Wednesday, March 14, 2012

Description / Proposal

Improvements in GPU performance have made it possible to create visual effects that were formerly reserved for offline renderers. Two such developments that interest me are reflections and refractions. Intuitively, these effects are not particularly complicated. Reflection involves bouncing a ray around an object's surface normal and refraction involves changing the angle of a ray based on the refractive indexes of two media. Unfortunately, these visual effects can be very difficult to compute efficiently because they require many intersection tests for multiple rays. As a result, there has been a great deal of research lately into developing real-time solutions. As I discuss such techniques, I will be referring to reflections only because refractions are sufficiently similar.

One of the most common and earliest techniques for simulating reflections is to put a reflective object in the center of a cube map. A cube map is a six sided texture that, for conceptual purposes, is infinitely large. The fragment shader simply takes the eye vector and reflects it off of the fragment’s surface normal. Next, it finds the texture coordinate where the ray intersects the cube map and draws that color onto the fragment. This approach creates a mostly realistic visual effect, but it cannot reflect arbitrary objects in a dynamic scene. More info on reflection mapping here:

Another interesting approach uses billboard impostors to simulate reflected geometry. This technique involves projecting an object onto a texture and intersecting rays with that texture during the reflection process, akin to what we do in the cube map. This approach has obvious speed limitations, especially for scenes with numerous objects. More info on billboard impostors here:

My goal for this project is to simulate reflections and refractions between many different objects that move and deform. One novel approach that has minimal dependence on scene complexity and detail is screen space reflections (SSR). Although there are a few ways to achieve this effect, the most understandable is to do two separate render passes. First, render the scene with no reflections. Second, use depth and color information from the previous pass to determine the reflected colors. Interestingly, the second pass is accomplished with ray-tracing techniques. We convert the view-space reflection vector to screen space and advance the ray incrementally. For each step, compare the screen space depth with the existing depth from the previous render pass. If the depth of the reflected ray is less than the existing depth, then we take the color at that position and apply it to the original reflective fragment. One drawback to this technique is it cannot reflect geometry that is not visible in the screen. 

I'm excited to start work on this project because it's applicable to many different graphics programs I see myself working on in the future. As I learn more about SSR I will update this post to fix any inaccuracies. 

References (not all are relevant to this post):

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