# Rain drops on screen

I am trying to make simple rain drop effect on screen.Something like this
http://fc00.deviantart.net/fs20/f/2007/302/5/6/Rain_drops_by_rockraikar.png

My idea is to:

Create small drop shaped normal textures, randomly put few on screen, apply texture perturbation and mix with current frame pixels.

Here are my questions:

• Does this idea even have sense? How professionals do this effect? Everything from text to code will be appreciated

• I am new to shaders but I think you could do that without offscreen rendering by passing the raindrop overlay as uniform to the fragment shader. – danijar Jan 8 '13 at 12:00
• Please post a screenshot when you get this working. We would love to see how this plays out. – ashes999 Jan 8 '13 at 12:01
• youtube.com/watch?v=_4cQfNVUTM8 – user1075940 Jan 8 '13 at 12:55

Then, to render the drops, you'd use the normal map to calculate the sample point in the texture. You could just negate the xy components of the normal and use that as the UVs (scale-biased to the [0,1] range), but if you're feeling fancy you could try doing the actual refraction math. (There is even a refract function in HLSL that will do most of the math for you.)