tinygl.c

tinygl.c

This is an example of a trivial OpenGL program with a pair of GLSL shaders. It uses OpenGL 4.1 and GLFW 3. Check out the GitHub project for background and instructions on how to build and run this program.

#define GLFW_INCLUDE_GLCOREARB
#include <GLFW/glfw3.h>
#include <stdlib.h>
#include <math.h>
#include <stdio.h>
#include <assert.h>

const double PI = 3.141592;
const unsigned int NVERTICES = 13;
const unsigned int NDIMENSIONS = 3;
const unsigned int BUFSIZE = 1024;  // For error logs.

¶

The Shaders

A shader is a small program that runs on the GPU as part of the graphics rendering pipeline. The graphics driver JITs shader programs from source code, which are passed in as a string from the application running on the CPU. In OpenGL, shader programs are written in GLSL.

Despite the name, shaders are not just for “shading” shapes to determine the color of their pixels. One kind of shader (the “fragment” shader in OpenGL terminology) does that, but there are several other kinds that define other aspects of the rendering pipeline (such as a “vertex” shader, which determines the position of each vertex in 3D space). Nothing really gets done at all on modern GPUs without shaders.

Shader Error Checker

Check for errors when compiling or linking the shader program. We need this 3 times: compiling the vertex shader, compiling the fragment (pixel) shader, and linking the two together into a complete OpenGL “program.”

typedef void (*GetLogFunc)(GLuint, GLsizei, GLsizei *, GLchar *);
typedef void (*GetParamFunc)(GLuint, GLenum, GLint *);
void shader_error_check(GLuint object, const char *kind,
        GetLogFunc getLog, GetParamFunc getParam, GLenum param) {

Get the error/warning log using either glGetShaderInfoLog or glGetProgramInfoLog (as getLog).

  GLchar log[BUFSIZE];
  GLsizei length;
  getLog(object, BUFSIZE, &length, log);
  if (length)
    fprintf(stderr, "%s log:\n%s", kind, log);

¶

Get the status flag using either glGetShaderiv with the GL_COMPILE_STATUS parameter, or glGetProgramiv with GL_LINK_STATUS.
```
  GLint status;
  getParam(object, param, &status);
  if (status == GL_FALSE)
    exit(1);
}
```
¶

Now we compile and link the GLSL shader source.
```
GLuint create_shader() {
```
¶

Vertex Shader

This is the program that runs on the GPU for every vertex of the shape we want to draw.
```
  GLuint vshader = glCreateShader(GL_VERTEX_SHADER);
  const char *vertex_shader =
```
¶

Shader programs need to specify the version of the language they’re targeting in the source code. “410” corresponds to OpenGL 4.1, which is from 2010.
```
    "#version 410\n"
```
¶

The in annotation indicates that this is an input from the CPU to the vertex shader. We provide this below via a buffer. Confusingly, this is a vec4 while the data we provide is only 3-dimensional; there is a 4th “W” dimension that has something to do with clipping.
```
    "in vec4 position;\n"
```
¶

The out annotation here indicates that this variable will be used to communicate from this vertex shader to the fragment shader.
```
    "out vec4 myPos;\n"

    "void main() {\n"
```
¶

Send output to the fragment shader.
```
    "  myPos = position;\n"
```
¶

Assigning to the special variable gl_Position (a member of the built-in “named block” gl_PerVertex) constitutes the output of a vertex shader.
```
    "  gl_Position = position;\n"
    "}\n"
  ;
```

Compile the vertex shader.

  glShaderSource(vshader, 1, &vertex_shader, 0);
  glCompileShader(vshader);
  shader_error_check(vshader, "vertex shader", glGetShaderInfoLog,
                     glGetShaderiv, GL_COMPILE_STATUS);

¶

Fragment (Pixel) Shader

This shader program runs per pixel in the rendered shape. The fragment shader is invoked many times per vertex shader. The vertex shader program can communicate with the fragment shader to “fan out” data from one stage to the next.
```
  GLuint fshader = glCreateShader(GL_FRAGMENT_SHADER);
  const char *fragment_shader =
    "#version 150\n"
```
¶

The uniform annotation indicates an input from the CPU to the fragment shader that is “global” for the invocation; it is not per-vertex or per-pixel. We provide this value below using a glUniform* call.
```
    "uniform float phase;\n"
```
¶

The in annotation here matches with the out annotation on the variable of the same name in the vertex shader.
```
    "in vec4 myPos;\n"
```

The out-annotated variable declared for a fragment shader is implicitly the color of the pixel.

