For font rendering I render my text to a texture surface and then just use the texture to display the text. I'd like to be able to render onto a surface without needing to clear the surface first, so that the rendered text is drawn on top of the existing texels.

In order to do this I:

-Start RTS
-Render the text to the surface
-End RTS

But I notice that this causes low-alpha texels around the edges of the rendered text, which is not good. So I try this:

-Create new texture with the dimensions of the existing one
-Start RTS
-Clear it
-Render the text to it
-End RTS
-Start RTS on the original texture surface
-Draw the new surface on top of it
-End RTS
-Dispose of the 'temporary' surface

But I'm still getting the same problem.

I checked my render states and I have separate alpha blending enabled. The op is ADD and the src/dest are the src/dest alpha, but the problem remains.

D3DPMISCCAPS_SEPARATEALPHABLEND is true.

So all I can think of is that maybe RTS ignores separate alpha blending?

Has anyone else run into this kind of thing?

Thanks in advance for any advice.