I worked a bit more on tsGL over the last few days, and managed to clean up a few things that were really bothering me! So far progress has been relatively smooth and it’s actually turning out quite well! So, what changed?
TSGL’s lighting code is much cleaner now, and seems to work pretty well! Lighting is broken down into a multi-pass system which allows for an arbitrary number of lights to be applied to each object. Take this scene, for example…
This scene features three realtime lights, a red light in the back left corner, a blue light behind the camera, and a white light above the scene. All of these lights are drawn as “point-lights”, meaning that they act as omnidirectional sources.
First, the scene is drawn with no lights applied. This is important to capture shader-specific details on each object, such as emissive textures, reflections, and unlit details.
Next, lights are sorted based on their “importance”. This is calculated based on distance to the object being rendered and the intensity of the light. If there’s a large, bright light shining on an object, it will be drawn first, followed by all other lights until we’ve either reached the maximum allowed number.
Then, light parameters are packed into a 4×4 matrix. This may seem odd, but it also means that all attributes of a light can be passed as a single input to the GLSL shader. This allows for a large amount of flexibility in designing shader programs, as well as the convenience of not requiring several uniform variables to be defined in each one.
The Vertex Shader calculates a set of values useful for lighting, mainly the per-vertex direction of incident light, and the attenuation of brightness over distance. These are calculated per-vertex because it reduces the number of necessary calculations significantly, and small imperfections due to interpolation over the triangle are largely imperceptible!
By scaling the intensity of the light with the square of the distance from the source, lights will appropriately grow dimmer as they are moved farther from an object. The diffuse component of the light is also calculated per-fragment using the typical Lambertian reflectance model, and ensures that only the “light-facing side” of objects are shaded. In the above image, the intensity of a red light throughout the scene is visualized in false color, and the final diffuse light calculation is shown on the bottom right.
Awesome, but at this point our scene is just an unlit void! How do we combine the output of each light pass into a final image?
By exploiting OpenGL blend-modes, we can produce exactly the effect we want! OpenGL allows the programmer to specify an active “blend-mode”, essentially determining how new data is written to the display buffer! This is primarily used for rendering transparent objects. A window pane for instance, would need to be rendered over top of the rest of a scene, and would mix the background color with the color of the glass itself to produce a final color! This is no different!
For these lights, the OpenGL Blend Mode is set to “additive”. This will literally add together the colors of every object drawn, which in the case of lights is just what we need. Illumination is a purely additive process, and it is impossible for a light to make things darker. Because of this, simply adding the effects of several lights together will output the illuminated scene as a whole! The best part is that it works without having to pass an array of lighting information to the shader, or arbitrarily limiting the number of available lights based on hardware! While the overhead of rendering an additional pass is non-trivial, it’s a small price to pay for the flexibility allowed by this approach.
Here, we can see the influence of each of the three lights.
At the end of the day, we end up with a process that looks like this.
- Clear your drawing buffers. (erases the previous frame so we have a clean slate.)
- Draw the darkened scene.
- Sort the lights based on their “importance”.
- Set the blend mode to “additive”.
- For each light in the scene (in order of importance)
- Draw the scene again, illuminated by the light.
- We’re done! Display the buffer!
This solution isn’t perfect, and more powerful techniques have been described in recent years, but given the restrictions of WebGL, I find this technique to work quite well. One feature I would like to add is for the scene to only draw objects effected by a light in the additive pass, rather than the entire scene over again. This allows us to skip any calculations that would not effect an object in some way, and may increase performance, though without further testing, it’s difficult to say for sure.
This is always a fun feature to add, because it can have incredibly apparent results. Cubemaps are essentially texture-maps that exist on all sides of a cube. Rather than sampling a single point for a color, you would sample a direction, returning the color at that “angle” within the cube. By providing an image for each face, a cubemap can be built to represent lighting information, the surrounding environment, or whatever else would require a 360 texture lookup!
tsGL now supports cubemaps as a specific instance of a “texture”, and they can be mapped to materials and used identically in the engine! One of the clever uses of a cubemap is called “environment mapping”, which essentially boils down to emulating reflection by looking up the color of the surrounding area in a precomputed texture. This is far more efficient than actually computing reflections dynamically, and plays much more nicely within the paradigms of traditional computer graphics! Here’s a quick example of an environment-mapped torus running in tsGL!
Now that cubemaps are supported, it’s also possible to make reflective and refractive materials efficiently, so shader programs can be made much more interesting within the confines of the engine!
Another nifty feature is the addition of render textures! By essentially binding a “camera” object to a texture, it is possible to render the scene into that texture, instead of onto the screen! This texture can then be used like any other anywhere in the drawing process, which means it’s possible to do things like draw a realtime security camera monitor in the scene, or have a mirror with realtime reflections! This can get quite costly, so it is best used sparingly, but the addition of this feature opens the door to a wide variety of other cool effects!
With the addition of both cubemaps and render textures, I hope to get shadow-mapping working in the near future, which would allow objects to appropriately cast shadows when illuminated in the scene, which was previously infeasible!
And now, the boring stuff – HTML
The custom HTML tag system has been improved immensely, and now makes much more sense. Entity tags may now be nested to define object hierarchies, and arbitrary parameters can be provided as child-tags, rather than attributes. This generally makes the scene documents far more legible, and makes adding new features in the future much easier.
Here’s a “camera” object, for example.
<tsgl-property type=”number” name=”fov” value=”80″></tsgl-property>
<tsgl-property type=”number” name=”aspect” value=”1.6″></tsgl-property>
<tsgl-property type=”vector” name=”position” value=”0 2 0″></tsgl-property>
Previously, the camera parameters would have been crammed into a single tag’s attributes, making it much more difficult to read, and much more verbose. With the addition of tsgl-property tags, attributes of each scene entity can now be specified within the entity’s definition, so all of those nice editor features like code-folding can now be exploited!
This part isn’t exactly fun compared to the rendering tests earlier, but it certainly helps when attempting to define a scene, and add new features!
That’s all for now! In the meantime, you can check out the very messy and very unstable, tsGL on GitHub if you want to try it for yourself, or experiment with new features!