    "out vec4 color;\n"

    "void main() {\n"
    "  float r2 = (myPos.x + 1.) * (myPos.x + 1.) +\n"
    "             (myPos.y + 1.) * (myPos.y + 1.);\n"
    "  color = vec4((myPos.x + 1.) / r2,\n"
    "               (myPos.y + 1.) / r2,\n"
    "               phase,\n"
    "               1.);\n"
    "}\n"
  ;

Compile the fragment shader.

  glShaderSource(fshader, 1, &fragment_shader, 0);
  glCompileShader(fshader);
  shader_error_check(fshader, "fragment shader", glGetShaderInfoLog,
                     glGetShaderiv, GL_COMPILE_STATUS);

Shader Program

Create a program that stitches the two shader stages together.

  GLuint shader_program = glCreateProgram();
  glAttachShader(shader_program, vshader);
  glDeleteShader(vshader);
  glAttachShader(shader_program, fshader);
  glDeleteShader(fshader);

Link the program so it’s ready to apply during drawing.

  glLinkProgram(shader_program);
  shader_error_check(shader_program, "program", glGetProgramInfoLog,
                     glGetProgramiv, GL_LINK_STATUS);
  return shader_program;
}

¶

Draw Stuff

Set the vertex positions of our shape according to the current time step. We call this inside the draw loop to make the shape animate.

void update_vertices(float *points, float t) {
  for (int i = 0; i < NVERTICES; ++i) {
    float *coords = points + NDIMENSIONS * i;
    coords[0] = cos(360. / (NVERTICES - 1) * PI / 180. * i + t);
    coords[1] = sin(360. / (NVERTICES - 1) * PI / 180. * i + t);
    coords[2] = 0.;
  }
}

int main(int argc, char **argv) {

Setup

Set up the OpenGL context and the GLFW window that contains it. We’ll request a reasonably modern version of OpenGL, >= 4.1.

  glfwInit();
  glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 4);
  glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 1);
  glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
  glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
  GLFWwindow* window = glfwCreateWindow(512, 512, "Look at Me!", NULL, NULL);
  glfwMakeContextCurrent(window);
  glfwSwapInterval(1);

Which OpenGL version did we actually get?

  printf("OpenGL version %s\n", glGetString(GL_VERSION));

¶

Shaders and Shader Variables
¶

Compile the shader program.
```
  GLuint program = create_shader();
```
¶

To communicate with the shader program, we need to get the “locations” (just IDs, really) of the shader-language variables based on their names. We will use these IDs to send data to the shader inside the draw loop.

First, the variable phase, for the fragment shader.

  GLuint loc_phase = glGetUniformLocation(program, "phase");
  assert(loc_phase != -1 && "could not find `phase` variable");

Then, the variable position, for the vertex shader.

  GLuint loc_position = glGetAttribLocation(program, "position");
  assert(loc_position != -1 && "could not find `position` variable");

¶

Vertex Position Buffer

We next need to set up a buffer to hold the positions of the vertices in the shape we want to draw. We actually need two regions of memory: one on the CPU (just an ordinary float[]) and one on the GPU. The GPU-side one is called a Vertex Buffer Object (VBO) and it is wrapped in a structure called a Vertex Array Object (VAO) for communication between the CPU and GPU.
¶

An array for the vertices of the shape we will to draw. We need 3 coordinates per point for a 3-dimensional space.
```
  float points[NVERTICES * NDIMENSIONS];
```
¶

