On Fri, Mar 15, 2019 at 5:00 PM Ilia Mirkin <imir...@alum.mit.edu> wrote: > > On Fri, Mar 15, 2019 at 11:52 AM Connor Abbott <cwabbo...@gmail.com> wrote: > > > > On Fri, Mar 15, 2019 at 3:46 PM Ilia Mirkin <imir...@alum.mit.edu> wrote: > > > > > > On Fri, Mar 15, 2019 at 10:29 AM Alyssa Rosenzweig < aly...@rosenzweig.io> wrote: > > > > > > > > > This is needed if you can only handle point sprites on certain > > > > > varyings but not others. This is the case for nv30- and nvc0-based > > > > > GPUs, which is why the cap was introduced. > > > > > > > > > > If your GPU does not have such restrictions, you can safely remove the cap. > > > > > > > > Ah-ha, I see, thank you. > > > > > > > > I can't handle point sprites on any varyings (they all have to get > > > > lowered to gl_PointCoord), so.. :) > > > > > > Yeah, that's not extremely surprising. Point sprites weren't a thing > > > in GLES 1, and only available as gl_PointCoord in GLES 2. With GL 1.x > > > + multitexture, you can have up to 8 tex coords used in your fragment > > > pipeline, and so need a fixed-function way of setting them to point > > > coords when drawing points. Hence the current TEXCOORD semantics of > > > just bumping generic varyings by 8, so that we can be sure that any > > > shaders with true gl_TexCoord[] usage get the low 8 varyings assigned > > > (and which, on nvc0+ go through "special" shader outputs, subject to > > > the replacement). > > > > > > Note that e.g. Adreno A3xx+ is capable of doing the replacement > > > everywhere, despite being a GLES-targeted GPU. So it's not out of the > > > realm of possibility that you just haven't figured out the full > > > mechanism for doing it yet. > > > > Can you replace more than one varying and output multiple points vertex shader, or something crazy like that? On Mali gl_PointCoord is just a normal varying whose descriptor points to a special point-coord buffer that gets fed to the tiler, so we should be able to make an arbitrary varying "turn into" gl_PointCoord just by changing its descriptor at draw time. > > Yeah, any (gl_TexCoord) varying may get replaced by a point sprite > coord (in GL 1.x / GL 2.x). Note that there's additional complication > from coordinate inversion, when you're drawing on the winsys fb vs an > fbo. So you have to be able to generate both S,T and S,1-T. [Or > perhaps there's an explicit control for it, I forget.] With GLES2+, > it's just gl_PointCoord though. > > The way gallium handles this is that you get a mask of varyings to > replace in https://cgit.freedesktop.org/mesa/mesa/tree/src/gallium/include/pipe/p_state.h#n188 > + whether to invert or not (sprite_coord_mode).
Oh, whoops... I guess I was thinking of gl_PointSize. gl_PointCoord in the fragment shader does indeed come through an extra-special varying, at least the way the blob does it.
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