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2022-09-27Cycles: add Path Guiding on CPU through Intel OpenPGLSebastian Herhoz
This adds path guiding features into Cycles by integrating Intel's Open Path Guiding Library. It can be enabled in the Sampling > Path Guiding panel in the render properties. This feature helps reduce noise in scenes where finding a path to light is difficult for regular path tracing. The current implementation supports guiding directional sampling decisions on surfaces, when the material contains a least one diffuse component, and in volumes with isotropic and anisotropic Henyey-Greenstein phase functions. On surfaces, the guided sampling decision is proportional to the product of the incident radiance and the normal-oriented cosine lobe and in volumes it is proportional to the product of the incident radiance and the phase function. The incident radiance field of a scene is learned and updated during rendering after each per-frame rendering iteration/progression. At the moment, path guiding is only supported by the CPU backend. Support for GPU backends will be added in future versions of OpenPGL. Ref T92571 Differential Revision: https://developer.blender.org/D15286
2022-09-23Cleanup: use lowercase function calls & macros in for CMakeCampbell Barton
This is already the case for most CMake usage. Although some find modules are an exception to this, as they were originally maintained externally they use some different conventions. Also corrected bad indentation in: intern/cycles/CMakeLists.txt
2022-09-13Cycles: Make OSL implementation independent from SVMPatrick Mours
Cleans up the file structure to be more similar to that of the SVM and also makes it possible to build kernels with OSL support, but without having to include SVM support. This patch was split from D15902. Differential Revision: https://developer.blender.org/D15949
2022-09-13Fix oneAPI compilation on modern Linux and CentOS 7 librariesSergey Sharybin
2022-09-02Cleanup: split surface/displacement/volume shader eval into separate filesBrecht Van Lommel
2022-09-02Cleanup: refactoring of kernel film function names and organizationBrecht Van Lommel
2022-08-24Merge branch 'blender-v3.3-release'Brecht Van Lommel
2022-08-24Fix WITH_CYCLES_ONEAPI_BINARIES issues with make releaseBrecht Van Lommel
Fix typo in blender_release.cmake, and ensure that "make release" still works when ocloc is not available. While a fatal error is useful for debugging, the current convention is to disable features, especially in cases like this where there is no simple way to make the feature work. Differential Revision: https://developer.blender.org/D15774
2022-08-19Cycles: add Sobol-Burley sampling patternNathan Vegdahl
Based on the paper "Practical Hash-based Owen Scrambling" by Brent Burley, 2020, Journal of Computer Graphics Techniques. It is distinct from the existing Sobol sampler in two important ways: * It is Owen scrambled, which gives it a much better convergence rate in many situations. * It uses padding for higher dimensions, rather than using higher Sobol dimensions directly. In practice this is advantagous because high-dimensional Sobol sequences have holes in their sampling patterns that don't resolve until an unreasonable number of samples are taken. (See Burley's paper for details.) The pattern reduces noise in some benchmark scenes, however it is also slower, particularly on the CPU. So for now Progressive Multi-Jittered sampling remains the default. Differential Revision: https://developer.blender.org/D15679
2022-08-09Merge branch 'blender-v3.3-release'Brecht Van Lommel
2022-08-09Cycles: make transform inverse match Embree exactlyBrecht Van Lommel
Helps improve ray-tracing precision. This is a bit complicated as it requires different implementation depending on the CPU architecture.
2022-08-09Cycles: add new Spectrum and PackedSpectrum typesAndrii Symkin
These replace float3 and packed_float3 in various places in the kernel where a spectral color representation will be used in the future. That representation will require more than 3 channels and conversion to from/RGB. The kernel code was refactored to remove the assumption that Spectrum and RGB colors are the same thing. There are no functional changes, Spectrum is still a float3 and the conversion functions are no-ops. Differential Revision: https://developer.blender.org/D15535
2022-08-08Merge branch 'blender-v3.3-release'Xavier Hallade
2022-08-07Cycles: re-enable zebin format for Intel GPUs on LinuxXavier Hallade
zebin format is critical for the compatibility of AoT graphics binaries across driver versions. It was previously disabled on Linux due to runtime issues that are now fixed in https://github.com/intel/compute-runtime/releases/tag/22.31.23852. The minimum supported driver version isn't bumped to this one yet as current codebase with current IGC compiler does actually run fine on earlier drivers and is not running into these issues anymore.