Create an OpenGL Vertex Array Object (VAO), which contains a bundle of state to be passed with one draw call to the vertex shader.
```
  GLuint array;
  glGenVertexArrays(1, &array);
```
¶

Also create a Vertex Buffer Object (VBO), which (as the name implies) holds the actual data to be passed to the vertex shader. A VAO can have multiple VBOs, but we just use one here (for the position).
```
  GLuint buffer;
  glGenBuffers(1, &buffer);
```
¶

To set up the buffer, we “bind” to the “target” GL_ARRAY_BUFFER, which lets us use other calls to manipulate it.
```
  glBindBuffer(GL_ARRAY_BUFFER, buffer);
```
¶

Allocate space for the buffer and fill it with zeros. (GL_DYNAMIC_DRAW indicates that we may change the contents of this buffer later. This is just a performance hint; nothing will break if you change this to GL_STATIC_DRAW.)
```
    glBufferData(GL_ARRAY_BUFFER, sizeof(points), NULL, GL_DYNAMIC_DRAW);

    glBindVertexArray(array);
```
¶

Associate the position variable with our buffer object in the array object. This call implictly refers to the currently-bound array object and buffer object, and explicitly refers to the position variable in the shader program via its “location”. (A “vertex attribute” is just a fancy name for an in-annotated variable in a vertex shader.)
```
      glVertexAttribPointer(loc_position, NDIMENSIONS, GL_FLOAT,
                            GL_FALSE, 0, 0);
```

For some unknowable reason, you also have to “enable” the variable (“attribute”) in the array object to make it actually work.

      glEnableVertexAttribArray(loc_position);
    glBindVertexArray(0);  // Unbind.

  glBindBuffer(GL_ARRAY_BUFFER, 0);  // Unbind.

¶

Draw Loop
¶

Initialize the time to zero. We’ll update it on every trip through the loop.
```
  float t = 0;
```
¶

The main draw loop terminates when the user closes the window.
```
  while (!glfwWindowShouldClose(window)) {
```
¶

Poll for user input.
```
    glfwPollEvents();
```
¶

Update Phase Variable
¶

Assign to a shader “uniform” variable. A “uniform” is a value passed from the CPU to the GPU that is the same for all invocations (i.e., all vertices). Morally: phase = sin(4 * t);
```
    glUniform1f(loc_phase, sin(4 * t));
```
¶

Update Vertex Positions
¶

Position the shape by updating the points array.
```
    update_vertices(points, t);
```
¶

Set the contents of the position vertex list. This is a bit more complicated than the uniform above: we have to “bind” the buffer first so we can manipulate it. Morally: position = points;
```
    glBindBuffer(GL_ARRAY_BUFFER, buffer);
```

Copy the contents of our CPU-side points to the bound GPU buffer.

      glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(points), points);
    glBindBuffer(GL_ARRAY_BUFFER, 0);  // Unbind.

¶

Advance the time counter for the next frame.
```
    t += 0.01;
```
¶

Draw

Actually draw something!
¶

Clear the frame so we can draw to it.
```
    glClear(GL_COLOR_BUFFER_BIT);
```
¶

Use our shader program for the next draw call.
```
    glUseProgram(program);
```
¶

Use the VAO to communicate with the shaders.
```
    glBindVertexArray(array);
```

Draw!

      glDrawArrays(GL_TRIANGLE_FAN, 0, NVERTICES);
    glBindVertexArray(0);  // Unbind.

¶

Display the frame to the screen.
```
    glfwSwapBuffers(window);
  }
```
¶

Teardown

Tear down the windowing system and deallocate the OpenGL resources.

  glfwDestroyWindow(window);
  glDeleteProgram(program);
  glDeleteBuffers(1, &buffer);
  glDeleteVertexArrays(1, &array);
  glfwTerminate();
  return 0;
}

tinygl.c

The Shaders

Shader Error Checker

Vertex Shader

Fragment (Pixel) Shader

Shader Program

Draw Stuff

Setup

Shaders and Shader Variables

Vertex Position Buffer

Draw Loop

Update Phase Variable

Update Vertex Positions

Draw

Teardown