[Bf-cycles] Environment/lightprobe importance sampling patch

[Mike Farnsworth]

I've updated the patch to include the task split (max of 16384 shader
evaluations per task), as well as turn off the background
sample_as_light switch by default to not adversely impact performance
for those who haven't set a custom environment.  I don't know of any
other outstanding issues.

-Farny

On Tue, Jan 17, 2012 at 6:18 AM, Brecht Van Lommel
<brechtvanlommel at pandora.be> wrote:
> Great work! I'll do a proper review of it later.
>
> Regarding the task size, there's a function to split up tasks in the
> device code. It's used in device_opencl.cpp, task_add, if there's a
> CUDA issue with maximum size being exceeded a similar thing could be
> done there.
>
> Brecht.
>
> On Mon, Jan 16, 2012 at 6:16 PM, Mike Farnsworth
> <mike.farnsworth at gmail.com> wrote:
>> Someone tried the patch and had a crash when using an importance
>> resolution greater than 1023.  (It's really just eating it with a
>> cuda_assert(cuLaunchGrid(cuDisplace, xblocks, 1)) when launching the
>> shader task.)  I had the same issue with my GTX 260, and I thought it
>> was just because that was an old(er) graphics card.  On my GTX 460 it
>> doesn't seem to have any issues with the importance resolution.  If
>> the GTX 560 Ti is crashing too, though, then I should probably to do
>> something about it.
>>
>> The issue is that when running a cuda task, you can't have 65536
>> blocks in any dimension.  We use a block size of 16, and with a
>> resolution of 1024x1024, that results in 65536 blocks in the X
>> direction.  The near-term workaround is to keep the resolution under
>> 1024, perhaps limited in the UI.
>>
>> The long-term solution is that I need to break up the background
>> evaluation task into, say, chunks of 128x128 and go through them one
>> at a time in order to cover the whole background.
>>
>> -Farny
>>
>> On Sun, Jan 15, 2012 at 11:35 PM, Mike Farnsworth
>> <mike.farnsworth at gmail.com> wrote:
>>> This patch includes a full implementation of importance sampling for
>>> environments, including the MIS solution we talked about.  It adds two
>>> items for the user to control: on/off switch for whether to treat the
>>> environment as a light (off is old behavior), and a resolution control
>>> for the importance map.  Here are a few notes about the patch:
>>>
>>> * For environments that are a solid color (or very slowly changing),
>>> turning on the env light machinery is overkill, and will actually
>>> result in reduced performance.  For anything complex (textures, sky,
>>> noise, etc) it produces noticeably less noise for the same number of
>>> passes, so it should be a solid advantage.
>>>
>>> * Any HDR environment textures are chopped down to SDR range by the
>>> texture upload to device right now (nothing to do with my patch, of
>>> course); this reduces the effectiveness of this new environment
>>> sampling.  If we can fix that, the env light sampling will have an
>>> even greater effect, not to mention potentially improved visuals in
>>> general.  Just sayin'...
>>>
>>> * The patch includes a change where the displacement device function
>>> now outputs to a float4 buffer, not a float3 buffer, because on my
>>> older GPU that was causing completely screwed up sampling of the
>>> background when building the importance map.  So, displacement and
>>> shader tasks now have a slightly larger output buffer, but it now
>>> works on both my older GPU (GTX 260, sm_13) and my newer GPU (GTX 460,
>>> sm_21).
>>>
>>> * There is a commented #define that allows dumping the sampled
>>> background and both parts of the importance map to /tmp for debugging
>>> purposes.  Those sections of code can be removed, but they could come
>>> in handy in the future.
>>>
>>> I'm going to post some before/after images to morecycles.blogspot.com
>>> tonight, so you can see the difference in quality.  I apologize for
>>> this taking longer than I had anticipated.  I had left in a
>>> __int_as_float(f) where I should have just had an (int)f instead, and
>>> it manifested itself with a zero PDF for the environment light, just
>>> causing noise.  It took a while to dig through it and find it.
>>> Brecht, let me know if you have any issues with the code, and I'll try
>>> to turn around any suggestions you make ASAP so this can be included
>>> in 2.62.  I understand the deadline is the 22nd?
>>>
>>> -Farny
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