Shaders

Jeremy Nicholson

COMP30019: Graphics and Interaction05 Sep 2011

Jeremy Nicholson Shaders

Rendering Pipeline

(Foley Figure 14.41)

Jeremy Nicholson Shaders

Rendering Pipeline

(OpenGL Red Book, section 1)

Jeremy Nicholson Shaders

Rendering Pipeline

(Angel Figure 1.38)

Jeremy Nicholson Shaders

Rendering Pipeline

Vertex processing:

Vertex position:

co-ordinate transformations (model)projection transformations

Vertex colour:

Illumination model

Jeremy Nicholson Shaders

Rendering Pipeline

Clipping and primitive assembly:

Clipping:

Clipping volume (based on viewing frustum)projection transformations

Primitive assembly

Vertices → line segments, polygons, etc.

Jeremy Nicholson Shaders

Rendering Pipeline

Rasterisation:

Scan conversion of polygons

Line segment → fragments

Fragments: ”potential pixels”

Position and colour to be entered into frame buffer

Jeremy Nicholson Shaders

Rendering Pipeline

Fragment processing:

Hidden surface removal

Some fragments may not be visible because they are behindother fragments

Texturing, blending, etc.

Jeremy Nicholson Shaders

Shaders

Loosely speaking, programming on the GPU itself

Typically high-powered processor, lots of RAM

Parallel processing plays an important part

not massively paralleltypically only a couple of simple mathematical operationse.g. matrix multiplication of a single transformation matrix tomany vertices

Don’t typically do any “shading”

Jeremy Nicholson Shaders

Types of Shaders

Vertex Shader:

Input: set of primitives (vertices)

Output: set of primitives

Operate on vertex values and associated data

NormalsColourMaterial propertiesTexture?

Jeremy Nicholson Shaders

Types of Shaders

Geometry Shader:

Input: set of primitives

Output: set of primitives

Add/remove vertices or other primitives

Triangulation (why?)Hidden surface removal?Shadow volumes etc.

Jeremy Nicholson Shaders

Types of Shaders

Fragment Shader:

Input: set of fragments

Output: set of fragments

Apply operations to viewport pixels (primarily colour)

Texture mappingBump mappingPotentially lighting modelRasterisation/anti-aliasing?

Jeremy Nicholson Shaders

Types of Shaders

Pixel Shader:

Input: set of fragments

Output: set of pixel values

Actually rendering images to the display

Depth testing/occlusion/blendingHardware conversion: dithering, γ-correctionDouble buffering

Jeremy Nicholson Shaders

Shaders and the Rendering Pipeline

Vertex processing: Vertex Shader

Clipping and primitive assembly: Geometry Shader

Rasterisation: Fragment Shader

Fragment processing: Pixel Shader

(Loosely)

Jeremy Nicholson Shaders

Shaders and the Rendering Pipeline

Vertex Shader: ???

Geometry Shader: ???

Fragment Shader: ???

Pixel Shader: ???

Jeremy Nicholson Shaders

Shaders and the Rendering Pipeline

Vertex Shader: Modelling transform, Lighting, Viewing transform

Geometry Shader: Trivial accept/reject, some Lighting

Fragment Shader: Rasterisation

Pixel Shader: Display

(Typically, but Clipping and Map to 3D viewport are ”missing”)

Jeremy Nicholson Shaders

Shaders in OpenGL

Contrast with ”legacy” OpenGL

Gives you the power (and hassle) of specifying your ownmodelling/viewing operations, illumination model, shading model

Only Vertex/Geometry/Fragment Shaders implemented in OpenGL

And geometry only recently!

Manipulated using a pared-down programming language...

Jeremy Nicholson Shaders

GLSL: The OpenGL shading language

”GL slang”

Reference: Orange book

Not many in-builts

Some mathematical functions (e.g. Trigonometric)I/O partly limited: specific streams for vertex position, colour,etc.

Jeremy Nicholson Shaders

GLSL: The OpenGL shading language

Common key words:

in - input (from OpenGL or earlier in the Shader pipeline)

out - output (to next Shader in the pipeline, or occasionallyOpenGL)

uniform - ”constant” variable: same across many inputs(iterationsof Shader) — allowing preprocessing and optimisation

varying - actual variable: for intermediate calculation etc.

Jeremy Nicholson Shaders

Examples

Trivial Example(Angel 2.2)

Jeremy Nicholson Shaders

Examples

Illumination Model (in Vertex Shader)(Angel 5.3)

Jeremy Nicholson Shaders

Examples

Illumination Model (in Fragment Shader)(Angel 5.6)

Jeremy Nicholson Shaders