Scanlines... How?

[Xorcist]

I'm using SDL for all my blitting/rendering etc. What I wanted to know, is if there is a standard way for creating scanlines? Is it easier to blit over the entire screen after you've drawn, or only drawn line per line skipping every other? Or is there some even better way? I've never done this type of thing before, but I have seen it used in many emulators (25%, 50%, 75%, Full scanlines, etc.). I like the effect and would hope to incorporate it into one of my projects.

[Paulius]

It is easier to draw an image containing lines on top, but if you do drawing in system memory(like a lot of emulators do to apply some postprocessing filters) then skipping every other line should be faster.