2022-07-28Cleanup: simplifications and consistency for vector typesBrecht Van Lommel
* OneAPI: remove separate float3 definition * OneAPI: disable operator[] to match other GPUs * OneAPI: make int3 compact to match other GPUs * Use #pragma once * Add __KERNEL_NATIVE_VECTOR_TYPES__ to simplify checks * Remove unused vector3
2022-07-25Cycles: add math functions for float8Andrii Symkin
This patch adds required math functions for float8 to make it possible using float8 instead of float3 for color data. Differential Revision: https://developer.blender.org/D15525
2022-07-25Cleanup: move device BVH code to kernel/device/*/bvh.hBrecht Van Lommel
Having the OptiX/MetalRT/Embree/MetalRT implementations all in one file with many #ifdefs became too confusing. Instead split it up per device, and also move it together with device specific hit/filter/intersect functions and associated data types.
2022-07-15Cycles: Apple Silicon optimization to specialize intersection kernelsMichael Jones
The Metal backend now compiles and caches a second set of kernels which are optimized for scene contents, enabled for Apple Silicon. The implementation supports doing this both for intersection and shading kernels. However this is currently only enabled for intersection kernels that are quick to compile, and already give a good speedup. Enabling this for shading kernels would be faster still, however this also causes a long wait times and would need a good user interface to control this. M1 Max samples per minute (macOS 13.0): PSO_GENERIC PSO_SPECIALIZED_INTERSECT PSO_SPECIALIZED_SHADE barbershop_interior 83.4 89.5 93.7 bmw27 1486.1 1671.0 1825.8 classroom 175.2 196.8 206.3 fishy_cat 674.2 704.3 719.3 junkshop 205.4 212.0 257.7 koro 310.1 336.1 342.8 monster 376.7 418.6 424.1 pabellon 273.5 325.4 339.8 sponza 830.6 929.6 1142.4 victor 86.7 96.4 96.3 wdas_cloud 111.8 112.7 183.1 Code contributed by Jason Fielder, Morteza Mostajabodaveh and Michael Jones Differential Revision: https://developer.blender.org/D14645
2022-07-15Cycles: keep track of SVM nodes used in kernelsMichael Jones
To be used for specialization in Metal, to automatically leave out unused nodes from the kernel. Ref D14645
2022-07-15Cycles: refactor to move part of KernelData definition to template headerBrecht Van Lommel
To be used for specialization on Metal in a following commit, turning these members into compile time constants. Ref D14645
2022-07-12Cycles: fix and enable JIT oneAPI CentOS7 builds for drivers 23570+Xavier Hallade
The current specific CentOS7 workaround we have for AoT, which is to disable __FAST_MATH__ by using -fhonor-nans, now also fixes the compilation issue for JIT as well since at least driver 23570.
2022-07-06Cycles oneAPI: Remove direct dependency on Level-ZeroXavier Hallade
We used it only to access device id for explicitly allowing Arc GPUs. It made the backend require ze_loader.dll which could be problematic if we end up using direct linking. I've replaced filtering based on PCI device id by using other HW properties instead (EUs, threads per EU), that are now available through Level-Zero.
2022-06-29Cycles: Add support for rendering on Intel GPUs using oneAPIXavier Hallade
This patch adds a new Cycles device with similar functionality to the existing GPU devices. Kernel compilation and runtime interaction happen via oneAPI DPC++ compiler and SYCL API. This implementation is primarly focusing on Intel® Arc™ GPUs and other future Intel GPUs. The first supported drivers are 101.1660 on Windows and 22.10.22597 on Linux. The necessary tools for compilation are: - A SYCL compiler such as oneAPI DPC++ compiler or https://github.com/intel/llvm - Intel® oneAPI Level Zero which is used for low level device queries: https://github.com/oneapi-src/level-zero - To optionally generate prebuilt graphics binaries: Intel® Graphics Compiler All are included in Linux precompiled libraries on svn: https://svn.blender.org/svnroot/bf-blender/trunk/lib The same goes for Windows precompiled binaries but for the graphics compiler, available as "Intel® Graphics Offline Compiler for OpenCL™ Code" from https://www.intel.com/content/www/us/en/developer/articles/tool/oneapi-standalone-components.html, for which path can be set as OCLOC_INSTALL_DIR. Being based on the open SYCL standard, this implementation could also be extended to run on other compatible non-Intel hardware in the future. Reviewed By: sergey, brecht Differential Revision: https://developer.blender.org/D15254 Co-authored-by: Nikita Sirgienko <nikita.sirgienko@intel.com> Co-authored-by: Stefan Werner <stefan.werner@intel.com>
2022-06-20Cleanup: renaming and consistency for kernel dataBrecht Van Lommel
* Rename "texture" to "data array". This has not used textures for a long time, there are just global memory arrays now. (On old CUDA GPUs there was a cache for textures but not global memory, so we used to put all data in textures.) * For CUDA and HIP, put globals in KernelParams struct like other devices. * Drop __ prefix for data array names, no possibility for naming conflict now that these are in a struct.
