factor per unit distance the light travels through the object. The trans parameter tells QRT how much light to remove from shadows cast by this object. In addition, specular highlights are specified with two parameters: reflect = .50, sreflect = 45 "REFLECT" is the percentage of light reflected in the specular highlight, and sreflect is the Phong spectral reflection coefficient. Again, an explanation of the Phong coefficient is beyond the scope of this document; however, a higher value for the coefficient will result in smaller, tighter highlights for a more metallic looking object. Lower values of sreflect should be accompanied by lower values in reflect, and will produce duller looking surfaces, such as paper. QRT also accepts a dithering amount for each object: QRT Ray Tracer Page 7 User Manual dither = 3 Dithering is a mechanism for simulating colors not available on the display by blending other colors. The default dithering coefficient is 3; it should be kept small. Values of 1 to 6 are good. Larger objects should employ more dithering, and mirrored or glass objects should have little or no dithering. QRT also accepts an attribute "FUZZ", but this is not used in the present implementation. In a future implementation of QRT, FUZZ will effect small, random perturbations of the normal vector at a given location on an object. This will simulate rough surfaces, matted glass, or imperfectly reflecting mirrors. All of the above light characteristics may be attached to any object. The default is for a white, non reflecting, non transmitting dull surface with an average amount of dithering. In addition, any of the above color information (which will be referred to from now on as COLORINFO) may also be attached to a pattern structure (see the PATTERN section of this document). QRT Ray Tracer Page 8 User Manual LIGHT SOURCES Light sources in QRT are called "LAMPS". The lamp structure is simple: LAMP ( loc = (12, 34, 56), dist = 50, radius = 10 ) The default is for a bright white lamp, but if you wish to change this, you can specify different values for the lamp's ambient light. The radius value is ignored by this implementation of QRT, but it must be supplied. In a future version, the radius may be used to implement penumbral shadows. I didn't include them in this version, since they take a very long time to compute, and don't add much to the image quality. The "dist" entry in the lamp structure specifies at what distance the light from the lamp is at full intensity. Beyond this value, the light will decrease in intensity. This value should be set so that the nearest object in the scene is slightly more than "dist" distance units from the lamp. This does not have to be exact. If objects are nearer than dist" units, they will be so bright that shading will not take place. Objects very far away will be dimly illuminated. By the way, QRT distance units are not tied to any real world unit. They can stand for feet, nautical miles, or furlongs, at your choice. QRT Ray Tracer Page 9 User Manual THE OBSERVER After defining the world, you must tell QRT the position and orientation of the observer. This is done as follows: OBSERVER ( loc = (0, 10, 20), lookat = (5, 5, 120), up = (0, 1, 0) ) The x,y, and z are the location of the observer. The "lookat" variables give a location in space that the observer is looking at. Most ray tracers require you to give two angles for the observers view direction, but it is MUCH easier to know the location of an object or point in space you wish the observer to look at. This will be the center of your scene. The "up" variables define which direction is up. Usually, you will wish to use the values given above. QRT uses a right hand coordinate system: positive y is up, positive z is out of the screen, and positive x is to the right if you are looking in the negative z direction. The "up" parameter is optional, and if omitted, defaults to (0,1,0). QRT will generate an error message if no observer is defined. QRT Ray Tracer Page 10 User Manual OUTPUT FILENAME QRT places its output in a file, so you must give it the name of this file: FILE_NAME = outfile.tmp This file must observe any file naming restrictions of the operating system and computer you are using. See the Machine Dependent Information section of this manual for information on file naming restrictions. FOCAL LENGTH This is the focal length of the "lens" used by the observer. Think of it as a 35mm camera lens - higher numbers produce a telephoto effect, and smaller numbers are for wide angle lenses. Note that small numbers may produce some distortion of the image around the edges. FOC_LENGTH = 60 QRT Ray Tracer Page 11 User Manual SKY AND GROUND QRT has facilities for generating the sky and ground. To define the sky, you must give it two colors - one for the sky overhead (zenith), and one for the horizon: SKY ( zenith = (.10, .2, .4), horiz = (.10, .2, .65), dither = 6 ) This will produce a blue sky, with a brighter color near the horizon. (The sky need not be blue - it could be red or hot pink, or vary from red to green). QRT will smoothly blend the colors from the zenith to the horizon. Since the sky is so large, you may want to specify a greater amount of dithering to compensate for the displays color resolution limit. The SKY structure will also produce sky colors below the horizon; any ray that does not strike an object will strike the SKY. To fix this, define a ground. There is no dedicated GROUND command, since you can define a very large parallelogram with the same effect. You can make it brown with patches of green using a PATTERN, or checkered green and yellow in the classic ray-tracing ground pattern. A very large sphere can also be used for the ground, which yields better results with mirrored objects. QRT Ray