使用 SDL 加载 OpenGL 纹理

Question

I have started learning OpenGL using NeHe tutorials for a little while.This is the code for lesson 6.It should load a bmp image and use it as texture for the cube I'm drawing.But It doesn't work properly and the cube remains white at all.The function for loading the image is "loadGLTextures".Can anyone help? My image bit depth is 24.I'm working with Visual Studio 2010.

#include <Windows.h>
#include <stdio.h>
#include <gl\GL.h>
#include <gl\GLU.h>
#include <SDL\SDL.h>


#pragma comment(lib , "SDL.lib")
#pragma comment(lib , "SDLmain.lib")
#pragma comment(lib , "OPENGL32.lib")
#pragma comment(lib , "glu32.lib")


//height , width and bit depth
#define SCREEN_WIDTH 800
#define SCREEN_HEIGHT 600
#define SCREEN_BPP 16

//SDL surface
SDL_Surface* surface;

//Texture storage.
GLuint texture[1];

//Quit func.
void Quit(int returnCode)
{
    SDL_Quit();
    exit(returnCode);
}

//This function will load a bitmap image.
bool loadGLTextures(void)
{
    SDL_Surface* textureImage;
    textureImage = SDL_LoadBMP("123.bmp");
    if(!textureImage)
    {
        fprintf(stderr , "Couldn't load %s.\n" , "123.bmp");
        return false;
    }

    else
    {
        //Create the texture.
        glGenTextures(1 , &texture[0]);

        //Typical texture generation using data from the bitmap.
        glBindTexture(GL_TEXTURE_2D , texture[0]);

        //Generate the texture.
        glTexImage2D(GL_TEXTURE_2D , 0 , 3 , textureImage->w , 
            textureImage->h , 0 , GL_RGB , GL_UNSIGNED_BYTE , 
            textureImage->pixels);

        //Linear filtering.
        glTexParameteri(GL_TEXTURE_2D , GL_TEXTURE_MIN_FILTER , GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D , GL_TEXTURE_MAG_FILTER , GL_LINEAR);

        //Free up the memory.
        if(textureImage)
            SDL_FreeSurface(textureImage);

        return true;
    }

}

//All of the drawing goes throw this.
int drawGLScene(void)
{
    static float xrot = 0 , yrot = 0 , zrot = 0;
    //Clear screen and depth buffer.
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glLoadIdentity();

    glTranslatef(0.0f , 0.0f , -5.0f);
    glRotatef(xrot , 1.0f , 0.0f , 0.0f);
    glRotatef(yrot , 0.0f , 1.0f , 0.0f);
    glRotatef(zrot , 0.0f , 0.0f  ,1.0f);

    //Select the texture.
    glBindTexture(GL_TEXTURE_2D , texture[0]);

    glBegin(GL_QUADS);
    //Front:
    //Bottom left of the texture and quad.
    glTexCoord2f(0.0f , 0.0f); glVertex3f(-1.0f , -1.0f , 1.0f);
    //Bottom right fo the texture and quad.
    glTexCoord2f(1.0f , 0.0f); glVertex3f(1.0f , -1.0f , 1.0f);
    //Top right of the texture and quad.
    glTexCoord2f(1.0f , 1.0f); glVertex3f(1.0f , 1.0f , 1.0f);
    //Top left of the texture and quad.
    glTexCoord2f(0.0f , 1.0f); glVertex3f(-1.0f , 1.0f , 1.0f);

    //Back:
    //Bottom left of the texture and quad.
    glTexCoord2f(0.0f , 0.0f); glVertex3f(1.0f , -1.0f , -1.0f);
    //Bottom right of the texture and quad.
    glTexCoord2f(1.0f , 0.0f); glVertex3f(-1.0f , -1.0f , -1.0f);
    //Top right of the texture and the quad.
    glTexCoord2f(1.0f , 1.0f); glVertex3f(-1.0f , 1.0f , -1.0f);
    //Top left of the texture and the quad.
    glTexCoord2f(0.0f , 1.0f); glVertex3f(1.0f , 1.0f , -1.0f);

    //Top:
    //Top right of the texture and quad.
    glTexCoord2f(1.0f , 1.0f); glVertex3f(1.0f , 1.0f , -1.0f);
    //Top left of the texture and quad.
    glTexCoord2f(0.0f , 1.0f); glVertex3f(-1.0f , 1.0f , -1.0f);
    //Bottom left of the texture and quad.
    glTexCoord2f(0.0f , 0.0f); glVertex3f(-1.0f , 1.0f , 1.0f);
    //Bottom right of the texture and quad.
    glTexCoord2f(0.0f , 1.0f); glVertex3f(1.0f , 1.0f , 1.0f);

