c++ - glGetTexImage reads too much data with texture format GL_ALPHA

Question

I'm trying to retrieve the pixel information for an alpha-only texture via glGetTexImage.

The problem is, the glGetTexImage-Call seems to read more data than it should, leading to memory corruption and a crash at the delete[]-Call. Here's my code:

int format;
glGetTexLevelParameteriv(target,0,GL_TEXTURE_INTERNAL_FORMAT,&format);
int w;
int h;
glGetTexLevelParameteriv(target,0,GL_TEXTURE_WIDTH,&w);
glGetTexLevelParameteriv(target,0,GL_TEXTURE_HEIGHT,&h);
if(w == 0 || h == 0)
    return false;
if(format != GL_ALPHA)
    return false;
unsigned int size = w *h *sizeof(unsigned char);
unsigned char *pixels = new unsigned char[size];
glGetTexImage(target,level,format,GL_UNSIGNED_BYTE,&pixels[0]);
delete[] pixels;

glGetError reports no errors, and without the glGetTexImage-Call it doesn't crash.

'target' is GL_TEXTURE_2D (The texture is valid and bound before the shown code), 'w' is 19, 'h' is 24, 'level' is 0.

If I increase the array size to (w *h *100) it doesn't crash either. I know for a fact that GL_UNSIGNED_BYTE has the same size as an unsigned char on my system, so I don't understand what's going on here.

Where's the additional data coming from and how can I make sure that my array is large enough?

Answer 1

Each row written to or read from by OpenGL pixel operations like glGetTexImage are aligned to a 4-byte boundary by default, which may add some padding.

To modify the alignment, use glPixelStorei with the GL_[UN]PACK_ALIGNMENT setting. GL_PACK_ALIGNMENT affects operations that read from OpenGL memory (glReadPixels, glGetTexImage, etc.) while GL_UNPACK_ALIGNMENT affects operations that write to OpenGL memory (glTexImage, etc.)

The alignment can be any of 1 (tightly packed with no padding), 2, 4 (the default), or 8.

So in your case, run glPixelStorei(GL_PACK_ALIGNMENT, 1); before running glGetImage2D.