Tracer Page 12 User Manual BOUNDING BOXES QRT supports the use of bounding boxes to speed the ray tracing process. For images composed of only a few (1 to 3) objects, bounding boxes will not do much to increase speed. However, for images where there are groups of objects physically close to each other, they can greatly reduce execution times. A bounding box is a conceptual structure that encloses a group of objects. When the ray tracer is finding line/object intersections, if a line does not strike a bounding box, it cannot possibly strike any objects within that bounding box. This saves the ray tracer the trouble of checking intersections with all objects within the box. In the case where the ray DOES enter the box, some additional overhead is incurred; however, this cost is easily justified by reduced times for negative tests. Bounding boxes can occasionally be useful for complex objects, such as quadratic surfaces, when the object is fairly small. Since the time to find the intersection with a quadratic surface is large, but bounding box intersections are fast, the ray tracer can save time for all the negative tests. Bounding boxes can contain other bounding boxes, in a recursive manner. This recursive structure defines an "object tree". There are two keywords that define the beginning and end of a bounding box: BEGIN_BBOX and END_BBOX. Here is an example: { ** QRT Code for a chessman ** } BEGIN_BBOX QUADRATIC ( { ** quadr defn ** } ) QUADRATIC ( { ** quadr defn 2 ** } ) SPHERE ( { ** sphere defn ** } ) BEGIN_BBOX SPHERE ( ) { ** two spheres ** } SPHERE ( ) END_BBOX END_BBOX QRT Ray Tracer Page 13 User Manual This structure may be nested to an arbitrary level; if you had a closely spaced group of chessmen, you could enclose all of them with a bounding box. There is no simple algorithm for knowing exactly where to place bounding boxes. You have to use common sense. Typically, bounding boxes do the most good for closely spaced objects, but if the entire group of objects is less than a third of the image area, it will be useful to enclose them all in a bounding box. QRT Ray Tracer Page 14 User Manual PATTERNS QRT permits user defined patterns which can be mapped to the surface of any object. A pattern is basically an organized method for changing an object's COLORINFO over the surface of that object. For example, a checkered surface can be created, or a surface given the appearance of brick or tile. The current patterning in QRT is limited, yet powerful enough to describe many common patterns. A pattern is composed of a series of sub-patterns. Each sub-pattern defines a region on the surface of an object which will contain a given COLORINFO. These regions are presently limited to rectangles, but this may be expanded in a future implementation of QRT. A pattern can be defined once, and used for many objects. For example, a brick pattern might be defined: PATTERN ( name = BRICK, { ** other pattern info here ** } ) SPHERE ( { ** sphere definition ** } pattern = BRICK ) PARALLELOGRAM ( { ** parallelogram definition ** } pattern = BRICK ) This example ignores what is actually in the pattern definition to demonstrate how patterns are attached to objects. A pattern must be defined before it can be used. It is given a name, such as BRICK, and any object can specify this pattern with "PATTERN = BRICK". There are no practical restrictions on the length of pattern names (if your computer has 1 megabyte of ram, you cannot have a pattern name longer than 1 million characters). Multiple patterns may be defined at the top of an input file, and used for any object in the file. What exactly makes up a pattern, you ask? A pattern can be viewed as a rectangle that is repeated over the surface of an object. If the pattern size is 10 x 10, and you have an object (say a parallelogram) that is 50 x 50, the pattern will repeat 5 times in each direction. Within this repeating rectangle, sub patterns can be defined. A sub-pattern is a rectangle or circle within the pattern rectangle with a certain COLORINFO. Any number of these sub-patterns can be QRT Ray Tracer Page 15 User Manual created within one pattern definition. To create a brick wall, you might define several sub-patterns, each with a slightly different color of red or brown. The sub patterns need not cover the entire pattern rectangle; if they do not, the COLORINFO you defined for the object is used instead of the pattern COLORINFO. If you defined sub-patterns for bricks, the area not covered would be mortar, and the objects COLORINFO would define the color of the mortar. An actual brick pattern is given in appendix C. The proper use of patterns can be very effective in producing realistic looking scenes. However, complicated patterns will slow image creation, so it is recommended that you first display test scenes with no patterns, and then add pattern information when the scene is right. Patterns have one other use, in the REMOVE command. A pattern can be defined and thought of as a cut out from a surface. For example, suppose you wish to make a flat surface in the shape of a grand piano top. Rather than creating the piano top out of many parallelograms and triangles, define a pattern that includes everything in a parallelogram but NOT in the piano top; that is, the pattern maps the area to remove, not the area to keep. Attach this pattern to a planar object with the REMOVE command. In this case, since the area is removed, any associated COLORINFO is ignored. QRT Ray Tracer Page 16 User Manual INSTANCES Often, several copies of a complex object must be created. Specifying their component primitives each time would be difficult. There is a solution to both these problems: INSTANCES. An INSTANCE is a method by which several primitives (actually, an arbitrary object