What are Shaders? - Villanova Computer Sciencemdamian/Past/csc4300sp15/notes/02-GLSL.pdf ·...
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1 CSC 4300 Shaders and GLSL Programs that run on the GPU instead of the CPU Optional in older OpenGL versions, but required in modern OpenGL Written in OpenGl Shading Language (GLSL) What are Shaders?
Transcript of What are Shaders? - Villanova Computer Sciencemdamian/Past/csc4300sp15/notes/02-GLSL.pdf ·...
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CSC 4300
Shaders and GLSL
q Programs that run on the GPU instead of the CPU q Optional in older OpenGL versions, but required in
modern OpenGL q Written in OpenGl Shading Language (GLSL)
What are Shaders?
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A Simplified OpenGL Pipeline
Vertex shader
Rasteriza.on & Interpola.on
Fragment shader Final pixels
(plus some fixed
transforma.on)
q Operates on individual vertices, one at a time q Has no knowledge of other vertices that make up a primitive q At a minimum, it calculates projected position of the input
vertex in screen space q Invoked once per vertex
Vertex Shader
Vertex shader
fixed
transform
(perspec(ve divide
and view
port
transforma(on)
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Understanding Coordinate Spaces
http://www.theamazingking.com/ogl-matrix.php
Vertex Transformations
vertex shader input
vertex shader output
position the model
position the camera
adjust the zoom
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Projection Transform
vertex shader output gl_Position!
q Here is the GLSL code for a simple vertex shader:
q Line 1: this shader is written in GLSL version 1.5 q Line 2: this shader takes a single vertex as input in variable vert!q gl_Position is a global vec4 variable that stores shader output q The input vertex is sent to the output without any change at all
Simplest Vertex Shader
#version 150!!in vec3 vert;!!void main() {! // does not alter the vertices at all! gl_Position = vec4(vert, 1);!}!
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From Vertices to Fragments q Note that the output of the vertex shader is not in screen
coordinates. This is because there is a fixed-function stage after the vertex shader and before the rasterizer
q We will discuss the details of this stage later in the semester
Vertex shader
Rasteriza.on & Interpola.on
Fragment shader Final pixels
fixed
transform
gl_Position!!
Built-in output variable gl_FragCoord!
!
Built-in input variable
q A fragment is basically a screen position (x,y), a depth value (z), plus all interpolated attributes (color) from previous stages
q Fragment shaders can access the fragment position (stored in read-only built-in variable gl_FragCoord), but cannot change it
Fragment Shader
Fragment shader
Rasteriza.on & Interpola.on
gl_FragCoord!!
Built-in input variable
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q Here is the GLSL code for a simple fragment shader:
q Line 1: this shader is written in GLSL version 1.5 q Line 2: declare an output variable that will hold the pixel color q Line 4: set the pixel color to white
q Note: Built-in variable gl_FragCoord stores current fragment position
Simplest Fragment Shader
#version 150!!out vec4 finalColor;!!void main() {! //set every drawn pixel to white! finalColor = vec4(1.0, 1.0, 1.0, 1.0);!}!
Passing Data Between Shaders
#version 150!!in vec4 vertColor;!out vec4 finalColor;!!void main() {! //set every drawn pixel to white! finalColor = vertColor;!}!
#version 150!!in vec3 vert;!out vec4 vertColor;!!void main() {! // does not alter the vertices at all! gl_Position = vec4(vert.x, vert.y, vert.z, 1);! vertColor = vec4(0.0, 0.5, 0.0, 1.0);!}!
Vertex shader
Fragment shader
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1. Modify the shaders in 01_project_skeleton (under resources) to change the output color to dark red, using the method shown on the previous slide.
Hands-On Session
2. Modify the vertex shader in 01_project_skeleton so that the triangle is upside down.
Hands-On Session
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3. Output the vertex position to the fragment shader and set the fragment's color equal to this vertex position (see how the color position values are interpolated across the triangle). Why is the bottom-left side of our triangle black?
Hands-On Session
4. Modify the fragment shader to bring the fragment’s color in the range (0, 1). The shader should still use the vertex position in deriving the fragment’s color. Here is one example:
Hands-On Session
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GLSL Variable Overview
q http://relativity.net.au/gaming/glsl/Variables.html
Required Reading
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Scalar types: float, int, bool! Vector types: vec2, vec3, vec4! ivec2, ivec3, ivec4! bvec2, bvec3, bvec4! C++ style constructors:
vec3 a = vec3(1.0, 2.0, 3.0);!
GLSL Data Types
1.02.03.0
!
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###
$
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Matrix types: mat2, mat3, mat4!
mat3 rotate30 = mat3(0.86, 0.5, 0.0, !! ! ! ! ! -0.5, 0.86, 0.0, !! ! ! ! ! 0.0, 0.0, 1);!!!!!mat3 scale2 = mat3(2);!
GLSL Data Types
0.86 −0.5 0.00.5 0.86 0.00.0 0.0 1
"
#
$$$
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&
'''
2.0 0.0 0.00.5 2.0 0.00.0 0.0 2.0
!
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§ Standard C/C++ arithmetic and logic operators § Operators overloaded for matrix and vector operations
mat4 m;!vec4 a, b, c;!!b = a*m;!c = m*a;!
GLSL Operators
For vectors can use [ ], xyzw, rgba or stpq!
Example: ! !vec3 v(1.0, 2.0, 3.0);!! !!!Then!! !v[1], v.y, v.g, v.t !
all refer to the same element 2.0
Components and Swizzling
1.02.03.0
!
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$
%
&&&
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For vectors can use [ ], xyzw, rgba or stpq!
Swizzling: !vec3 a, b, c;!!a.xy = vec2(2.3, 1.7);!!b.yx = a.xy;!!c.xyz = a.rrr;!
Swizzling Examples
2.31.7?
!
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1.72.3?
!
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2.32.32.3
!
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a b c
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