Thursday, March 29, 2012

Progress Report

Here is my progress report a couple weeks into the project:

I've been doing a lot of reading on frame buffers (FBO's). I found a nice tutorial about shadow mapping that uses FBO's and basically explains how they are constructed, written, and accessed. Although shadow mapping it not what I am doing for this project, there are several technical similarities. You can read the tutorial here: http://ogldev.atspace.co.uk/www/tutorial23/tutorial23.html .

Anyway, I now have a better sense of how I'll implement screen space reflections.

How to use FBO's:

1. Give each object a Material. This would include diffuse color, specular color, specular intensity, transparency, reflectivity, refractivity, and maybe more.
2. Create an FBO that stores depth and color information.
3. The first render call will write to the FBO. This will use my regular material shader but with reflections turned off (set through a unform buffer object). I might split the two render calls into two different shaders since the lighting computations are redundant the second time.
4. Render a second time but use color texture data from the FBO to determine reflected pixel colors.

How to calculate reflections in a shader:

1. Reflect the view vector off of the fragment's surface normal.
2. Reflected ray is marched at a pixel length interval across the screen until it hits the end of the window or collides with an object (explained in step 4).
3. Convert view vector to screen space by dividing the clip space value by its w component, followed by scaling by .5 and shifting by .5 (to get it into screen space coordinates for texture access).
4. If the sampled texture depth value falls between the old and the new ray depth values, there has been an intersection. Take the color at that position from the FBO's color texture and apply it to the original fragment.
5. Mix reflected color value with existing color value based on the object's reflectivity constant.

This is the high level breakdown of how I will implement this. I'm sure there is something I'm missing or wrong about, but as of now it seems like a pretty good approach that shouldn't take too long to at least get an initial working version.


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