Using the Interlace Extension

Using the Interlace Extension

To turn interlacing on or off, call glEnable() or glDisable() with the cap parameter GL_INTERLACE_SGIX. When enabled, it modifies the behavior of glDrawPixels(), glCopyPixels(), glTexImage2D(), glTexSubImage2DEXT(), glCopyTexImage2DEXT() and glCopyTexSubImage2DEXT() as well as the 3D versions of texture calls.

If GL_INTERLACE_SGIX is enabled, all the groups that belong to a row m are treated as if they belonged to the row 2m. If the source image has a height of h rows, this effectively expands the height of the image to 2h rows.

After interlacing, only every other row of the image is defined:

If the interlaced pixel rectangle is rasterized to the framebuffer, then only those rows are converted to fragments.
If the interlaced pixel rectangle is a texture image, then only those rows are written to texture memory.

In cases where errors can result from the specification of invalid image dimensions, the resulting dimensions--not the dimensions of the source image--are tested.

For example, when you use glTexImage2D() with GL_INTERLACE_SGIX enabled, the source image you provide must be of height (texture_height + texture_border)/2.