OpenGL texturing using C++

Question

GLuint createTexture(const char* filename, int width = 128, int height = 128)
{
    GLuint texture;
    char bitmap[width][height][3];
    FILE* fp = fopen(filename, "rb");
    assert(fp);
    assert(fread(bitmap, 3*sizeof(char), width*height, fp) == width*height);
    fclose(fp);
    glGenTextures(1, &texture);
    glBindTexture(GL_TEXTURE_2D, texture);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, bitmap);
    glBindTexture( GL_TEXTURE_2D,0);

    return texture;
}

Hi people, I would like to know how the above code works. Supposedly it is a coding that can be used to texture solid objects in OpenGL (ex: glutSolidSphere). Can someone please explain to me how the above coding works?

As it's answered below: start by reading beginner tutorials. For more info on commands try looking openGL library reference like this one http://www.opengl.org/sdk/docs/man/ — Protheus, Jul 23 '14 at 15:40

Dacobi · Answer 1 · 2014-07-23T15:55:37.963

4

You're sort of asking "How OpenGL works" and that depends on which version you're using. I would recommend: http://www.opengl-tutorial.org/beginners-tutorials

Or better yet, read the "Red Book" (8th edition covers OpenGL 4.3)

http://www.opengl-redbook.com/

edited Jul 23 '14 at 15:55

answered Jul 23 '14 at 15:26

Dacobi

417
4
14

score 2 · Answer 2 · answered Jul 23 '14 at 16:35

GLuint createTexture(const char* filename, int width=128, int height=128)
{
    GLuint texture;

Use C99 variadic array to allocate width*height*3 bytes of memory on the stack. If there's not enough stack left, the program crashes with a stack overflow.

    char bitmap[width][height][3];

Try opening the file to fetch the texture from. The file must be a headerless, raw, tight array of 8 bit per channel RGB data.

    FILE* fp=fopen(filename, "rb");

Terminate the program if opening the file failed. Technically using assert for this purpose is a programming error, because a) assert does nothing in release builds and b) this is an easy to recover from error. The function should just return 0 if the file couldn't be opened.

    assert(fp);

Read width*height chunks of 3 bytes. sizeof(char) indicates the writer of this had no clue about C, because the C standard defines sizeof(char) == 1, so there's no point in doing that multiplication.

If not the expected amount of data could be read, crash the program if in debug mode. In case of error and a release build do strange things.

    assert(fread(bitmap, 3*sizeof(char), width*height, fp) == width*height);

Close the file.

    fclose(fp);

Allocate a OpenGL texture object handle.

    glGenTextures(1, &texture);

Bind the texture handle so that subsequent OpenGL operations affecting textures are operating on the given object.

    glBindTexture(GL_TEXTURE_2D, texture);

Set the texture magnification and minification filters. Use linear, so that mipmap levels are not required.

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

Copy the data into the OpenGL texture. Since this uses a newly allocated texture object handle, the texture is also initialized by this.

A lot of important steps are missing here. Namely glPixelStorei must be used to prepare OpenGL to read data from the array. Line alignment, strides and so on.

    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE,bitmap);

Unbind the texture object.

    glBindTexture( GL_TEXTURE_2D,0);

Return texture object handle.

    return texture;
}

Major issues with this code: No OpenGL error checks and crashing the program if in debug builds. In release builds no file operation errors are detected and handled.

DON'T USE THIS CODE!

OpenGL texturing using C++

2 Answers2