    //Bottom:
    //Top left of the texture and quad.
    glTexCoord2f(0.0f , 1.0f); glVertex3f(-1.0f , -1.0f , 1.0f);
    //Bottom left of the texture and quad.
    glTexCoord2f(0.0f , 0.0f); glVertex3f(-1.0f , -1.0f , -1.0f);
    //Bottom right of the texture and quad.
    glTexCoord2f(1.0f , 0.0f); glVertex3f(1.0f , -1.0f , -1.0f);
    //Top right of the texture and quad.
    glTexCoord2f(1.0f , 1.0f); glVertex3f(1.0f , -1.0f , 1.0f);

    //Right:
    //Bottom right of the texture and quad.
    glTexCoord2f(1.0f , 0.0f); glVertex3f(1.0f , -1.0f , -1.0f);
    //Top right of the texture and quad.
    glTexCoord2f(1.0f , 1.0f); glVertex3f(1.0f , 1.0f , -1.0f);
    //Top left of the texture and quad.
    glTexCoord2f(0.0f , 1.0f); glVertex3f(1.0f , 1.0f , 1.0f);
    //Bottom left of the texture and quad.
    glTexCoord2f(0.0f , 0.0f); glVertex3f(1.0f , -1.0f , 1.0f);

    //Left:
    //Bottom left of the texture and quad.
    glTexCoord2f(0.0f , 0.0f); glVertex3f(-1.0f , -1.0f , -1.0f);
    //Bottom right of the texture and quad.
    glTexCoord2f(1.0f , 0.0f); glVertex3f(-1.0f , -1.0f , 1.0f);
    //Top right of the texture and quad.
    glTexCoord2f(1.0f , 1.0f); glVertex3f(-1.0f , 1.0f , 1.0f);
    //Top left of the texture and quad.
    glTexCoord2f(0.0f , 1.0f); glVertex3f(-1.0f , 1.0f , -1.0f);
    glEnd();

    SDL_GL_SwapBuffers();

    xrot += 0.1;
    yrot += 0.1;
    zrot += 0.1;

    return true;

}

//This function will reset our viewport after a windows resize.
int resizeWindow(int width , int height)
{
    //Height / width ration.
    float ratio;

    //Protect against a division by zero.
    if(height == 0)
        height = 1;

    ratio = width / height;

    //Setup viewport
    glViewport(0 , 0 , width , height);

    //Change to the projection matrix and reset it.
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    //set perspective.
    gluPerspective(45.0f , ratio , 0.1f , 100.0f);

    //Change to model view matrix and reset it.
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    return true;
}

//Toggle fullScreen.
void toggleFullscreen(SDL_Surface* screen)
{
    int videoFlags = screen->flags;
    (videoFlags & SDL_FULLSCREEN) == SDL_FULLSCREEN ? videoFlags ^= SDL_FULLSCREEN : videoFlags |= SDL_FULLSCREEN;//NICE!!
    screen = SDL_SetVideoMode(SCREEN_WIDTH , SCREEN_HEIGHT , SCREEN_BPP , videoFlags);
    resizeWindow(surface->w , surface->h);
    drawGLScene();
}

//OpenGL initialization.
int initGL(void)
{
    if(!loadGLTextures())
        return false;

    glShadeModel(GL_SMOOTH);
    glEnable(GL_TEXTURE_2D); //Enable texture mapping.
    glClearColor(0.0f , 0.0f , 0.0f , 0.5f);
    glClearDepth(1.0f);
    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LEQUAL);
    //Nice perspective.
    glHint(GL_PERSPECTIVE_CORRECTION_HINT , GL_NICEST);
    return true;
}

//This func will handle any key inputs.
void handleKeyPress(SDL_keysym* keysym)
{
    switch(keysym->sym)
    {
    case SDLK_ESCAPE:
            Quit(0);
            break;
    case SDLK_F1:
        toggleFullscreen(surface);
        break;
    case SDLK_r:
        drawGLScene();
        break;
    default:
        break;
    }
    return;
}


int main(int argc , char* argv[])
{
    //Flags to pass to SDL_SetVideoMode : awsome!! ints can be compiled.
    int videoFlags;
    //Event
    SDL_Event event;
    //Holds information about display.
    const SDL_VideoInfo* videoInfo;
    //Is window active?
    bool isActive = true;

