3501 #4: Pixel and Vertex Shaders

Assignment 4: Shaders

Due date: Wednesday, Oct 20

This assignment is to get some practice with creating and modifying pixel and vertex shaders. You will use the vertex shader to make custom transformations, and the pixel shader to make custom lighting calculations. In particular, we will make a "wobble" effect in the vertex shader and a "haze" effect in the pixel shader.

Part 1: setup

Download the illumination demo from the 3501 site.
Run it to make sure it works before starting to modify it.
Read through the code, with particular attention to the shader itself and to the shader initialization in the Game.cs file.

We need two additional parameter for the shaders: wobbliness and haziness, both floats. We will also add a command to turn on and off the wobbling, which will be passed in as the boolean parameter doIwobble. Add all of these to the shader and bind them to EffectParameters in the main program.

Part 2: Wobbly Vertex Shader

You can do custom position calculations on a per-vertex basis. We will get the models to distort their shapes by moving their vertices according to a function.

After the output position has been transformed, increment its x component by this quantity: wobbliness*sin(5*time+real_pos.y/2). The magic numbers 2 and 5 are chosen to give a certain speed and size to the distortion -- you can experiment to find ones that you like better, if you are so inclined.

Set wobbliness to some value, say 5, in the main program. Run the program. You should see the distorted sphere.

Put the wobbling modification in an if statement, to be executed only when "doIwobble" is true.

Part 3: Distance Haze

A lighting effect we discussed but did not implement is a "dust haze" or "fog" where things become obscured as they get further away. Now, we will actually implement this in the pixel shader.

Compute the value exp(-haziness*d) where "d" is the z distance of the pixel. This is trickier than it sounds, because you do not have access to the position in the pixel shader. You will need to calculate the z distance in the vertex shader and add it to the VertexShaderOutput structure, to be interpolated and thus available on a per-pixel basis in the pixel shader.

Once you have the value, multiply it with the computed illumination value to get your hazy lighting. This will produce a lighting effect where the objects are darker the further they are from the eye.

Note that the "haziness" value will have to be quite small, around 0.05 or perhaps even smaller, in order for the haze to give a reasonable dimming. Experiment a bit to find a value that you like.

Part 4: User Control

Add controls over the parameters to the program. Have "H" and "J" increment and decrement haziness, respectively; have "W" and "E" increment and decrement wobbliness, respectively; and have the spacebar turn the wobbling on and off. Don't let any of these parameters go below zero. In the draw method, update the shader parameters with the main program's values (with the .SetValue method of your EffectParameter variables).

Part 5: bonus

For a bonus of up to +5%, implement a proper "fog" effect where the object fades into the fog instead of just becoming black. You can do this by computing the haziness attenuation as before, and then using the value to linearly interpolate between the object color and your fog color.

For a bonus of a further +5%, implement a channel-specific distance haze, where red is attenuated most, green a medium amount, and blue the least. This produces the familiar effect where distant mountains appear bluish.

Handing it in

Hand in your project using WebCT. The easiest thing to do is to create a single zipped folder and submit that, rather than all the individual files. It might take a while to upload, so be prepared to take a little break while the files are in transit.

Reference art

Two distorted spheres: