OpenGL Shader Language (GLSL)

Part of OpenGL since OpenGL 2.0 (i.e. available in compatibility profile)

OpenGL API introduces new functions to deal with shaders

• Compile, link, install, feed data
• Shaders can easily obtain OpenGL states

#version 450 core /* version number profile */

Syntax

Primitives

• float
• double
• int
• uint
• bool
• _vec# (dvec, ivec, uvec, bvec. by default vec is for floats)
• _mat# (only dmat)

Constructors

• vec3 rgb = vec3(vec4(1.0, 0.5, 0.5, 0.0))
• vec4 rgba = vec4(rgb, 1.0)
• vec3 white = vec3(1.0)
• mat3(4.0) $ = \left[\begin{matrix} 4.0 & 0.0 & 0.0 \ 0.0 & 4.0 & 0.0 \ 0.0 & 0.0 & 4.0 \ \end{matrix} \right]$​

Matrices are specified in column-major order.

Accessors

mat4 m = mat4(2.0);
vec4 zVec = m[2];
float yScale = m[1][1]; // or m[1].y

Swizzling

vec3 red3 = color.rrr; // multiplying
color = color.abgr; // reordering

Arrays

float coeff[3] = float[3](2.38, 3.14, 0.0);

for (int i = 0; i < coeff.length(); i++)
    coeff[i] *= 2.0;

Structures

struct Particle {
	float lifetime;
	vec3 position;
	vec3 velocity;
}
Particle p = (1.0, vec3(1.0, 0.0, 0.0), vec3(0.0, 1.0, 1.0));
pos = p.position;

Storage Qualifiers

• const read only variable
• in, out
• uniformvalue is passed to shader from application and is constant across a single primitive (e.g. one triangle)
• buffer read-write memory shared with application
• shared (compute shaders only)

How to pass uniforms:

GLint uniformLocations[n];
uniformLoations[0] = glGetUniformLocation(CurrShaderProgObj, "NameOfMatrixUniform");
uniformLoations[1] = glGetUniformLocation(CurrShaderProgObj, "NameOfFloatVecUniform");
...

glUniformMatrix4fv(uniformLocations[0], 1, isTranspose, &matrix[0][0]);
glUniform4fv(uniformLocations[1], 1, &vector[0]);

Overloaded Operators

important: matrix vector multiplication

vec3 v, u;
mat3 m;

u = v * m; // u.x = dot(v, m[0]), u.y = dot(v, m[1]), u.z = dot(v, m[2])
mat3 n;
n = m * v; // usual Mv matrix * vector

Control flow

• if-else, switch
• for, while, do-while
• break, continue, return
• discard: discards a fragment

No recursion allowed.

Return Types

• void
• GLSL data type
• struct
• array (size must be explicitly specified)

GLSL has no concept of pointer/reference. Functions are thus called by value-return (copied in/out, read-only/read-write).

Built-in Variables

Vertex Shader

in int gl_VertexID;
in int gl_InstanceID;

out gl_PerVertex { // this is not a struct!
	vec4 gl_Position; // most important. is the *clip space position*
    float gl_PointSize;
    float gl_ClipDistance;
}

Fragment Shader

in vec4 gl_FragCoord; // window space coordinates
// z is the z-depth value.
// w is -z_e (z_e is the z coordinate of the fragment in eye space)
in bool gl_FrontFacing;

out float gl_FragDepth; // default: gl_FragCoord.z