hi

WTF

is a

GPU

@omershapira

Raytracing

Accurate

RayTracing

• Striaghtforward simulation
• Mathematically correct
• Can approximate physical phenomena
• Is really, really slow
• Can't currently be used for real time (one day it will).

GPUs Are Fast

CPU

Like 4*

Apex of technology

Like 8GB*

Entire program at once

GPU

Like 400*

Pretty much a microbe

Like 1GB*

One element (pixel, vertex etc) at once

Cores

Complexity

RAM

​Program

* For computers in 2014

How pixels are put on screens

Vertex Shader

Small program that processes each vertex in the model geometry.

Can do:

• Scaling/Rotation/Translation
• Move polygons to screen space
• Read texture and geometry info
• Paint vertices

//Bad vertex shader

//set vertex color to magenta
gl_FrontColor =  vec4(1,0,1,1);
//set the UV coordinate
gl_TexCoord[0] = gl_MultiTexCoord0;
//set the XYZ coordinate
gl_Position = modelViewProjectionMatrix * gl_Vertex;

Vertex Shader

Example: Normal Extrusion

gl_Position.xyz += normal * ExtrusionAmount;

Images (c) Unity Technologies

Fragment Shader

Paints pixel components (fragments) in screen space.

Can do:

• Read textures and (poor) geometry info
• Move nothing
• Paint every single pixel on the screen

//read the UV coords from a texture
vec4 img = texture2D(texture, uv);

//premultiply alpha
img.rgb *= img.a;

//assign to the output fragment
gl_FragCoord = img

Fragment Shader

Example: Discarding Pixels

Example: Normals To Colors

gl_FragColor *= abs(normal);

Images (c) Unity Technologies

float value= frac((pos.y + pos.z*.1) * 5);
if(value < 0.5) {
   discard;
}

Tesselation

The tesselator determines where tesselation needs to occur and:

• Subdivides iffy triangles to less iffy triangles
• Attemps to reduce curvature
• Avoids large triangles in screen space

The Z-Buffer Pipeline

1. Buffer VBOs
2. Load textures
3. Bind a specific shader program (vert + tess + frag)
4. Draw calls:
   1. Vertex shader, then
   2. Tesselation shader, then
   3. Rasterizer, then
   4. Fragment shader
5. After all draw calls, Depth Sorting.

Depth Sorting

This is easy in Raytrcing

but hard in Realtime

Realtime Artists worry about:

• Number of draw calls
• Number of triangles / vertices on screen
• Time it takes shaders to run (shaders can time out)
• Number of transparent / blended layers
• Scripts that may clog the pipeline
• WHY IS EVERYTHING AROUND ME ON FIRE

Non-RT Artists Worry about:

• Occasionally, too many photons make the render long
• That cyan is physically accurate, but isn't exactly my cup of Kombucha

Lighting

Expensive: Deferred Lighting

Diffuse Pass

Depth Pass

Normal Pass

Final Composite

Lighting

Cheaper: Forward 

Calculate all of the color information from a list of lights, one after another, directly on the material.

Lighting

Cheapest: Baked

Baked lighting can get raytracer-quality textures for elements that don't change.

Gears of War

Shadows

Expensive: ShadowMapping

Pretend each light source is a camera, and paint that "camera"'s screenspace with light. 

The remaining dark pixels are the shadows.

Expensive because it requires multiple geo->screen renders.

Shadows

Better: Blob shadows, Cookies

Create a "Dark light" from a directional source with a small footprint, and add tags to specific materials affected.

Images (c) Unity Technologies

Shadows

Cheap: Baked

Raytrace the whole thing.

Far Cry 2

Animation

Spritesheets are still good.  Sometimes videos are too.

Braid

Animation

Principles

Game animation should be:

• Looping
• Stateful (run->walk etc)
• Centered around a pivot

Tools:

• GPUs support skinned meshes
• Bones are native to most programs
• Cloths are not

Procedural

If it:

• Has too many states that don't really matter
• Acting kinda predictably, but hard to keyframe
• Depends on exact interaction

Maybe it's easier to do in code.

Boids

Postprocessing

Glow

Unreal Engine 4

Postprocessing

Lens Distortion

Unreal Engine 3

Postprocessing

• No cheap way for pixels to know about their neighbors.
• No cheap way to oversample.
• Involves copying every single pixel to a new buffer. Expensive.

Antialiasing

• In advanced cards, happens automatically in pixel space.
• Never happens in geometry space. If a triangle is too small, it's either in or out.

MIP Maps

• The GPU solution for seamless zooming
• Each MIP level is 1/2 the size of the previous one
• Takes another 1/3 of the image space
• Precomputed when loading the texture
• Relatively cheap to read

Faking it

• Flares as textures
• Glows as textures
• Noise as textures
• You get the deal
• Sometimes, all of the above can be generative, but sparingly

Faking it

Volumetric effects can be faked with use of the depth buffer

Inigo Quilez

Faking IT

Screen Space Ambient Occlusion

Splinter Cell