    //SDL initialization.
    if(SDL_Init(SDL_INIT_VIDEO) < 0)
    {
        fprintf(stderr , "SDL video initialization failed : %s\n" , SDL_GetError());
        Quit(1);
    }

    //Fetch the video info.
    videoInfo = SDL_GetVideoInfo();

    if(!videoInfo)
    {
        fprintf(stderr , "Video query failed : %s\n" , SDL_GetError());
        Quit(1);
    }

    //Add flags to pass to SDL_SetVideoMode.
    videoFlags = SDL_OPENGL;              //Enable OpenGL in SDL.
    videoFlags |= SDL_GL_DOUBLEBUFFER;    //Enable double buffering.
    videoFlags |= SDL_HWPALETTE;          //Store the palette in hardware.
    videoFlags |= SDL_RESIZABLE;          //Enable window resizing.

    //This checks to see if surfaces can be stored in hardware.
    videoInfo->hw_available ? videoFlags |= SDL_HWSURFACE : SDL_SWSURFACE;

    //This checks if harware blits can be done.
    if(videoInfo->blit_hw)
        videoFlags |= SDL_HWACCEL;

    //Set OpenGL double buffering.
    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER , 1);

    surface = SDL_SetVideoMode(SCREEN_WIDTH , SCREEN_HEIGHT , 16 , videoFlags);

    //verify the surface.
    if(!surface)
    {
        fprintf(stderr , "Video mode set failed : %s\n" , SDL_GetError());
        Quit(1);
    }

    SDL_WM_SetCaption("OpenGL-Sample" , 0);

    //initialize OpenGL
    if(initGL() == false)
    {
        fprintf(stderr , "Could not initialize OpenGL.\n");
        Quit(1);
    }

    //Main loop
    while(1)
    {
        //Handle the events in the queue.
        if(SDL_PollEvent(&event))
        {
            switch(event.type)
            {
            case SDL_ACTIVEEVENT:
                if(event.active.gain == 0)
                    isActive = false;
                else
                    isActive = true;
                break;
            case SDL_VIDEORESIZE:
                //Handle resize event.
                surface = SDL_SetVideoMode(event.resize.w , event.resize.h , SCREEN_BPP , videoFlags);
                if(!surface)
                {
                    fprintf(stderr , "Could not get a surface after resize : %s\n" , SDL_GetError());
                    Quit(1);
                }
                resizeWindow(event.resize.w , event.resize.h);
                break;
            case SDL_KEYDOWN:
                handleKeyPress(&event.key.keysym);
                break;
            case SDL_QUIT:
                Quit(0);
            default:
                break;
            }
        }
        if(isActive)
            drawGLScene();
    }

}

This is the image I'm trying to load.

Answer 1

I use the SOIL library, http://www.lonesock.net/soil.html ,

It's a very easy to use OpenGL texture loader library, you don't have to worry about the image format or make any loading code yourself, it will do it all for you.

Loading a texture is as easy as this:

int textureID = SOIL_load_OGL_texture("img.png", SOIL_LOAD_AUTO, 
    SOIL_CREATE_NEW_ID, SOIL_FLAG_MIPMAPS);
if(textureID == 0) { cout << "Failed to load texture!" << endl };

Answer 2

I was able to get your program to produce a texture (although it was mirrored) by changing:

        textureImage->h , 0 , GL_RGB , GL_UNSIGNED_BYTE , 

to:

        textureImage->h , 0 , GL_RGBA , GL_UNSIGNED_BYTE , 

Getting that field wrong unfortunately does not produce any error. It may cause your program to crash if you attempt to tell the GL that the image data is bigger than it really is, but passing GL_RGB instead of GL_RGBA has the effect of saying it's smaller than it really is.

Keep in mind that SDL_LoadBMP() doesn't attempt to convert the image data, so you have to make sure the format of the BMP file is what the program expects. you probably need to use GL_RGBA or GL_RGB.

Answer 3

Everything depends on your image's format. As Michael said SDL_LoadBMP does not convert the image data. So you can't be sure which flag you should pass.
I recommend you to use SDL_Image library. Which converts all the image formats to one specific format. Then you can use (for example) GL_RGBA flag and make sure everything works!