How to down/up sample a render target

You can read up on image scaling if you want some background info. The basic idea is that you sample one or more texels from the source render target, apply some sort of filter to those texels, and output the result. You're probably already familar with the "point" and "linear" filtering modes that are built-in to the hardware, which you can use just by taking a single texture sample with the appropriate sampler settings. But you can also implement more complex filters manually in your shader, if you wish.

For downscaling depth, things are a bit more tricky since some of the conventional wisdom used for scaling color images won't necessarily apply. Most people just end up using point filtering to downscale depth, which preserves edges but increases aliasing. To implement that, it really is as easy as you think it is: just use a pixel shader to sample the full-size render target, and output the value to your half-size render target.

For downscaling depth, things are a bit more tricky since some of the conventional wisdom used for scaling color images won't necessarily apply. Most people just end up using point filtering to downscale depth, which preserves edges but increases aliasing. To implement that, it really is as easy as you think it is: just use a pixel shader to sample the full-size render target, and output the value to your half-size render target.

My image seems distorted now though. Do I need to change the viewport while rendering in half res ?

edit: Ah yes that was it

Aniso decimation is something that may also be interesting for you.

solved.