COMP 175 | COMPUTER GRAPHICS
Lecture 13:
OpenGL Shading Language (GLSL)
COMP 175: Computer Graphics
April 12, 2016
Remco Chang
13 – GLSL
1/XX
COMP 175 | COMPUTER GRAPHICS
Motivation
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Last week, we discussed the many of the new “tricks” in
Graphics require low-level access to the Graphics pipeline
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For example, using textures, which is a pain in regular
OpenGL.
Remco Chang
13 – GLSL
2/XX
COMP 175 | COMPUTER GRAPHICS
Graphics History
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Software Rendering
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Fixed-Function
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Immediate Mode (What we have used)
Programmable Pipeline (~2001)
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Old-fashioned way (Use ray casting)
GL Shading Language (What we will learn)
CUDA/OpenCL (GPGPU) (~2008)
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General Purpose GPU Programming
Remco Chang
13 – GLSL
3/XX
COMP 175 | COMPUTER GRAPHICS
Ray Casting (Software Rendering)
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Wolfenstein
Quake 1
Remco Chang
13 – GLSL
4/XX
COMP 175 | COMPUTER GRAPHICS
Fixed-Function
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Quake 3
Half-Life
Remco Chang
13 – GLSL
5/XX
COMP 175 | COMPUTER GRAPHICS
Programmable Pipeline (Shaders)
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Doom 3
Far Cry
Remco Chang
13 – GLSL
6/XX
COMP 175 | COMPUTER GRAPHICS
GPGPU
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General Purpose Graphics Processing Unit
Utilize graphics card for computation
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Do work other than graphics
Remco Chang
13 – GLSL
7/XX
COMP 175 | COMPUTER GRAPHICS
Old Fixed-Function
Remco Chang
13 – GLSL
8/XX
COMP 175 | COMPUTER GRAPHICS
Programmable Pipeline
Remco Chang
13 – GLSL
9/XX
COMP 175 | COMPUTER GRAPHICS
10
4/15/2014
Accessing Programmable
Pipeline
Introducing Shaders!
Remco Chang
13 – GLSL
10/XX
COMP 175 | COMPUTER GRAPHICS
CPU + GPU
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CPU: Typical C++ code
GPU: OpenGL Shading Language (GLSL)
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CPU passes data to GPU
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Some processing happens on the CPU:
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Get mouse movement
Load and parse data from disk
Some processing happens on the GPU:
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Transform point from Object to World space
Solve lighting equation
Render
Remco Chang
13 – GLSL
11/XX
COMP 175 | COMPUTER GRAPHICS
Types of Shaders
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Most common:
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Vertex
Fragment (Pixel)
Newer:
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Geometry
Tessellation
Compute
Remco Chang
13 – GLSL
12/XX
COMP 175 | COMPUTER GRAPHICS
Two Things to Learn:
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OpenGL Shading Language (GLSL)
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Syntax
Sequence and processes
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Communication between CPU and GPU
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Vertex Shader
Pixel Shader
What type of information can be passed
How to pass information
Note that you also need to know how to load a shader
program, but that’s “magic incantation” that (mostly)
stays the same every time.
Remco Chang
13 – GLSL
13/XX
COMP 175 | COMPUTER GRAPHICS
Remco Chang
13 – GLSL
14/XX
COMP 175 | COMPUTER GRAPHICS
Interfacing with the GPU
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Modern OpenGL
Mobile/Web Programming
Direct3D
Compute Languages
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Stream, OpenCL, CUDA
Future
Remco Chang
13 – GLSL
15/XX
COMP 175 | COMPUTER GRAPHICS
Client/Server Architecture
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Think of like a network
Client = the CPU
Server = the GPU
The less they talk, the better
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Less waiting for memory to transfer
Bandwidth is almost always the bottleneck
Send data over once, and have it ready to go
Remco Chang
13 – GLSL
16/XX
COMP 175 | COMPUTER GRAPHICS
Optimizations Available
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Vertex Array
Display Lists
Vertex Buffer Objects
Pixel Buffers
Frame Buffer Objects
Michael Shah
Remco Chang
13 – GLSL
17/XX
COMP 175 | COMPUTER GRAPHICS
Immediate Mode
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Every ‘gl’ command is being sent to the GPU every
iteration.
