CODEWITHSUNDEEP
c++opengl
Pat
Pat

Reputation: 331

Issue with glGetTexLevelParameter

I'm having an issue trying to get the width and height of a texture using the glGetTexLevelParameter function. No matter what I try, the function will not set the value of the width or height variable. I checked for errors but keep getting no error. Here is my code (based off of the NeHe tutorials if that helps):

int LoadGLTextures()
{
//load image file directly into opengl as new texture
GLint width = 0;
GLint height = 0;
texture[0] = SOIL_load_OGL_texture("NeHe.bmp", SOIL_LOAD_AUTO, SOIL_CREATE_NEW_ID,   SOIL_FLAG_INVERT_Y);       //image must be in same place as lib
if(texture[0] == 0)
{
    return false;
}
glEnable(GL_TEXTURE_2D);
glGenTextures(3, &texture[0]);

glBindTexture(GL_TEXTURE_2D, texture[0]);
glGetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_WIDTH, &width);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);  //no filtering bc of GL_NEAREST, looks really bad
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

const GLubyte* test = gluErrorString(glGetError());
cout << test << endl;
return true;
}

I'm using visual studio 2010 also if that helps. The call to load texture[0] is from the SOIL image library.

Upvotes: 0

Views: 1347

Answers (3)

datenwolf
datenwolf

Reputation: 162164

Let's break this down:

This call loads an image, creates a new texture ID and loads the image into the texture object named by this ID. In case of success the ID is returned and stored in texture[0].

texture[0] = SOIL_load_OGL_texture(
     "NeHe.bmp",
     SOIL_LOAD_AUTO,
     SOIL_CREATE_NEW_ID,
     SOIL_FLAG_INVERT_Y);

BTW: The image file is not to be in the same directory as the library, but in the current working directory of the process at time of calling this function. If you didn't change the working directory, it's whatever directory your process got called from.

Check if the texture was loded successfully

if(texture[0] == 0)
{
    return false;
}

Enabling texturing here makes only little sense. glEnable calls belong in the rendering code.

glEnable(GL_TEXTURE_2D);

Okay, here's a problem. glGenTextures generates new texture IDs and places them in the array provided to it. Whatever was stored in that array before is overwritten. In your case the very texture ID generated and returned by SOIL_load_OGL_texture. Note that this is just some handle and is not garbage collected in any way. You now have in face a texture object dangling in OpenGL and no longer access to it, because you threw away the handle.

glGenTextures(3, &texture[0]);

Now you bind a texture object named by the newly created ID. Since this is a new ID you're effectively creating a new texture object with no image data assigned.

glBindTexture(GL_TEXTURE_2D, texture[0]);

All the following calls operate on an entirely different texture than the one created by SOIL.

How to fix the code: Remove glGenTextures. In your case it's not only redundant, it's the cause of your problem.

Upvotes: 2

Nicol Bolas
Nicol Bolas

Reputation: 473192

This line:

texture[0] = SOIL_load_OGL_texture("NeHe.bmp", SOIL_LOAD_AUTO, SOIL_CREATE_NEW_ID,   SOIL_FLAG_INVERT_Y);

Creates a texture, storing the OpenGL texture in texture[0].

This line:

glGenTextures(3, &texture[0]);

Creates three textures, storing them in the texture array, overwriting whatever was there.

See the problem? You get a texture from SOIL, then you immediately throw it away by overwriting it with a newly-created texture.

This is no different conceptually than the following:

int *pInt = new int(5);
pInt = new int(10);

Upvotes: 1

Ahk4iePaiv8u
Ahk4iePaiv8u

Reputation: 118

Hm, doesn't glGenTextures(howmany,where) work just like glGenBuffers? Why do you assign three textures to one pointer, how it's expected to work?

I think it shoud be

int textures[3];
glGenTextures(3,textures);

this way three generated texture buffers will be placed in texture array. Or

int tex1, tex2, tex3;
glGenTextures(1,&tex1);
glGenTextures(1,&tex2);
glGenTextures(1,&tex3);

so you have three separate texture buffer pointers

Upvotes: 0

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