GLSL Fragment Shader not modifying output colour

Question

The shaders compile (I have confirmed this by purposefully making the GLSL code error). In addition, the program is created and linked correctly, as I have checked by printing the value of program and it was non-zero.

However, I am still faced with a blue window (from glClearColor), and not a red one, as I would expect due to the fragment shader I have created.

Code is below:

#include <iostream>

#include <GL/glew.h>
#include <SDL2/SDL.h>
#include <SDL2/SDL_opengl.h>

GLuint compile_shader(const char *shader, GLenum type)
{
  GLuint shader_id = glCreateShader(type);

  glShaderSource(shader_id, 1, &shader, NULL);
  glCompileShader(shader_id);
  GLint compiled;

  glGetShaderiv(shader_id, GL_COMPILE_STATUS, &compiled);

  if (!compiled) {
    int length;

    glGetShaderiv(shader_id, GL_INFO_LOG_LENGTH, &length);
    char* message = new char[length];
    glGetShaderInfoLog(shader_id, length, &length, message);

    std::cerr << "Error compiling shader: " << message << std::endl;;
    delete[] message;
    glDeleteShader(shader_id);

    return 0;
  }

  return shader_id;
}

GLuint link_program(GLuint vertex_shader, GLuint frag_shader)
{
  GLuint program = glCreateProgram();

  if (vertex_shader != 0)
    glAttachShader(program, vertex_shader);

  if (frag_shader != 0)
    glAttachShader(program, frag_shader);

  glLinkProgram(program);
  GLint linked;

  glGetProgramiv(program, GL_LINK_STATUS, &linked);

  if (!linked) {
    std::cerr << "Error linking program." << std::endl;
    glDetachShader(program, vertex_shader);
    glDetachShader(program, frag_shader);
    glDeleteProgram(program);

    return 0;
  }

  return program;
}

int main(void)
{
  SDL_Init(SDL_INIT_VIDEO);

  SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
  SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1);
  SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);

  SDL_Window *window = SDL_CreateWindow("Title", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 800, 600, SDL_WINDOW_OPENGL);

  SDL_GLContext context = SDL_GL_CreateContext(window);
  (void) context;

  glewInit();

 // const char *vert_shader_src = R"GLSL(
 //   #version 430
 //   in vec3 pos;
 //   void main() {
 //     gl_Position = vec4(pos, 1);
 //   }
 //   )GLSL";

  const char *frag_shader_src = R"GLSL(
    #version 130
    void main() {
      gl_FragColor = vec4(1, 0, 0, 1);
    }
  )GLSL";

  glClearColor(.0f, .3f, .7f, 1.f);

  GLuint frag_shader = compile_shader(frag_shader_src, GL_FRAGMENT_SHADER);
  GLuint program = link_program(0, frag_shader);

  SDL_Event event;
  int quit = 0;

  glUseProgram(program);

  while (!quit) {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    while (SDL_PollEvent(&event)) {
      switch (event.type) {
        case SDL_QUIT:
          quit = 1;
          break;
      }
    }

    SDL_GL_SwapWindow(window);
  }

  SDL_DestroyWindow(window);
  SDL_Quit();

  return 0;
}

Here is also my Makefile, in case this has something to do with the compilation process:

CPPFLAGS=-Wall -Wextra -Werror -pedantic -g -std=c++20 $(shell pkg-config --cflags sdl2 glew)
LIBS=$(shell pkg-config --libs sdl2 glew)

sdl_noise: sdl_noise.cpp
    clang++ $(CPPFLAGS) -o sdl_noise sdl_noise.cpp $(LIBS)

Where is the vertex shader located? A shader program consists of a vertex shader and a fragment shader. — Rabbid76, Mar 15 '23 at 16:44
The clear color can be changed with [`glClearColor`](https://registry.khronos.org/OpenGL-Refpages/gl4/html/glClearColor.xhtml), which must be called before `glClear` — Rabbid76, Mar 15 '23 at 16:47
And agreed, at the very least there also needs to be a vertex shader — AnalyticaL, Mar 15 '23 at 16:48
I suggest reading a good tutorial. e.g.: https://learnopengl.com/ — Rabbid76, Mar 15 '23 at 16:48
@AnalyticaL Ah I see, I was under the impression that I could have a fragment shader without a vertex shader, since in this case I just wanted to apply it to every pixel on the screen rather than specific vertices of a mesh etc. Thanks! — Basil, Mar 15 '23 at 16:52

score 2 · Accepted Answer · answered Mar 15 '23 at 16:47

2

You are not rendering anything in the code..

You just linked a shader, cleared the framebuffer and that is it.

The shader will only apply to stuff you actually render. Clearing the framebuffer does not count as rendering.

answered Mar 15 '23 at 16:47

AnalyticaL

501
2
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GLSL Fragment Shader not modifying output colour

1 Answers1