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GLSL Core Tutorial – Creating a Shader

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The following figure shows the necessary steps to create a shader.

The first step is creating an object which will act as a shader container. The function available for this purpose returns a handle for the container.

The syntax for this function is as follows:

GLuint glCreateShader(GLenum shaderType);

Parameter:

  • shaderTypeGL_VERTEX_SHADER, GL_GEOMETRY_SHADER, GL_TESS_CONTROL_SHADER, GL_TESS_EVALUATION_SHADER, or GL_FRAGMENT_SHADER.

Return Value:

  • the shader handler

A shader program can be decomposed in many shader containers, each having some functions of the program. However, there can only be a main function for the set of each shader type in each single program.

The following step is to add some source code. The source code for a shader is a string array, although you can use a pointer to a single string.

The syntax of the function to set the source code for a shader is:

void glShaderSource(GLuint shader, int numOfStrings, const char **strings, int *lengthOfStrings);

Parameters:

  • shader – the handler to the shader.
  • numOfStrings – the number of strings in the array.
  • strings – the array of strings.
  • lengthOfStrings – an array with the length of each string, or NULL, meaning that the strings are NULL terminated.

Finally, the shader must be compiled. The function to achieve this is:

void glCompileShader(GLuint shader);

Parameters:

  • shader – the handler to the shader.

Considering the source code is defined as a string inside the application code itself then the following snippet could be used to setup a shader:

std::string source = "#version 330\n\
\
in vec4 Color;\n\
\
void main()\
{\
	gl_FragColor = Color;\
} ";

GLuint f;
const char * ff = source.c_str();

f = glCreateShader(GL_FRAGMENT_SHADER);
glShaderSource(f, 1, &ff, NULL);
glCompileShader(f);

Assuming the code is in a file, then the source code to achieve this could be:

char *vs;
GLuint v;

// get a shader handler
v = glCreateShader(GL_VERTEX_SHADER);
// read the shader source from a file
vs = textFileRead(vertexFileName);
// conversions to fit the next function
const char *vv = vs;
// pass the source text to GL
glShaderSource(v, 1, &vv,NULL);
// free the memory from the source text
free(vs);
// finally compile the shader
glCompileShader(v);

The above code uses an auxiliary function to read the text, textFileRead. The code for this function is now provided:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char *textFileRead(char *fn) {

	FILE *fp;
	char *content = NULL;

	int count=0;

	if (fn != NULL) {
		fp = fopen(fn,"rt");

		if (fp != NULL) {

      fseek(fp, 0, SEEK_END);
      count = ftell(fp);
      rewind(fp);

			if (count > 0) {
				content = (char *)malloc(sizeof(char) * (count+1));
				count = fread(content,sizeof(char),count,fp);
				content[count] = '\0';
			}
			fclose(fp);
		}
	}
	return content;
}

 

Prev: OpenGL Setup Next: Creating a Program
 

4 Responses to “GLSL Core Tutorial – Creating a Shader”

  1. Remember, sizeof(char) is defined always to be 1, no matter what the platform is (the C and C++ standards both define 1 byte to be the size of a char, and sizeof to return the number of bytes needed to hold a given type). You never need to use it, unless you’re doing it for documentation purposes (and even then, it reads like, “and then multiply by the number of bytes in a byte”).

  2. Typo: **lenghtOfStrings** – an array with the length of each string, or NULL, meaning that the strings are NULL terminated.”

    Thanks for the articles.

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