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Re: [Public WebGL] Proposed change to WebGL spec section 4.2 (Security Origin Restrictions)



2010/10/7 Kenneth Waters <kwaters@google.com>:
>> Alpha test happens after the fragment shader AFAIK, so I think it
>>
>> shouldn't have an effect on the execution time.
>
> There is no alpha test in WebGL, you use discard in the shader to achieve
> the same effect.
>
>>
>> Are there other possible leaks? Ways to use texture values to control the
>> amount of fragments a fragment shader is called on.
>
> You can radically effect render time by controlling texture access patterns.
>  If you have a bit of information you want to leak in "bit", draw a bunch of
> full screen point sprites and draw with,
> medp vec2 coord = gl_PointCoord;
> coord = texture2D(random_tex, coord).xy * bit;
> coord = texture2D(random_tex, coord).xy * bit;
> coord = texture2D(random_tex, coord).xy * bit;
> gl_FragColor = vec4(coord.rg, 0., 1.);
> If bit is 1. the shader will be memory bandwidth limited (virtually every
> texture hit will be a cache miss), if bit is 0. the shader will be compute
> limited.

Oh nice, that's very hard to defend against. Guess the non-SO textures
should be banned.

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