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GLSL Core Tutorial – Data Types

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The following simple data types are available in GLSL:

  • float
  • double
  • bool
  • int
  • uint

These all behave like regular C types, apart from bool.

Vectors with 2,3 or 4 components are also available for each of the simple data types mentioned above. These are declared as:

  • vec{2,3,4} a vector of 2, 3, or 4, floats
  • dvec{2,3,4} vector of doubles
  • bvec{2,3,4} bool vector
  • ivec{2,3,4} vector of integers
  • uvec{2,3,4} vector of unsigned integers

Square matrices 2×2, 3×3 and 4×4 are provided, for floats and doubles, since they are heavily used in graphics. The respective data types are:

  • mat2, dmat2
  • mat3, dmat3
  • mat4, dmat4

There are also non-square matrices, also for float and doubles which have the generic form:

  • mat{2,3,4}x{2,3,4}
  • dmat{2,3,4}x{2,3,4}

If columns and rows, first and second number respectively, are equal then these are equivalent to the previous definition of square matrices.

A set of special types are available for texture access. These are called samplers and are required to access texture values, also known as texels.

Some of the most common data types for texture sampling are:

  • sampler1D – for 1D textures
  • sampler2D – for 2D textures
  • sampler3D – for 3D textures
  • samplerCube – for cube map textures
  • sampler2DShadow – for shadow maps

Atomic counters are a new feature in OpenGL 4 hardware. Have a look at the Atomic Counter Tutorial for more details on this.

In GLSL, arrays can be declared using the same syntax as in C. However, arrays can’t be initialized when declared. Accessing array elements is done as in C.

Structures are also allowed in GLSL. The syntax is the same as C.

struct dirlight {

	vec3 direction;
	vec3 color;
};

Variables

Declaring a simple variable is pretty much the same as in C, we can even initialize a variable when declaring it.

float a,b;		// two vector (yes, the comments are like in C)
int c = 2;		// c is initialized with 2
bool d = true;	// d is true

Declaring the other types of variables follows the same pattern. GLSL relies heavily on constructor for initialization and type casting. However, it adopts a relaxed policy regarding implicit conversion. A type can be implicitly converted to a more general type, for instance an int to a float.

float b = 2;		// implicit conversion
int a = 2;
float c = float(a); // also correct. c is 2.0

vec3 f;		// declaring f as a vec3
vec3 g = vec3(1.0,2.0,3.0); // declaring and initializing g

GLSL is pretty flexible when initializing variables using other variables. All that it requires is that you provide the necessary number of components. Look at the following examples.

vec2 a = vec2(1.0,2.0);
vec2 b = vec2(3.0,4.0);

vec4 c = vec4(a,b) // c = vec4(1.0,2.0,3.0,4.0);

vec2 g = vec2(1.0,2.0);

float h = 3.0;

vec3 j = vec3(g,h);

Matrices also follow this pattern. You have a wide variety of constructors for matrices. For instance the following constructors for initializing a matrix are available:

mat4 m = mat4(1.0) // initializing the diagonal of the matrix with 1.0

vec2 a = vec2(1.0,2.0);
vec2 b = vec2(3.0,4.0);

mat2 n = mat2(a,b); // matrices are assigned in column major order

mat2 k = mat2(1.0,0.0,1.0,0.0); // all elements are specified

The declaration and initialization of structures is demonstrated below:

struct dirlight {		// type definition
	vec3 direction;
	vec3 color;
};

dirlight d1;

dirlight d2 = dirlight(vec3(1.0,1.0,0.0),vec3(0.8,0.8,0.4));

In GLSL a few extras are provided to simplify our lives, and make the code a little bit clearer. Accessing a vector can be done using letters as well as standard C selectors.

vec4 a = vec4(1.0,2.0,3.0,4.0);

float posX = a.x;
float posY = a[1];

vec2 posXY = a.xy;

float depth = a.w

As shown in the previous code snippet, it is possible to use the letters x,y,z,w to access vectors components. If you’re talking about colors then r,g,b,a can be used. For texture coordinates the available selectors are s,t,p,q. Notice that, by convention, texture coordinates are often referred as s,t,r,q. However r is already being used as a selector for “red” in RGBA. Hence there was a need to find a different letter, and the lucky one was p.

Matrix selectors can take one or two arguments, for instance m[0], or m[2][3]. In the first case the first column is selected, whereas in the second a single element is selected.

As for structures the names of the elements of the structure can be used as in C, so assuming the structures described above the following line of code could be written:

d1.direction = vec3(1.0,1.0,1.0);

Constant Qualifier

A declaration of a local variable can also used the const keyword, as in

const int gravity = 9.8;

 

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