Question

I have image data and i want to get a sub image of that to use as an opengl texture.

glGenTextures(1, &m_name); glGetIntegerv(GL_TEXTURE_BINDING_2D, &oldName); glBindTexture(GL_TEXTURE_2D, m_name); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, m_width, m_height, 0, GL_RGBA, GL_UNSIGNED_BYTE, m_data); 

How can i get a sub image of that image loaded as a texture. I think it has something to do with using glTexSubImage2D, but i have no clue how to use it to create a new texture that i can load. Calling:

glTexSubImage2D(GL_TEXTURE_2D, 0, xOffset, yOffset, xWidth, yHeight, GL_RGBA, GL_UNSIGNED_BYTE, m_data); 

does nothing that i can see, and calling glCopyTexSubImage2D just takes part of my framebuffer. Thanks

Answer 1

Edit: Use glPixelStorei. You use it to set GL_UNPACK_ROW_LENGTH to the width (in pixels) of the entire image. Then you call glTexImage2D (or whatever), passing it a pointer to the first pixel of the subimage and the width and height of the subimage.

Don’t forget to restore GL_UNPACK_ROW_LENGTH to 0 when you’re finished with it.

Ie:

glPixelStorei( GL_UNPACK_ROW_LENGTH, img_width ); char *subimg = (char*)m_data + (sub_x + sub_y*img_width)*4; glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, sub_width, sub_height, 0, GL_RGBA, GL_UNSIGNED_BYTE, subimg ); glPixelStorei( GL_UNPACK_ROW_LENGTH, 0 ); 

Or, if you’re allergic to pointer maths:

glPixelStorei( GL_UNPACK_ROW_LENGTH, img_width ); glPixelStorei( GL_UNPACK_SKIP_PIXELS, sub_x ); glPixelStorei( GL_UNPACK_SKIP_ROWS, sub_y );  glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, sub_width, sub_height, 0, GL_RGBA, GL_UNSIGNED_BYTE, m_data );  glPixelStorei( GL_UNPACK_ROW_LENGTH, 0 ); glPixelStorei( GL_UNPACK_SKIP_PIXELS, 0 ); glPixelStorei( GL_UNPACK_SKIP_ROWS, 0 ); 

Edit2: For the sake of completeness, I should point out that if you’re using OpenGL-ES then you don’t get GL_UNPACK_ROW_LENGTH. In which case, you could either (a) extract the subimage into a new buffer yourself, or (b)…

glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, sub_width, sub_height, 0, GL_RGBA, GL_UNSIGNED_BYTES, NULL );  for( int y = 0; y < sub_height; y++ ) {     char *row = m_data + ((y + sub_y)*img_width + sub_x) * 4;     glTexSubImage2D( GL_TEXTURE_2D, 0, 0, y, sub_width, 1, GL_RGBA, GL_UNSIGNED_BYTE, row ); }