Very inefficient
Think of it like having an interpreter versus a
compiler for C++
Michael Shah
Remco Chang
13 – GLSL
18/XX
COMP 175 | COMPUTER GRAPHICS
Vertex Array
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No more glBegin/glEnd
Store Vertices, indices, normals, colors, texture
coordinates in an array
Minimizes OpenGL calls on Client (CPU)
Data still being passed from Client to Server every
frame
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A little better because it’s all at once
Stylistically, much cleaner code as well
Michael Shah
Remco Chang
13 – GLSL
19/XX
COMP 175 | COMPUTER GRAPHICS
20
4/22/2014
Display List
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Compiled geometry
Send it to the Server(GPU) once, and it is stored in
memory on Server
Geometry cannot be updated, without deleting
display list.
someGeometryDisplayList = glGenLists(1);
glNewList(someGeometryDisplayList,GL_COMPILE);
drawShape();
glEndList();
Michael Shah
Remco Chang
13 – GLSL
20/XX
COMP 175 | COMPUTER GRAPHICS
21
4/22/2014
Vertex Buffer Object (VBO)
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Improves upon Vertex Array
We can store data on the Server (GPU)
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Just like display lists!
We get a pointer into the Server (GPU) so that we
can update it!
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As good as display lists!
Because we have a pointer to the data, multiple clients
can access data.
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Multiple instances of a program can access data
Michael Shah
Remco Chang
13 – GLSL
21/XX
COMP 175 | COMPUTER GRAPHICS
Modifiers to Variables Uniform/Varying/Attribute
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Uniform – Read only data passed into vertex/fragment
shaders
Attributes – Input value which change every vertex.
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Read only – Used only in vertex shader
Depending on GLSL version:
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GLSL <3.0: Varying – Pass data from vertex to fragment
shader.
GLSL >=3.0: in / out – in is the variable being passed in, out is
the variable being passed out
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Read-only in fragment shader.
Read/Write in Vertex Shader
Remco Chang
13 – GLSL
22/XX
COMP 175 | COMPUTER GRAPHICS
OpenGL Shading Language
Remco Chang
13 – GLSL
23/XX
COMP 175 | COMPUTER GRAPHICS
GLSL
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C-Like Language
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Made for GPU Architecture
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Because GPU’s are different than CPU’s
Still Cross Platform
Debugging – Still as difficult
Domain Specific Language
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Compiled at runtime
Remco Chang
13 – GLSL
24/XX
COMP 175 | COMPUTER GRAPHICS
GLSL Primitives
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float, int, bool
Example:
float f = 10.0;
or
float f = float(10);
(Casting not allowed, choose your type wisely!)
Remco Chang
13 – GLSL
25/XX
COMP 175 | COMPUTER GRAPHICS
GLSL Types
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Vector: vec2, vec3, vec4
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Overloaded to take in each type
Example: vec4 temp= vec4(1.0, 1.0, 1.0, 1.0)
Matrix: mat2, mat3, mat4
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Example: glModelViewMatrix is a mat4 type
Access first column with: mat[0]
Access second column with: mat[1]
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Example: mat[1] = vec3(1.0, 1.0, 1.0);
Access individual element: mat[2][1];
Remco Chang
13 – GLSL
26/XX
COMP 175 | COMPUTER GRAPHICS
Some more examples
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Pass in vec3 into a vec4
vec3 color = vec3(1.0,0.5,0.25)
vec4 temp = vec4(color, 1.0)
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Get components of vector
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color.x = 1.0; or color[0] = 1.0
color.y = 1.0; or color[1] = 1.0;
Remco Chang
13 – GLSL
27/XX
COMP 175 | COMPUTER GRAPHICS
Swizzling [1]
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Swizzling is the ability to rearrange a vectors
components
vec2 pos = temp.xy; // retrieve x and y component,
or any of xyzw
vec2 pos = temp.yz;
vec4 = color.agbr; // opposite of rgba
Remco Chang
13 – GLSL
28/XX
COMP 175 | COMPUTER GRAPHICS
More GLSL Keywords [1][2]
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Note that some of these default keywords might not be there depending on
GLSL version number (for example: gl_ModelViewMatrix isn’t there any more)
Remco Chang
13 – GLSL
29/XX
COMP 175 | COMPUTER GRAPHICS
Write a Simple Shader
Basic Vertex and Fragment Shader
Remco Chang
13 – GLSL
30/XX
COMP 175 | COMPUTER GRAPHICS
Writing a Shader
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Writing a shader, compiling a shader, and loading a
shader
Compile at runtime
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So we do not need to recompile entire source, if we only
modify shader.
Remco Chang
13 – GLSL
31/XX
COMP 175 | COMPUTER GRAPHICS
Remco Chang
13 – GLSL
32/XX
COMP 175 | COMPUTER GRAPHICS
Simple Vertex Shader
Remco Chang
13 – GLSL
33/XX
COMP 175 | COMPUTER GRAPHICS
Simple Fragment Shader
Remco Chang
13 – GLSL
34/XX
COMP 175 | COMPUTER GRAPHICS
The cpp file
Remco Chang
13 – GLSL
35/XX
COMP 175 | COMPUTER GRAPHICS
Drawing with immediate mode for now
Remco Chang
13 – GLSL
36/XX
COMP 175 | COMPUTER GRAPHICS
Final Image
Remco Chang
13 – GLSL
37/XX