Question

I plan on making a game (in SDL) where, if one character moves, the part of the image it was on turns alpha, thus allowing me to place a scrolling image underneath the original scene.

1) Is this possible?
2) If yes to #1, how can I go about implementing this (not to give me code, but to guide me in the right direction).

Answer 1

It sounds like you want to learn about image compositing.

A typical game these days will have a redraw function somewhere to redraw the entire screen. The entire scene is always redrawn each frame.

void redraw()
{
    drawBackground();
    drawCharacters();
    drawHUD();
    swapBuffers();
}

This is as simple as it gets: by using the right blending modes, each time you draw something it appears on top of what was drawn before. Older games are much more complicated because they don’t redraw the entire screen at a time (or don’t use a framebuffer), and newer games are much more complicated because they draw the world front-to-back and back-to-front in multiple passes for different types of objects.

SDL has software image compositing functions which you can use, or you can use OpenGL (which may use a combination of software and hardware). I personally use OpenGL because it is more powerful (lets you draw more complicated scenes), but the SDL compositing functions are easier to use. There are many excellent tutorials and many more mediocre or terrible tutorials online.

I’m not sure what you mean when you say “the part of the image it was on turns alpha”. The alpha channel does not appear on screen, you cannot see it, it just affects how two images are composited.