CODEWITHSUNDEEP
openglglslshader
DontCareBear
DontCareBear

Reputation: 834

how to pass a custom c++ matrix class to glUniformMatrix

is it possible to create a c++ custom matrix class such that we can pass instances of this class to glUniformMatrix? in other words passing instances of this class to the vertex shader.

i ask this question because when i did a computer graphics course we were given a code skeleton for opengl and glsl shader programming. in the code skeleton we had a custom c++ matrix class named mat4. this class contained lots of methods and user defined constructors. but still instances of mat4 used as parameters to glUniformMatrix and this instances were correctly converted to type mat4 in the vertex shader.

in the future i want to use GLM library to create an interface between opengl code to vertex/fragment shaders, but i want to understand the rules and semantics of what data types can be passed to glUniformMatrix, such that instances of this types will be correctly converted to uniform mat4 in the vertex shader.

i think that the key to this behavior is defining operators

operator GLfloat* (); 
operator const GLfloat* () const;

that return static_cast<const GLfloat*>(...) to the begining of the matrix data structure such that it will point to all the matrix in column wise fashion. is this in broad strokes how the GLM library works?

Upvotes: 3

Views: 3095

Answers (1)

derhass
derhass

Reputation: 45322

The various glUniformMatrx*fv() functions expect the matrix is a series of GLfloat values (16 for mat4), consecutively laid out in memory, so passing a pointer to a C/C++ one- or two-dimensional array of floats will work.

GLM uses the following two variants

glm::mat4 m;
glUniform4fv(location, 1, GL_FALSE, &m[0][0]);
// or
glUniform4fv(location, 1, GL_FALSE, glm::value_ptr(m));

They explicitely avoided to implement some mechanism to hide that operation, altough it would be easy to implement.

Upvotes: 3

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