Can you store negative numbers with XNA HLSL?

Question

I am trying to make a ripple effect with a HLSL shader, it works so far but I need to output negative numbers and that won't work for the calculations. Is there some way to do this with the SurfaceFormat class? By the way I am rendering 2 RenderTarget2D classes. This is how I calculate it:

float4 add = float4(0,0,0.5,0);
float4 input = tex2D( BaseSampler, Tex);
float4 cur = tex2D( RippleSampler, Tex);
float curY = 0;
float curX = cur.x;
float tempCurY = cur.y;

curY = (tex2D( RippleSampler, float2(Tex.x - pixelSizeX, Tex.y)).x + tex2D( RippleSampler, float2(Tex.x + pixelSizeX, Tex.y)).x + tex2D( RippleSampler, float2(Tex.x , Tex.y - pixelSizeY)).x +  tex2D( RippleSampler, float2(Tex.x , Tex.y + pixelSizeY)).x) * 0.5f - tempCurY;
curY *= damping;

if(curY < 0.01f){
    cur = float4(curX,curY  + input.x,0.5,1);
}else{
    cur = float4(curX,curY,0.5,1);
}

return cur;

Answer 1

The trick is to encode the negative number as positive one, by transforming it into a different positive range.

For instance if you need to store values in the [-1,1] range in the texture, you'd start by mapping that into the [0-1] range before rendering to the texture.

You can do this for instance, by taking the original number, adding one to it and then dividing by two.

float originalValue = /* some number between -1 and 1 you want to store in the texture */
float encodedValue = (originalValue + 1.0f) * 0.5f; /* now it's between 0 and 1 */

Then inside your pixel shader, after reading from the texture, convert it back into the original [-1,1] range by multiplying by two and subtracting by one.

float encodedValue = /* value in 0 to 1 range read from the texture */
float originalValue = encodedValue * 2.0f - 1.0f; /* now it's back between -1 and 1 */