[Bf-cycles] Cycles development newbie
Lukas Stockner
lukas.stockner at freenet.de
Sun Feb 11 20:21:12 CET 2018
Hi,
regarding the overall layout, there is a Wiki article
(https://wiki.blender.org/index.php/Dev:Source/Render/Cycles/SourceLayout)
but it doesn't really say much, so here's my attempt on an overview:
Overall, Cycles can be divided into four layers:
- Kernel code - the computations that are executed on the chosen device,
doing the actual rendering
- Device code - handles memory management and is able to execute tasks
on the different compute devices by calling the kernel
- "Host" code - handles data synchronization to the device and control
flow during rendering, exposes an API
- Blender code - Glue logic between Blender and Cycles, synchronizes
data from Blender into the Cycles-specific formats
Of those four, the first three are independent of Blender and also used
in e.g. the standalone renderer.
When you hit F12, Blender internally starts a rendering job (not going
into details here). Effectively, it ends up calling functions in the
Cycles addon (blender/addon/, all paths relative to intern/cycles/).
These in turn call the Cycles Python API (as defined in
blender/blender_python.cpp), which wraps the BlenderSession (in
blender/blender_session.cpp).
This Blender session allocates a Cycles Session (render/session.cpp) and
Scene (render/scene.cpp) and then fills the Cycles-internal scene
representation (consisting of Shaders, Meshes, Lights etc., all defined
in render/ and part of the Scene class) from Blender-internal data
through the code in BlenderSync (blender/blender_sync.cpp).
Then, it calls the Session to start the rendering. The Session in turn
calls the internal Scene synchronization, which then loops over all the
components of a Scene and calls their device_update functions, which are
responsible for converting the internal host representation into the
format that the kernel expects. Once that is done, it divides the image
into tiles (render/tile.cpp) and then starts a DeviceTask
(device/device_task.cpp) for rendering.
The device (device/device_{cpu, cuda, opencl, multi}.cpp) then calls the
acquire_tile callback of the task, which makes the Session return tiles
to render. Then, it calls the rendering kernel for that particular
device until the tile is finished, and goes back to acquiring another
tile until they're all rendered.
In the kernel, most of the code is shared between devices, which means
that we can only use the common subset of C++, CUDA and OpenCL (which is
generally the limiting factor). To archieve this, whenever any language
needs a particular keyword, a ccl_ keyword is defined. Then, for each of
the three languages, these ccl_keywords are #defined to the proper
keywords in kernel/kernel_compat_<device>.h. For example, CUDA requires
each function that's supposed to be executed on the GPU to be prefixed
by __device__. Therefore, we have the ccl_device keyword, which gets
#defined to __device__ for CUDA and to nothing for CPU and OpenCL. This
is all there is to the "cycles compute language" - it's essentially just
a bunch of defines that allows to write code that compiles in all three
languages.
As a result, for many changes you can completely ignore the fact that
your code will also be compiled on GPUs. The main downside is that it
completely screws up code completion in most IDEs.
Also, here's the wiki article on the kernel language (a bit outdated
unfortunately):
https://wiki.blender.org/index.php/Dev:Source/Render/Cycles/Kernel
Generally, to get an overview of the control flow, I'd recommend looking at:
- BlenderSession::create_session, BlenderSession::reset_session,
BlenderSession::render
- BlenderSync in general
- Session::run_cpu
- Scene::device_update
- CPUDevice::thread_render
- Session::acquire_tile
- kernel_path_trace
And ultimately, the best approach for figuring out control flow of
course always is searching and a debugger.
Hope this helps :)
- Lukas
On 11.02.2018 13:09, Bob Geller wrote:
> Hi,
>
> My name is Bob and I'm trying to get into cycles development to
> hopefully contribute in the future. Is there any documentation to
> understand the system better and especially understand the flow of
> control in the renderer? Also, I'm struggling to find enough info
> about cycles kernel development and the cycles compute language. I
> would appreciate if someone can point me in the right direction to
> help understand this better. Thank you for your time and sorry for
> mailing the whole group, I wasn't sure who to contact.
>
> Regards,
> Bob
>
>
> _______________________________________________
> Bf-cycles mailing list
> Bf-cycles at blender.org
> https://lists.blender.org/mailman/listinfo/bf-cycles
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