Unit 5: Graphics Pipeline

Rasterization

Rasterizations: The process of taking a triangle and figuring out which pixels it covers

• It is done by the GPU

• Can be accelerated using some tricks other than ray tracing

• You can generate multiple fragment in the same pixel

 

Difference between GP Rasterization and Ray Casting

Scan conversion

Given a triangle’s vertices, figure out which pixels to turn on:
    Compute illumination values to fill in pixels within the primitive
    At each pixel, keep track of the closest primitive (z-buffer)
        Only overwrite if triangle being drawn is closer than the previous triangle in that pixel

Rasterization Limitation

Ray Tracing

Modern Interactive Graphics Pipeline

• It’s an organization of all algorithmic operations in one flow

• Steps (high-level):
• Project vertices to 2D (image)
• Rasterize triangle: find which pixels should be lit
• For each pixel, test 3 edge equations.
• if all pass, draw pixel (generate fragment)
• Compute per-pixel color
• Test visibility (Z-buffer), update frame buffer color
• Store minimum distance to camera for each pixel in “Z-buffer”
• ~same as in ray casting!

if newz < zbuffer[x,y]
	zbuffer[x,y]=new_z
	framebuffer[x,y]=new_color

• We use a technique called double buffering: display one buffer (front buffer) and render the other (back buffer), and swap these going back and forth (this is handled in OpenGL using glfwSwapBuffers function.



 

• Rasterization Pseudocode

For each triangle
	transform into eye space
	(perform projection)
	setup 3 edge equations
	for each pixel x,y
		if passes all edge equations
			compute z
			if z<zbuffer[x,y]
				zbuffer[x,y]=z
				framebuffer[x,y]=shade()

Projection

• Triangles geometry is preserved in 2D and 3D
• It looks like a triangle in both coordinate systems and are kept mapped to triangles

Ray Casting vs Rasterization