tree) can be grouped and given a name. Copies of these objects may be easily created with one command. Here is an example of an instance definition. BEGIN_INSTANCES NAME = object1 BEGIN_BBOX { ** a bunch of primitives here ** } END_BBOX NAME = object2 BEGIN_BBOX { ** a bunch more primitives here ** } NAME = spheres BEGIN_BBOX SPHERE ( ) { ** two sphere definitions ** } SPHERE ( ) END_BBOX END_BBOX END_INSTANCES There can be only one of these instance definitions, and it must appear before any instances are used. However, any arbitrary object tree can appear within the begin/end instance statements. Several instances are now available for use. For example: INSTANCE_OF ( name = object1, loc = (100, 10, 20), ) INSTANCE_OF ( name = spheres, loc = (12.3, 24.5, 999) ) This example shows that any named portion of the object subtree can function as an instance (the "spheres" object is at a lower level than the "object1' object). The "offset" parameters are required. They specify a new position for the instance (offset from the origin). In the instance definition QRT Ray Tracer Page 17 User Manual segment, all objects are defined relative to 0,0,0. They can then be moved in the INSTANCE_OF statement with the offset command. In addition, the instance can be given a new size: INSTANCE_OF ( name = object2, loc = (200, 100, 50), scale = (1.5, 2.0, .25) ) The scale factors are optional, and are given the default of 1. The instance will be resized by the indicated amount in each of the directions. Note that there are some restrictions to this: if spheres are given a different scale factor in each direction, they will remain spheres (not change to QUADRATIC types). The new radius will be the old radius scaled by the smallest of the scale factors. QRT Ray Tracer Page 18 User Manual DEFAULTS Each object in QRT is given certain default surface light characteristics (see the section on surface characteristics). If these defaults are not suitable, they can be changed with the DEFAULT command: DEFAULT ( diff = (1.00, .1, .1) no_shadow ) This will make all future objects red unless specified otherwise. More than one default command can be used in a file: each one affects all the objects created after it, but before the next default command. Any light characteristics can be changed (MIRROR, DITHER, etc). In addition, the keyword "no_shadow" can be included (as above). Ordinarily, QRT computes shadow information for all objects. This takes a lot of time, especially for scenes composed of many objects and many lamps. The "no_shadow" command causes QRT to bypass the shadow routines. This will result in a much faster image generation time, but the picture will not look as realistic. X and Y screen resolution and aspect ratio can also be changed with this command. If you wish to generate a preliminary image in a smaller resolution, say, 127 x 127, you would use the following default: DEFAULT ( x_res = 127, y_res = 127, aspect = .56 ) The aspect ratio of .56 is correct for the Amiga in 4096 color mode; use 1.0 for high resolution mode. QRT Ray Tracer Page 19 User Manual PROBLEMS WITH QRT There are several limitations in this version of QRT: o Patterning does not work well with some quadratic surfaces. This is a problem with the 3d to 2d mapping function used for quadratics. o User defined texturing is not implemented - all objects in QRT appear smooth. I have some ideas on how to add textures to objects, but these have not been tested and are not incorporated into this version of QRT. QRT Ray Tracer Page 20 User Manual FUTURE ENHANCEMENTS TO QRT The following are some things I'd like to add to QRT: o Enhanced patterning capability. The ability to use enumerated (bit-mapped) patterns as well as analytic patterns would be useful. o Anti-dithering routines. These routines are very computationally expensive (they can increase image generation times by a factor of 3 to 5), so they were not included in this version. o Penumbral shadows. This is also very computationally expensive for a minimal utility level, so I didn't bother to include this feature. o Fractal generation. Fractals are, again, computationally expensive, but some things, such as mountains, cannot be modeled with a ray tracer any other way. o Wavy surfaces. This is useful for modeling water, rippled mirrors, etc. I know how to do it, I just didn't have time to add it yet. It would be nice if the waves were user definable in amplitude and x and y wavelength. Wavy surfaces are very similar in implementation to textures. o Snowy surfaces. This is an idea from DBW, which adds snow to a surface depending on its slope and altitude. o Interpolated normal surfaces. This is a mechanism to model arbitrary curved surfaces by using a polygonal approximation to the surface, and interpolating the normal vector between surfaces to avoid angular looking surfaces. o Image plane object lists. This is a technique for increasing the speed for positive line/object intersection test. (Bounding boxes increase speed for negative line/object intersection tests). o Heuristics for faster completion of images using area coherence. o An interactive editor. This is another large scale project in itself, and I can't forsee having time to do it for a long time, but it would be useful. The editor would display a preliminary image of the scene, and when the user had placed objects to his satisfaction, QRT Ray Tracer Page 21 User Manual it would write a QRT input file and call QRT as a background task. There are certain problems here - for instance, how do you quickly generate the outline of a user defined quadratic that can take many forms (cone, spheroid, etc)? And how can the user easily specify the object tree structure (bounding boxes) with an interactive editor? QRT Ray Tracer Page 22 User Manual