# Identify difference in lighting between sheep it and local render in these two images

This image is the result of the exact same file, one rendered on my computer and the other on sheep-it. The quality of lighting in the sheep-it version is superior, but I cannot understand what it actually is.

Rendered locally:

Rendered with sheep-it:

Animated gif of the above (6.8M download)

The local render is GPU (CUDA) but I tried a small part of it on the CPU (because it would take weeks to try rendering it all) and the lighting is the same. I have also tried Windows on the same computer and a Mac.

For a specific difference look at this detail. There is distinct difference in the shadows, as is on the upper bar shelf.

Reducing the intensity of the overhead lights does introduce those shadows but changes the overall image in other ways. Plus changing the value of one node in one material doesn't make sense. It's probably some global variable that is different.

Blender version is 2.78a downloaded from blender.org or the Thomas Schiex ppa (same result).

• I'm thinking colour management - maybe different gamma? Look in the Scene properties under colour management. What settings do you have there? Do you have a non-standard device like having the filmic addon installed? Could the sheep-it blender not have OCIO in the build? – sambler Dec 25 '16 at 3:27
• I do. And that's what I thought at first too, so I removed it. The images above are without filmic (see the saturated spotlight cone over the counter — with filmic it becomes almost white). I also tried with all the different settings and it just becomes lighter-darker, no difference in the shadows. Exposure is 0 and Gamma is 1 in the file. – qwazix Dec 25 '16 at 10:35
• To me it looks like sheep-it does more or more accurate indirect bounces. – VSB Jan 27 '17 at 4:03
• I'll try tweaking that and report back. – qwazix Jan 27 '17 at 8:08

Doing a difference of the two images is a little more revealing:

What do you notice? For one, the light fixtures themselves are black. And there are no reflections of lights, only of other objects.Further, the direct shine from a light is seen through the glass (center right), but nowhere else. My guess is something in the sheep-it version has more indirect light. I'm not sure WHY this is the case, (I would expect it to render the file as is) but it may be using more diffuse/glossy bounces, or have increased/disabled the threshold for the "clamp indirect" setting.

• If not overall indirect, is it plausible it is a different transfer function being applied? The highest emissions look identical, yet the darker regions might be lifted? – troy_s Dec 24 '16 at 18:20
• @JtheNinja Hm... testing now. At first impression it seems you are correct. – qwazix Dec 24 '16 at 18:29
• @troy_s I have a curve under color management but I also tried without and the image still isn't the same. I don't think any amount of curves can change the relationship of the shadows as it happens in the detail above. – qwazix Dec 24 '16 at 18:29
• @qwazix Curves will influence the relationship of the shadow. The reason is that the display referred transform will bend the ratios more or less dramatically based on whatever transform is in use. In this case, I suspect JTheNinja may well be closer to the solution. There are a few different contexts here though. – troy_s Dec 24 '16 at 19:10
• @qwazix How are you encoding the render on Sheepit? – troy_s Dec 24 '16 at 19:34

## Found it!

Sheep it! instead of splitting a frame to parts and stitching, it splits the sample count and averages. And as @YOLOonline said here averaging different sample seeds isn't equivalent to just rendering with the total samples.

Kinda proof: I rendered 157 equal frames at 20% resolution and averaged them. I resized both the sheep-it image and the locally rendered image to 20% with gimp and here are the differences with the averaged frame.

The sheepit - averaged image is almost completely black except near edges (which are artifacts of resizing) while the local-averaged image looks like the local-sheepit image JtheNinja posted

I believe that the edges will not be visible if I render the 156 frames in full resolution but I don't intend to waste that much processing power just to make sure.

Now the question becomes, why?. I thought that cycles sampling was like dice roll, and the previous result didn't influence the next.