View Full Version : Texture dimensions not power of 2

01-21-2013, 04:23 AM
Red book 7th ed says:
"For OpenGL implementations that do not support version 2.0 or greater, both width and height must have the form 2^m + 2b, ... For OpenGL implementations supporting version 2.0 and greater, textures may be of any size."

My card support 3.3 but non-power-of-2 dimensioned textures (e.g., 480 x 480) don't come out right. Powers of 2 are fine though. My textures are in bmp format if that matters. Any idea why?

Thanks in advance,

01-21-2013, 05:05 AM
check your image loading algorithm. bmp uses padding. and specify what means "don't come out right", what happens and how does it look?

01-21-2013, 09:18 AM
Hi Nowhere,
Thanks for the response. Attached are grabs of what's drawn for a texture size 512x512 and then scaled to 480x480. Below is the part of the code that loads the image:

struct BitMapFile
int sizeX;
int sizeY;
unsigned char *data;

// Routine to read a bitmap file.
// Works only for uncompressed bmp files of 24-bit color.
BitMapFile *getBMPData(string filename)
BitMapFile *bmp = new BitMapFile;
unsigned int size, offset, headerSize;

// Read input file name.
ifstream infile(filename.c_str(), ios::binary);

// Get the starting point of the image data.
infile.read((char *) &offset, 4);

// Get the header size of the bitmap.
infile.read((char *) &headerSize,4);

// Get width and height values in the bitmap header.
infile.read( (char *) &bmp->sizeX, 4);
infile.read( (char *) &bmp->sizeY, 4);

// Allocate buffer for the image.
size = bmp->sizeX * bmp->sizeY * 24;
bmp->data = new unsigned char[size];

// Read bitmap data.
infile.read((char *) bmp->data , size);

// Reverse color from bgr to rgb.
int temp;
for (int i = 0; i < size; i += 3)
temp = bmp->data[i];
bmp->data[i] = bmp->data[i+2];
bmp->data[i+2] = temp;

return bmp;

BitMapFile *image[1];

// Load the texture.
image[0] = getBMPData("../Textures/launch.bmp");
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, image[0]->sizeX, image[0]->sizeY, 0,
GL_RGB, GL_UNSIGNED_BYTE, image[0]->data);

01-21-2013, 07:46 PM
// Read bitmap data.
infile.read((char *) bmp->data , size);

as i said, you have padding issues. the problem is not your texture is NPOT, but it is 4-byte aligned(so at the end of every row it has zero-bytes to make row size multiple of 4). while you reading it as a continuous RGB-array.
you can fix it dirty way, just to ensure i'm correct by placing these calls in your code(before passing image to opengl):

glPixelStorei ( GL_UNPACK_ALIGNMENT, 4 );
glPixelStorei ( GL_UNPACK_ROW_LENGTH, 0 );
glPixelStorei ( GL_UNPACK_SKIP_ROWS, 0 );
glPixelStorei ( GL_UNPACK_SKIP_PIXELS, 0 );

but you shouldn't use gl_unpack_* for that, you should parse bmp properly. that means you should take padding into account.
Loading Bitmaps (http://tipsandtricks.runicsoft.com/Cpp/BitmapTutorial.html), Sections #3 and #4

and also, this code is incorrect:

// Allocate buffer for the image.
size = bmp->sizeX * bmp->sizeY * 24;
bmp->data = new unsigned char[size];

it should be 3 instead of 24. 24 is bit depth, but you store data in bytes. 24 bit = 3 bytes, 1 per each RGB component.

01-22-2013, 03:33 AM
Thanks, Nowhere-01!