void PrepareRenderTarget()
{
if (RenderTarget == null)
{
var format = SurfaceFormat.Single;
if (shadowMapEffect.Form == ShadowMapForm.Variance)
format = SurfaceFormat.Vector2;
RenderTarget = new RenderTarget2D(
GraphicsDevice,
size, size, false, format,
DepthFormat.Depth24Stencil8, 0, RenderTargetUsage.PreserveContents);
}
}