Glome tutorial

From HaskellWiki
Revision as of 02:14, 26 April 2008 by Jsnow (talk | contribs)

Jump to: navigation, search

Installing Glome

First, you need to install the software. I will assume that you already have ghc and the Haskell OpenGL libraries installed, and are comfortable with "tar" and the like. See How to install a Cabal package. Familiarity with Haskell is not required, but it will help.

The source code is available from hackage. You must untar the file ("tar xvfz [filename]", "cd glome-hs[version]") , and then build the binary with:

runhaskell Setup.lhs configure --prefix=$HOME --user
runhaskell Setup.lhs build
runhaskell Setup.lhs install

Glome doesn't really need to be installed in order to run it. If you'd prefer, you can invoke it directly from the build directory as "./dist/build/glome/glome".

If everything works, a window should open and you should (after a pause) see a test scene with a variety of geometric shapes. If it doesn't work, then let me know.

Command line options

These are pretty sparse at the moment. You can specify an input scene file in NFF format with the "-n [filename]" option, and there is one such scene included with Glome, a standard SPD level-3 sphereflake.

NFF isn't very expressive (and it was never intended to be), so I won't say much about it here. Glome supports most of the basic features of NFF except for refraction. My approach to polygon tesselation is also questionable: the SPD "gears" scene, for instance, doesn't render correctly.

You may have to adjust the lighting to get satisfactory results (i.e. by adjusting the value of "intensity" in "shade" function in the Trace.hs file and recompiling). NFF doesn't define a specific intensity, and I'm not sure what sort of falloff (if any) Eric Haines used when he rendered the reference images.

Describing Scenes in Haskell

Ideally, using Glome would be a matter of firing up Blender and editing 3-d geometry in a graphical, interactive way and then exporting the scene to Glome, which would do the final render.

Unfortunately, Glome isn't able to import files from any standard 3-d format except NFF (which isn't typically used for anything but benchmark scenes).

So, with only limited import functionality, how do we model complex scenes?

One option we have left is to describe our scene directly in Haskell, and then compile the description and link it with the Glome binary. This is the approach we will be following for the remainder of this tutorial.

This isn't quite as difficult as it sounds. POV-Ray, for instance, has a very user-friendly scene description language (SDL), and many artists type their scenes in directly as text.

Glome was, in fact, quite heavily influenced by POV-Ray, so anyone familiar with POV's SDL and Haskell should be able to write scenes for Glome without much trouble.

Unlike POV-Ray, in which the SDL is separate from the implementation language (C, or more recently, C++), in Glome there is no distinction. In that sense, Glome is more of an API than a standalone rendering system.

The default scene, which is loaded if the user does not specify an input file on the command line, is defined in TestScene.hs. To define a new scene, you must edit this file and then recompile the source.

TestScene.hs: Camera, Lights, and the minimal scene

TestScene.hs import a number of modules at the beginning, and it contains a handful of objects and then defines a single function.

 scn :: IO Scene
 scn = return (Scene geom lits cust_cam (t_matte (Color 0.8 0.5 0.4)) c_sky)

"scn" is called from Glome.hs to specify a scene if there wasn't one passed in as a command line argument.

"scn" uses the IO monad, in case we want to load a file from disk. We won't be doing that in any of our examples, so you can safely ignore the "IO" part. It returns an object of type "Scene".

A Scene is described like this in Solid.hs:

data Scene = Scene {sld     :: Solid, 
                    lights  :: [Light], 
                    cam     :: Camera, 
                    dtex    :: Texture, 
                    bground :: Color} deriving Show

So, in order to construct a valid scene we need to put something into all the fields.

The first field takes a Solid. A Solid is any of the basic primitive types that Glome supports. These might be triangles, spheres, cones, etc...

You might wonder why we only need one primitive to define a scene. Certainly, we'd want to have a scene that contains more than a single sphere!

The solution is that Glome includes several primitive types that let us create more complex Solids out of simple ones. For instance, a Group is a list of Solids, and Glome treats that list as if it was a single Solid. This will be discussed in more detail later on.

A light is defined by a Color and a Vec:

data Light = Light {litpos :: !Vec,
                    litcol :: !Color} deriving Show

A "Vec" is a vector of three floating point numbers, while a "Color" is also three floats as red, green, and blue values. (This may change in the future: RGB isn't necessarily the best representation for colors.)

(See Vec.hs and Clr.hs for the definitions and useful functions for dealing with vectors and colors, respectively.)

We can define a light like so:

Light (Vec (-3) 5 8) (Color 1.5 2 2)

Note that the rgb values don't have to be between 0 and 1. In fact, we may wish to make them quite a bit larger if they're far away.

Also note that a decimal point isn't mandatory. Haskell is smart enough to infer that the Color constructor expects a float. The parentheses around the "-3", on the other hand, are required.

The square brackets is the definition of Scene tells us that we need a list of lights rather than a single light, and we can turn our single light into a list simply by enclosing it in square brackets. We could also use the empty list [], but then our scene would be completely black except the background.

A camera describes the location and orientation of the viewer. There is a function for creating a camera defined in Solid.hs:

camera :: Vec -> Vec -> Vec -> Flt -> Camera
camera pos at up angle =
 let fwd   = vnorm $ vsub at pos
     right = vnorm $ vcross up fwd
     up_   = vnorm $ vcross fwd right
     cam_scale = tan ((pi/180)*(angle/2))
  Camera pos fwd
         (vscale up_ cam_scale) 
         (vscale right cam_scale)

It's arguments are: a point defining it's position, another point defining where it's looking, an "up" vector, and an angle. At this point, we need to decide which direction is up. I usually pick the "Y" axis as pointing up, So, to set up a camera at position <20,3,0> looking at the origin <0,0,0>, and a 45 degree field of view (measured from the top of the image to the bottom, not right to left or diagonal) we might write:

 camera (vec 20 3 0) (vec 0 0 0) (vec 0 1 0) 45

"dtex" and "background" define a default texture and a background color. The background color is the color if we miss everything in the scene. The defaults should work okay for the present.

Spheres, Triangles, Etc..




Bounding Objects

The Bounding Interval Hierarchy

Textures, Lighting