@Philipp Oeser (lichtwerk) @Jeroen Bakker (jbakker) Thanks for investigating this more. I also remember at some point when I was accessing the pixels directly through "Viewer Node" image block and stored them on disk. What I realized however was the pixels were sort of rotated. I remember the depth map rendering that I got was shown 90 degrees rotated. So I have a request from you: I would appreciate if you can provide a nice Python API that allows people to easily access the rendering results without having to store the renderings on disk. Maybe bpy.ops.render.render() could take an argument like keepInMemory=True. Then people can access the rendering results through something like renderings = np.array(bpy.ops.render.results). The should also account for having a couple of output nodes. For instance, if in the node editor I have 5 Viewer nodes, renderings should be a 5 x 4 x resolution x resolution tensor.

@Brecht Van Lommel (brecht) Just an update: I can get somehow around the forking issue using subprocess and doing one rendering at the time. However, the problem with using subprocess is I have to literally import everything I need from scratch. This makes the rendering much more time-consuming than doing the rendering normally. Sorry if it's too obvious that I'm just trying to convince you to prioritize this :) I hope that happens :)

This issue on PyTorch GitHub page might be helpful for the developers: https://github.com/pytorch/pytorch/issues/6194

@Brecht Van Lommel (brecht) I hope you will prioritize this in the future. Many people like me need to use Blender as a module for their research work. Anyways, I used gdb to get you the back trace after Python freezes. Here's what I get (I had to CTRL+C at the end):

@Brecht Van Lommel (brecht) I'm not sure if this could be very relevant but I used OpenEXR Python bindings package to do some tests. Here's what I found out: if I have a function that simply loads an exr file and separates its channels into (R, G,B) as shown below and I execute this function via multiprocessing.Process everything works fine. However, when I just do import bpy and execute the same function via multiprocessing.Process things do not work and Python freezes. I would say this is a bug in Blender. Could you please look into this? I would really appreciate it.

@Brecht Van Lommel (brecht) I also posted an issue on OpenEXR's GitHub repo (link here). Hopefully they can also make some clarification from their side.

@Brecht Van Lommel (brecht) You said it wouldn't surprise you if some parts of Blender of OpenEXr does not work well with forking. So I wonder, which one do you think is more likely to be the case given that I can store my renderings in other formats? Would you think it's more likely that OpenEXR is the one to blame here?

@Brecht Van Lommel (brecht) I tried with spawn but I keep getting weird errors and things do not work. It seems that I have to somehow make the process spawnable and I don't know how to do that precisely. I hope you can find a solution for this. It's so annoying for me since I have to millions of renderings and I didn't expect to do it sequentially :(

@Brecht Van Lommel (brecht) Sorry for the missing information. I just realized that the problem is being caused if I call the rendering function using multiprocessing.Process and updated the script in my bug report that shows how exactly I do rendering in my project, with all other irrelevant details removed. The strange thing is I can easily render my meshes if I do not want the rendering result to be stored in EXR files. So, if I replace OPEN_EXR with PNG and replace 32 with 8/16 I will definitely get the rendering results stored on disk. This does not happen when trying to store the result in OpenEXR format and I did not know that the multiprocessing package might be causing it. What should I do now? Why I cannot store EXR files when using multiprocessing.Process?

@Brecht Van Lommel (brecht) Sorry I should have been more clear. The problem is I am compiling Blender as Python module in Ubuntu 16.04. I don't think it is possible to import the Blender module given those arguments you mentioned. Could you please double-check this as well?