How can i map textures to objects in legacy OpenGL 1.x? - c++

Question

I want to draw some indicators for use in an older software using SDL2 and OpenGL 1.6 (therefore i can't switch to modern OpenGL for now) and have decided to build them from basic shapes. I've gotten the drawing logic working properly, but can't get the texturing to work. It actually completely broke the program. I want to keep an OOP approach and be able to draw any object/shape separately, by just calling a drawing method.
At first i create the window:
void SDLWindow::createWindow(const std::string windowTitle)
{
if (SDL_Init(SDL_INIT_VIDEO) < 0)
{
puts("Could not init SDL");
return;
}
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 1);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 6);
SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 1);
SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES, 4);
SDL_GL_SetAttribute(SDL_GL_ACCELERATED_VISUAL, 1);
Uint32 flags = SDL_WINDOW_OPENGL | SDL_WINDOW_ALWAYS_ON_TOP
| SDL_WINDOW_RESIZABLE;
this->mainWindow = SDL_CreateWindow(windowTitle.c_str(),
SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, this->width, this->height,
flags);
if (nullptr == mainWindow)
{
puts("Window could not be created");
return;
}
this->context = SDL_GL_CreateContext(this->mainWindow);
if (nullptr == this->context)
{
puts("Could not create context");
return;
}
SDL_GL_SetSwapInterval(1);
SDL_RaiseWindow(this->mainWindow);
}
Then i initialise OpenGL specifics:
void SDLWindow::initGL()
{
glClearDepth(1.0f);
glDepthFunc(GL_LEQUAL); // Type Of Depth Testing
// glEnable(GL_DEPTH_TEST);
glViewport(0, 0, this->width, this->height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0f, this->width, this->height, 0.0f, 0.0f, 1.0f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glEnable(GL_MULTISAMPLE);
}
These settings are the ones used in the main app and, until i started adding textures, everything was working properly.
Loading the texture from a file:
void SDLWindow::loadTextureFromFile(char* path){
SDL_Surface* Surface = SDL_LoadBMP(path);
glGenTextures(1, &this->textureID);
glBindTexture(GL_TEXTURE_2D, this->textureID);
glTexEnvf( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE );
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, Surface->w, Surface->h, 0, GL_RGB, GL_UNSIGNED_BYTE, Surface->pixels);
SDL_FreeSurface(Surface);
}
Drawing a primitive shape with the texture applied to it:
void SDLWindow::drawBasicShape()
{
glBindTexture(GL_TEXTURE_2D, this->textureID); // commented this but doesn't change anything
glBegin( GL_QUADS);
glTexCoord2f(0.0f, 0.0f);
glVertex2f(this->width / 4, this->height / 4);
glTexCoord2f(1.0f, 0.0f);
glVertex2f(3 * this->width / 4, this->height / 4);
glTexCoord2f(1.0f, 1.0f);
glVertex2f(3 * this->width / 4, 3 * this->height / 4);
glTexCoord2f(0.0f, 1.0f);
glVertex2f(this->width / 4, 3 * this->height / 4);
glEnd();
}
In the main loop, i make a call to renderRectCore() which first loads the texture and then draws the shape:
void SDLWindow::renderRectCore()
{
//Clear color buffer
glClear( GL_COLOR_BUFFER_BIT);
// glColor3f(1.0f, 0.0f, 0.0f);
glEnable(GL_TEXTURE_2D);
loadTextureFromFile("D:\Workspace\Eclipse\SDL_test\sample.bmp");
drawBasicShape();
}
As far as i can tell, the function that loads the texture somehow breaks the program and causes the window to open blank and immediately close. Removing the call to loadTextureFromFile simply draws the white rectangle, but keeps the window functioning as intended. When instantiating the class SDLWindow, textureID is initialized to 0.

Answer 1

Use a debugger to see which line of code is failing. But most likely SDL_LoadBMP returns NULL because
"D:\Workspace\Eclipse\SDL_test\sample.bmp"
is not the correct path.
You have to properly escape backslash characters in C++ string literals:
"D:\\Workspace\\Eclipse\\SDL_test\\sample.bmp"
(Or just use forward slashes as they are actually also supported by Windows).
Your code is also lacking the most basic error handling and will crash if the file is not found or not readable (or not the expected data format).

Answer 2

glEnable(GL_TEXTURE_2D);
loadTextureFromFile("D:\Workspace\Eclipse\SDL_test\sample.bmp");
There is no point in loading texture in the main loop.
Move it to init.