OpenGL with Shaders
OpenGL vs OpenGLES
• OpenGL built for desktop applications
• Fewer power/heat concerns
• OpenGLES created for embedded/
mobile applications
• Concerns about power/heat
• Has fewer features than OpenGL
Shaders
• Small arbitrary programs that
run on GPU
• Massively parallel
• Four kinds: vertex, geometry,
tessellation, fragment
• Now used for GPGPU
calculations, but we’re
focusing on the graphics
aspect!
Data Types
Allows for storage of:
• scalars: bool, int, uint, float,
double
• vectors: bvecn, ivecn, uvecn, vecn,
dvecn
• matrices: matnxm, matn
Note: matrices are always floating point
Functions
Users can define functions for greater flexibility
void method() { //code here }
Or use built-in functions:
sqrt, pow, abs, sin, step, length,
reflect, etc
http://www.shaderific.com/glsl-functions/
Flow Control
All the usual suspects:
• If-else/Switch
• For/While/Do-while (avoid!)
• Break/Return/Continue
• Discard (only in fragment shader)
Swizzling
Access vector components individually
vec4 a_vector;
a_vector.x + a_vector.y;
Any combination allowed: a_vector.xxyx;
Syntactic sugar masks: xyzw, rgba, stpq
Vertex Shader
• Runs in parallel on every vertex
• No access to triangles or other vertices
What can we use the vertex shader for?
Vertex Shader Uses
Per-vertex lighting
• Apply toon-shading or other NPR
techniques
Height-fields
• Adjust position of vertex based on function
or input data
Compute transforms
• Perform matrix calculations in shader
Transforming Vertices
gl_Position variable must be assigned for each
vertex
• vec4 (x, y, z, w)
• determines vertex transforms during shading
Vertex shader main method:
void main() {
gl_Position = MVP * vertex_position;
}
Providing Input to Shaders
Remember glVertexAttribPointer?
VAO tracks data between CPU and GPU
• Notifies what data to use and how to use it
• Knows whether data is coming or going
Qualifiers modify storage or behavior of
variables
Layout Qualifiers
Determines which buffer stores what values
Example:
layout(location = attribute index)
associates buffer to use with VAO index
(defined earlier)
Overrides glBindAttribLocation
Note: Currently not available in ELSL
Storage Qualifiers
in or out determines if assignment is
being inputted or outputted
in links into current shader
out links onto next shader stage
Expanded Layout Example
layout(location = 4) in vec3
position;
void main() {
gl_Position.xyz = position;
gl_Position.w = 1.0;
}
What is this doing?
Uniforms
Global GLSL variables
• Constant within the shader
• Same value for all verts/fragments
• Cannot be passed to in or out
Why might this be useful?
Specifying Uniforms
Can specify a variety (and number) of
scalars, vectors, and matrices:
glUniform3f
glUniform2i
glUniform4ui
glUniform3fv
glUniformMatrix4fv
Using Uniforms
uniform type variable_name;
Uniforms can be set using default initialization:
uniform float scale = 2.0;
Or pass in uniforms using glUniform:
GLint scale_location =
glGetUniformLocation(program_id, “scale”);
glUniform1f(scale_location, 2.0);
Expanded Uniform Example
layout(location = 0) in vec4
vertex_position;
uniform mat4 MVP;
void main() {
gl_Position = MVP *
vertex_position;
}
Vertex Shader Inputs and Outputs
Built-in attributes from old OpenGL (but still useful conceptually)
Processing Vertices
Must assemble a group of verts into a polygon
Primitives can be: points, lines, triangles,
patches
Assembly defined by glDrawArrays
Okay, Back to the Pipeline…
Tessellation Shader
Controls amount of tessellation per patch
• Lower poly models can be subdivided
into higher resolution models
• Values calculated for generated vertices
• Optional
Three Parts of Tessellation
Tessellation Uses
• GPU-based subdivision of geometry
• Can also perform smoothing algorithms
• Level of detail (LOD) controllable within
the shader pipeline
Geometry Shader
Takes primitives and outputs multiple
primitives
• Not optimized for subdivision
