[SDL] Difference between a 1-bit image and an indexed image when loaded into surface

[mal content]

Hi.

I'm writing a bit of code to convert SDL_Surface structures into OpenGL
textures.

Is it possible to programmatically tell the difference between:

  - 1-bit monochrome image
  - 1-bit monochrome image with alpha
  - indexed image
  - indexed image with alpha

The last two, I can distinguish between with:

   if (images[ind].img->flags & SDL_SRCCOLORKEY)
    gltxload.flags |= GLTEX_ALPHA;

So that's not a problem, but how can I tell the difference between
an indexed or monochrome image? The 'file' utility can tell for PNGs,
so at least for PNG files the images are structurally distinct:

IMAGES/1bit_alpha.png:      PNG image data, 128 x 128, 1-bit colormap,
non-interlaced
IMAGES/1bit_noalpha.png:    PNG image data, 128 x 128, 1-bit colormap,
non-interlaced
IMAGES/indexed.png:         PNG image data, 128 x 128, 2-bit colormap,
non-interlaced
IMAGES/indexed_alpha.png:   PNG image data, 128 x 128, 2-bit colormap,
non-interlaced

The point of this exercise is that I want to be able to load 1 bit
images into OpenGL as (memory) efficiently as possible, for things
such as bitmap fonts. The color can obviously be set at load time.

Any help would be appreciated.
MC