Uploading alternate rows of Pixel Data OpenGL - opengl

Question

I am uploading an interlaced image to an OpenGL texture using glTexImage2D which of course uploads whole image. What I need is to upload only alternate rows, so on first texture odd rows and on second even rows.
I don't want to create another copy of the Pixel Data on CPU.

Answer 1

You can set GL_UNPACK_ROW_LENGTH to twice the actual row length. This will effectively skip every second row. If the size of your texture is width x height:
glPixelStorei(GL_UNPACK_ROW_LENGTH, 2 * width);
glBindTexture(GL_TEXTURE_2D, tex1);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);
glPixelStorei(GL_UNPACK_SKIP_PIXELS, width);
glBindTexture(GL_TEXTURE_2D, tex2);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);
glPixelStorei(GL_UNPACK_ROW_LENGTH, 0);
glPixelStorei(GL_UNPACK_SKIP_PIXELS, 0);
Instead of setting GL_UNPACK_SKIP_PIXELS to skip the first row, you can also increment the data pointer accordingly.

Answer 2

There is an ancient SGI extension (GL_SGIX_interlace) for transferring interlaced pixel data, but it is probably not supported on your implementation.
An alternative you might consider is memory mapping a Pixel Buffer Object. You can fill this buffer over two passes and then use it as the source of image data in a call to glTexImage2D (...). You essentially do the de-interlacing yourself, but since this is done by mapping a buffer object's memory you are not making an unnecessary copy of the image on the CPU.
Pseudo code showing how to do this:
GLuint deinterlace_pbo;
glGenBuffers (1, &deinterlace_pbo);
// `GL_PIXEL_UNPACK_BUFFER`, when non-zero is the source of memory for `glTexImage2D`
glBindBuffer (GL_PIXEL_UNPACK_BUFFER, deinterlace_pbo);
// Reserve memory for the de-interlaced image
glBufferData (GL_PIXEL_UNPACK_BUFFER, sizeof (pixel) * interlaced_rows * width * 2,
NULL, GL_STATIC_DRAW);
// Returns a pointer to the ***GL-managed memory*** where you will write the image
void* pixel_data = glMapBuffer (GL_PIXEL_UNPACK_BUFFER, GL_WRITE_ONLY);
// Odd Rows First
for (int i = 0; i < interlaced_rows; i++) {
for (int j = 0; j < width; j++) {
//Fill in pixel_data for each pixel in row (i*2+1)
}
}
// Even Rows
for (int i = 0; i < interlaced_rows; i++) {
for (int j = 0; j < width; j++) {
//Fill in pixel_data for each pixel in row (i*2)
}
}
glUnmapBuffer ();
// This will read the memory in the object bound to `GL_PIXEL_UNPACK_BUFFER`
glTexImage2D (..., NULL);
glBindBuffer (GL_PIXEL_UNPACK_BUFFER, 0);