(tessellation shader’s job)
• Ability to work on entire primitive
• Optional
Geometry Input Primitives
Primitives input:
• points
• lines
• lines_adjacency
• triangles
• triangles_adjacency
Input type set in layout
Geometry Output Primitives
Zero or more primitives output:
• points
• line_strip
• triangle_strip
emitVertex() adds vertex to outputted
primitive
EndPrimitive() generates primitive
Geometry Shader Uses
Shader can be invoked multiple times for
multiple passes within geometry shader
• Layered rendering (dynamic cubemaps,
etc)
Shader can generate a lot of primitive data
output (and variety) from limited input
• Reduces CPU to GPU bandwidth
Fragment Shader
Many fragments per triangle…
GPU automatically applies
barycentric interpolation
UV coords, normals,
colors, …
Fragment Shader
Runs in parallel on each fragment (pixel)
• rasterization: one triangle -> many
fragments
Writes color and depth for one pixel (only)
Final texturing/coloring of the pixels
Fragment Shader Example
out vec4 frag_color;
void main() {
frag_color = vec4(1.0, 0.0,
0.0, 1.0);
}
Fragment Shader Uses
Per-fragment lighting
• Compute lighting on each fragment
rather than each vertex
Bump-mapping
Environment-mapping
Fragment Shader In and Outs
Open GL Tutorial
http://www.opengl-tutorial.org/beginners-
tutorials/tutorial-3-matrices/
http://www.opengl-tutorial.org/beginners-
tutorials/tutorial-4-a-colored-cube/
Using Shaders
Must compile and link shaders to make
executable:
Shaders GPU CPU
Inputs Vertex VBOs
Attributes
position vertPos[]
normal vertNormals[]
1
2
3 Uniforms
Uniforms
Global view
Memory lightPos
view
lightPos
1
2
3
Shaders GPU CPU
Inputs Vertex VBOs
Attributes
position vertPos[]
normal vertNormals[]
1
2
3 Uniforms
Uniforms
Global view
Memory lightPos
view
lightPos
1
2
3 when shader is compiled
Shaders GPU CPU
Inputs Vertex VBOs
Attributes
position vertPos[]
normal vertNormals[]
1
2
3 Uniforms
Uniforms
Global view
Memory lightPos
view
lightPos
1
2
3 glBindAttribLocation()
Shaders GPU CPU
Inputs Vertex VBOs
Attributes
position vertPos[]
normal vertNormals[]
1
2
3 Uniforms
Uniforms
Global view (2)
Memory lightPos (1)
view
lightPos
1
2
3 glGetUniformLocation()
Shaders GPU CPU
Inputs Vertex VBOs
Attributes
position vertPos[]
normal vertNormals[]
1
2
3 Uniforms
Uniforms
Global view (2)
Memory lightPos (1)
view
lightPos
1 at render time:
2 glVertexAttribPointer()
3
Shaders GPU CPU
Inputs Vertex VBOs
Attributes
position vertPos[]
normal vertNormals[]
1
2
3 Uniforms
Uniforms
Global view (2)
Memory lightPos (1)
view
lightPos
1 at render time:
2 glVertexAttribPointer()
3 glUniform**()
Shaders GPU CPU
Inputs Vertex VBOs
Attributes
position vertPos[]
normal vertNormals[]
1
2
3 Uniforms
Uniforms
Global view (2)
Memory lightPos (1)
view
lightPos
1
2
3 VAOs store the VBO state
Switching VAOs, Shaders, VBOs
Possible to switch between VAOs, shaders
and VBOs at any time
1. Set up all resources at beginning of
program
2. Bind or unbind as necessary within the
main loop (batch as much as possible)
3. Only update uniforms if values have
changed
GLEW
OpenGL Extension Wrangler
Library for loading pointers at runtime into
OpenGL core functions and extensions
Should work on all platforms
GLFW
OpenGL Framework
Handles windows, contexts, and receives
input and events
Should work on all platforms
Textures
Create and bind texture within application using
glGenTextures and glBindTexture
Take UV coordinates:
in vec2 uvcoords;
Return associated RGBA value:
texture(sampler, uvcoords);
Samplers
Allows reading from a particular texture and
fetching texels
Textures have type depending on purpose:
• sampler1D, sampler2D, samplerCube, etc
Sampling parameters determine how texture
is accessed
Shaders in Art Pipelines
Substance Designer
Shaders in Art Pipelines
Unreal Engine 4 Materials
Additional Resources
http://www.opengl-tutorial.org/beginners-
tutorials/tutorial-5-a-textured-cube/
https://www.shadertoy.com/
