Material System Details

Design and implementation ideas for our material system.

Phase 1

• Done: Add simple tests that verify the material by rendering a polygon. Currently, we don't have tests for most materials._
• Done: Explore materials implemented using procedural textures, i.e., brick, marble, granite, wood, asphalt, etc. Later, we'll procedurally shade a city with these building blocks._
• Done: Add better reference documentation._
• Done: Add an opacity/alpha map material.
• Done: Decouple diffuse and specular components.
  • Split agi_getMaterialColor into two separate GLSL functions: agi_getMaterialDiffuseComponent and agi_getMaterialSpecularComponent.
  • Add gloss/specular map material.
  • This will slightly impact the lighting code.
• Done: Allow the material to modify the surface normal.
  • Add a third GLSL function that materials can optionally implement called agi_getMaterialNormal that takes and returns a normal.
  • Implement a bump map material using this. Implement a normal map material too.
• Done: Add the world-space eye direction, i.e. the vector from the camera to the fragment in world coordinates, to agi_getMaterial* (or do we only need it for agi_getMaterialDiffuseComponent initially?). Use this to implement:
  • A diffuse reflective material that uses an environment map for reflection, and a 2D texture for the diffuse component. Blend these with a reflectivity parameter.
  • A diffuse refractive material that also uses an environment map. Expose the two indices of refraction.
  • A Fresnel material - approximate, of course.
  • Include optional reflection and refractive maps for the above reflective and refractive materials, maybe Fresnel.
• Done: All materials should return the specular exponent.
• Done: All materials should return the emission color.
  • Add a material for an emission map.

Phase 2

• Polylines
  • Polylines currently use very simple shaders, and are rendered in three passes using the stencil buffer to achieve an outline effect (turn ANGLE off; start Chrome with --use-gl=desktop).
  • Replace the three-pass algorithm with a single pass algorithm that, in a fragment shader, uses the distance from the fragment to the line to determine if the fragment is part of the outline. Read Tron, Volumetric Lines, and Meshless Tubes.
  • First hard-code the above in the Polyline, then factor it out into a new PolylineOutlineMaterial.
  • Create a PolylineGlowMaterial based on Tron, Volumetric Lines, and Meshless Tubes.
  • Make Polylines work with the rest of the materials as reasonable. Polylines will need to be able to compute at least 1D texture coordinates. I could see some potential for 2D and 3D coordinates as well. All materials will not work with all primitives. We'll need to document a feature matrix.

Phase 3

• How do we combine multiple materials, e.g.,
  • A diffuse map and an alpha map to render .png files, for example. See #43.
  • A diffuse map and a specular map.
  • Crumbling bricks that combine brick and bump map materials.
  • A bumpy diffuse reflective surface that combines the bump map and diffuse reflection materials.
  • A diffuse map, diffuse reflective, specular map, and bump map. A bumpy, diffuse lit and reflective surface with shiny areas.
  • Blend two diffuse maps based on a parameter, e.g., terrain height, or third map.
• Implement the CentralBody fragment shader using materials, instead of hard-coding bump, specular, etc.

Phase 4

Details to follow...

• Different lighting models. Consider Phong, Blinn-Phong, Gaussian, Cook-Torrance, Oren-Nayar, Strauss, Ward, and Ashikhmin-Shirley. Which are most useful? Which fit best into our engine? See Learning Modern 3D Graphics Programming and Programming Vertex, Geometry, and Pixel Shaders.
• Light types: point, direction, spot. Area?
• Multiple lights: turn lights on/off per primitive. This will replace affectedByLighting on Polygon and CentralBody.

Phase 5

• How does this fit with the effects framework for models? Can they work well together?

Questions

• How important is it to support multiple texture coordinates, i.e., different coordinates for different materials on the same object?
• Do we have a need for relief mapping, etc?
• How do screen-space techniques like fog, glow, and bloom fit in?

Other Material Systems

• Effects frameworks like CgFX, Direct3D 10 FX Files, and COLLADA FX have materials as does every graphics engine under the sun, such as:
  • Three.js
  • Unity
  • C4 Engine
  • Id tech 4