OpenGL with SDL, what am I doing wrong? - c

Question

I'm trying to display simple white cube in OpenGL with SDL. I've setup my VBO and IBO like this:
GLfloat vertexData[] =
{
-0.5f, -0.5f, -0.5f, // bot, left, back
-0.5f, 0.5f, -0.5f, // top, left, back
-0.5f, -0.5f, 0.5f, // bot, left, front
-0.5f, 0.5f, 0.5f, // top, left, front
0.5f, -0.5f, -0.5f, // bot, right, back
0.5f, -0.5f, 0.5f, // bot, right, front
0.5f, 0.5f, -0.5f, // top, right, back
0.5f, 0.5f, 0.5f // top, right, front
};
GLint indexData[] =
{
//back
0, 1, 6,
0, 5, 6,
//left
0, 2, 3,
0, 1, 3,
//right
4, 5, 7,
4, 6, 7,
//bot
0, 4, 5,
0, 2, 5,
//top
1, 3, 7,
1, 6, 7,
//front
2, 3, 7,
2, 5, 7
};
However, it looks like this:
What am I doing wrong?
Edit:
My init functions
bool App::OpenGLInit()
{
programID = glCreateProgram();
GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
std::string vertexString = getShaderFromFile("src/shader.vert");
const GLchar * vertexShaderSource = vertexString.c_str();
glShaderSource(vertexShader, 1, (const GLchar **)&vertexShaderSource, NULL);
glCompileShader(vertexShader);
glAttachShader(programID, vertexShader);
GLuint fragmentShader = glCreateShader( GL_FRAGMENT_SHADER );
//Get fragment source
std::string fragmentString = getShaderFromFile("src/shader.frag");
const GLchar* fragmentShaderSource = fragmentString.c_str();
//Set fragment source
glShaderSource( fragmentShader, 1, (const GLchar **)&fragmentShaderSource, NULL );
//Compile fragment source
glCompileShader( fragmentShader );
glAttachShader(programID, fragmentShader );
glLinkProgram(programID);
return true;
}
bool App::OnInit()
{
SDL_GL_SetAttribute( SDL_GL_CONTEXT_MAJOR_VERSION, 3 );
SDL_GL_SetAttribute( SDL_GL_CONTEXT_MINOR_VERSION, 2 );
SDL_GL_SetAttribute( SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE );
if(SDL_Init(SDL_INIT_EVERYTHING) < 0) {
return false;
}
if((screen = SDL_CreateWindow("Color Wars", 100, 100, 1024, 600, SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN)) == NULL) {
return false;
}
if ((gl_context = SDL_GL_CreateContext(screen)) == NULL)
{
return false;
}
if((renderer = SDL_CreateRenderer(screen, -1, 0)) == NULL)
{
return false;
}
if (SDL_GL_SetSwapInterval(1) < 0)
return false;
glewExperimental = GL_TRUE;
if (glewInit() != GLEW_OK)
{
return false;
}
if (!OpenGLInit())
return false;
game_screen = new GameScreen();
game_screen->Init(programID);
return true;
}
my Render functions:
void App::OnRender()
{
glClear( GL_COLOR_BUFFER_BIT );
glUseProgram( programID );
game_screen->Render(renderer);
glUseProgram( NULL );
SDL_GL_SwapWindow(screen);
}
void GameScreen::Render(SDL_Renderer *renderer)
{
//Enable vertex position
glEnableVertexAttribArray(vertex2DPosition);
//Set vertex data
glBindBuffer( GL_ARRAY_BUFFER, VBO );
glVertexAttribPointer(vertex2DPosition, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(GLfloat), NULL );
//Set index data and render
glBindBuffer( GL_ELEMENT_ARRAY_BUFFER, IBO );
glDrawElements(GL_TRIANGLE_FAN, 12, GL_UNSIGNED_INT, NULL );
//Disable vertex position
glDisableVertexAttribArray( vertex2DPosition );
}
Dummy text: asdasdasdasdasdasd

Answer 1

You have three things working against you in this example:
You are drawing a cube in Normalized Device Coordinates, which have the range XYZ: [-1,1].
You are drawing into a window that is not square, so the cube face is stretched because your coordinate space has nothing to do with the size of your window.
It appears to be a rectangle because you have not defined any viewing transformation and are viewing it face-on with no perspective.
You can correct all of this if you do this whenever your window is resized:
GLfloat aspect = (GLfloat)width / (GLfloat)height;
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
glOrtho ((-1.0f * aspect), (1.0f * aspect), -1.0f, 1.0f, -1.0f, 1.0f);
glViewport (0, 0, width, height);
glMatrixMode (GL_MODELVIEW);
glLoadIdentity ();
glRotatef (45.0f, 0.25f, 0.5f, 0.75f); // Let's rotate this sucker!
Where width and height are the width and height of your SDL window, respectively.
UPDATE:
See, now it is clear why this did not work. You never mentioned in your question that you were using shaders. Thank you for posting your updated code. In order for this to work with shaders you need to pass a projection and modelview matrix. You can use the same setup to build your matrices as I described using the old OpenGL matrix manipulation functions, and glm would probably make this easiest.