- From: Adam Barth <w3c@adambarth.com>
- Date: Wed, 14 Dec 2011 15:46:41 -0800
- To: Ralph Thomas <ralpht@gmail.com>
- Cc: "Gregg Tavares (wrk)" <gman@google.com>, James Robinson <jamesr@google.com>, Chris Marrin <cmarrin@apple.com>, Vincent Hardy <vhardy@adobe.com>, "public-fx@w3.org" <public-fx@w3.org>
On Wed, Dec 14, 2011 at 3:44 PM, Ralph Thomas <ralpht@gmail.com> wrote: > I think you could still branch on anything that wasn't derived from a > texture sample of HTML content -- branching on uniforms/fragment > position/other GL state wouldn't change runtime based on texture > contents. If you remove VTF then you only have the limitations in the > fragment shader. Out of curiosity, how would a dependent texture > lookup change runtime based on contents? Is it faster to sample edge > texels when using GL_LINEAR or something like that? My understanding is that there is a texture lookup cache. Cache hits and cache misses take different amounts of time. Adam > Internally pow has a conditional branch on a value derived from the > second argument, which in your example would be tainted, so that > should generate an error -- but a good point that the internal > functions would need to be considered too. > > And yes, this doesn't address any DOS concerns. You'd need something > else to cancel a frame if it's taking too long to rasterize. > > Ralph > > On Wed, Dec 14, 2011 at 1:07 AM, Gregg Tavares (wrk) <gman@google.com> wrote: >> Off the top of my head you'd have to remove "if", "while" and "?" and >> dependent texture lookups (sampling 1 texture based of sampling of another >> texture). You might be able to keep 'for' with its current WebGL limits of >> only a constant integer input since with no "if" there would be no way to >> break out early. >> >> I think you'd probably be able to write many useful shaders with those >> limitations >> >> No dependent texture lookups kind of sucks but probably not needed for most >> uses of CSS shaders. >> >> I think the confusion about limited GLSL not being useful is the DOS issue >> vs the timing issue. You can't limit shaders in any useful way and solve the >> DOS issue. In fact you don't even need shaders for the DOS issue. Just a >> large mesh. But for the timing issue, removing those 4 features might solve >> it. >> >> That assumes none of the math changes time based on input like for example >> >> float v = pow(1, texture2D(someTex, someUV).r * 10000.0); >> >> >> >> >> On Mon, Dec 12, 2011 at 11:10 PM, Adam Barth <w3c@adambarth.com> wrote: >>> >>> On Mon, Dec 12, 2011 at 9:56 PM, Ralph Thomas <ralpht@gmail.com> wrote: >>> > On the topic of "constant time GLSL", I was thinking that any value >>> > derived from a texture sample read from web content would be >>> > "poisoned" so it could not be used for conditionals and that the >>> > poison would propagate to any dependent value in the program. You >>> > could assign a poisoned value to gl_FragColor (obviously) but could >>> > not branch or loop on it. >>> > >>> > This would still let you write blur kernels, do lighting effects and >>> > warp texture coordinates, but you wouldn't be able to use any part of >>> > the texture as a lookup table, for example. >>> >>> In principle, this approach can work. To be fully correct, the >>> program should be restricted to performing constant-time operations on >>> tainted values. However, just avoiding branches is probably a good >>> place to start. >>> >>> > It should be possible to add a pass to ANGLE to poison values read >>> > from texture and those dependent on them and then validate that no >>> > selections or loops depend on a poisoned value. I think that a program >>> > that passed this test would then always execute in the same time for a >>> > given set of vertices regardless of the contents of any bound texture. >>> > >>> > I believe CSS Shaders would still be useful with these limitations >>> > added to GLSL -- what did I miss? >>> >>> I would encourage you to implement a prototype of this scheme to see >>> whether you can still write useful shaders. Some OpenGL experts I >>> talked with earlier claimed that this approach would be too >>> restrictive, but that's something that's easy to experiment with. >>> >>> Adam >> >>
Received on Wednesday, 14 December 2011 23:47:49 UTC