Unable to render a texture on a quad

Question

I am trying to program a simple game in C++ using OpenGL for graphics. In my game, I have objects that are rendered onscreen as a white square. I would like to be able to bind an image as a texture to these objects, so that I can render an image onscreen instead of the white square. There's no restriction on the format of the image, though I've been using .png or .bmp for testing.

One of my object classes, Character.h, stores a GLuint* member variable called _texture and an unsigned char* member variable called _data (as an image handle and pixel data, respectively). Within Character.h is this function meant to bind the image as a texture to the Character object:

void loadTexture(const char* fileName)
    {
        /* read pixel data */
        FILE* file;
        file = fopen(fileName, "rb");
        if (file == NULL) {cout << "ERROR: Image file not found!" << endl;}
        _data = (unsigned char*)malloc(_dimension * _dimension * 3);
        fread(_data, _dimension * _dimension * 3, 1, file);
        fclose(file);
        /* bind texture */
        glGenTextures(1, &_texture); // _texture is a member variable of type GLuint
        glBindTexture(GL_TEXTURE_2D, _texture);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, _dimension, _dimension, 0, GL_RGB, GL_UNSIGNED_BYTE, _data); // _dimension = 64.0f.
    }

Once the image is bound, I then attempt to render the Character object with this function (which is a static function found in a separate class):

static void drawTexturedRectangle(float x, float y, float w, float h, GLuint texture, unsigned char* data)
    {
        /* fractions are calculated to determine the position onscreen that the object is drawn at */
        float xFraction = (x / GlobalConstants::SCREEN_WIDTH) * 2;
        float yFraction = (y / GlobalConstants::SCREEN_HEIGHT) * 2;
        float wFraction = (w / GlobalConstants::SCREEN_WIDTH) * 2;
        float hFraction = (h / GlobalConstants::SCREEN_HEIGHT) * 2;
        glEnable(GL_TEXTURE_2D);
        glClear(GL_COLOR_BUFFER_BIT);
        glBindTexture(GL_TEXTURE_2D, texture);
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, w, h, 0, GL_RGB, GL_UNSIGNED_BYTE, data);
        glPushMatrix();
        glEnable(GL_BLEND);
        glBegin(GL_QUADS);
        glTexCoord2f(0.0f, 0.0f); glVertex2f(xFraction, yFraction); // bottom left corner
        glTexCoord2f(0.0f, 1.0f); glVertex2f(xFraction, yFraction + hFraction); // top left corner
        glTexCoord2f(1.0f, 1.0f); glVertex2f(xFraction + wFraction, yFraction + hFraction); // top right corner
        glTexCoord2f(1.0f, 0.0f); glVertex2f(xFraction + wFraction, yFraction); // bottom right corner
        glEnd();
        glDisable(GL_BLEND);
        glPopMatrix();
    }

No matter how much I mess around with this, I cannot seem to get my program to do anything other than render a plain white square with no texture on it. I have checked to ensure that variables file, _texture, _dimension, and _data all contain values. Can anybody spot what I might be doing wrong?

I don't think this is the cause of your problem, but you shouldn't call `glTexImage2D` every time you want to draw a texture - you call it once, when loading the texture. After that, you can discard the data from client memory, because you have handed it off to OpenGL, which will manage it, and you only need to bind the texture again to tell OpenGL you want to use it. — orost, Jun 14 '15 at 22:09
I don't know about the rest of your code but try to make sure that you have one `glEnable( GL_TEXTURE_2D );` call, before you call the `loadTexture()` function. — aslg, Jun 14 '15 at 22:58

agnu17 · Accepted Answer · 2015-06-15T09:26:54.903

i dont think you can just input a raw file to glTexImage2D, except if you store texture files in that format (which you probably dont). glTexImage2D expects a huge array bytes (representing texel colors), but file formats typically dont store images like that. Even bmp has some header information in it.

Use a 3rd party image loader library like DevIL or SOIL to load your textures! These libraries can convert most common image file formats (png, jpg, tga, ...) into a byte stream that you can input to OpenGL!

On a side note: After you call glTexImage2D(..., data), the content of data is copied into the GPU memory (VRAM). From this point you only have to bind the texture using it's id, as it is already in the VRAM. Calling glTexImage2D() in each frame will reupload the whole texture in each frame into the vram, causing a big performance penalty.

Thanks so much agnu17! I got it to work using SOIL. The loadTexture method simply becomes: _texture = SOIL_load_OGL_texture(fileName, SOIL_LOAD_AUTO, SOIL_CREATE_NEW_ID, SOIL_FLAG_INVERT_Y); I've read that this may only work with 8-bit .pngs for whatever reason. — tomdunkle0, Jun 15 '15 at 22:31

Unable to render a texture on a quad

1 Answers1