2022-05-04Nodes: Add general Combine/Separate Color nodesHallam Roberts
Inspired by D12936 and D12929, this patch adds general purpose "Combine Color" and "Separate Color" nodes to Geometry, Compositor, Shader and Texture nodes. - Within Geometry Nodes, it replaces the existing "Combine RGB" and "Separate RGB" nodes. - Within Compositor Nodes, it replaces the existing "Combine RGBA/HSVA/YCbCrA/YUVA" and "Separate RGBA/HSVA/YCbCrA/YUVA" nodes. - Within Texture Nodes, it replaces the existing "Combine RGBA" and "Separate RGBA" nodes. - Within Shader Nodes, it replaces the existing "Combine RGB/HSV" and "Separate RGB/HSV" nodes. Python addons have not been updated to the new nodes yet. **New shader code** In node_color.h, color.h and gpu_shader_material_color_util.glsl, missing methods hsl_to_rgb and rgb_to_hsl are added by directly converting existing C code. They always produce the same result. **Old code** As requested by T96219, old nodes still exist but are not displayed in the add menu. This means Python scripts can still create them as usual. Otherwise, versioning replaces the old nodes with the new nodes when opening .blend files. Differential Revision: https://developer.blender.org/D14034
2022-05-04Cycles: mark all CUDA 11.x versions as supportedBrecht Van Lommel
All released versions appear to work fine. Also slightly change wording.
2022-05-03CMake: fix error building when CUDA_NVCC_EXECUTABLE is missingCampbell Barton
2022-04-29Cleanup: fix various Cycles build warnings with non-default optionsBrecht Van Lommel
* Float/double promotion warnings were mainly meant for avoiding slow operatiosn in the kernel. Limit it to that to avoid hard to fix warnings in Hydra. * Const warnings in Hydra iterators. * Unused variable warnings when building without glog. * Wrong camera enum comparisons in assert. * PASS_UNUSED is not a pass type, only for pass offsets.
2022-04-06Add CMake option to control CUDA host compilerhowetuft
This revision allows to specify CUDA host compiler (nvcc's -ccbin command line option) when configuring the build. It addresses the case where the C/C++ compiler to be used in CUDA toolchain should be different from the default C/C++ compiler, for instance in case of compilers versions conflicts or multiple installed compilers. The new CMake option is named `CUDA_HOST_COMPILER` and can be used as follows: `cmake -DCUDA_HOST_COMPILER=<path-to-host-compiler>` If the option is not specified, the build configuration behaves as previously. Differential Revision: https://developer.blender.org/D14248
2022-04-01Cycles: approximate shadow caustics using manifold next event estimationOlivier Maury
This adds support for selective rendering of caustics in shadows of refractive objects. Example uses are rendering of underwater caustics and eye caustics. This is based on "Manifold Next Event Estimation", a method developed for production rendering. The idea is to selectively enable shadow caustics on a few objects in the scene where they have a big visual impact, without impacting render performance for the rest of the scene. The Shadow Caustic option must be manually enabled on light, caustic receiver and caster objects. For such light paths, the Filter Glossy option will be ignored and replaced by sharp caustics. Currently this method has a various limitations: * Only caustics in shadows of refractive objects work, which means no caustics from reflection or caustics that outside shadows. Only up to 4 refractive caustic bounces are supported. * Caustic caster objects should have smooth normals. * Not currently support for Metal GPU rendering. In the future this method may be extended for more general caustics. TECHNICAL DETAILS This code adds manifold next event estimation through refractive surface(s) as a new sampling technique for direct lighting, i.e. finding the point on the refractive surface(s) along the path to a light sample, which satisfies Fermat's principle for a given microfacet normal and the path's end points. This technique involves walking on the "specular manifold" using a pseudo newton solver. Such a manifold is defined by the specular constraint matrix from the manifold exploration framework [2]. For each refractive interface, this constraint is defined by enforcing that the generalized half-vector projection onto the interface local tangent plane is null. The newton solver guides the walk by linearizing the manifold locally before reprojecting the linear solution onto the refractive surface. See paper [1] for more details about the technique itself and [3] for the half-vector light transport formulation, from which it is derived. [1] Manifold Next Event Estimation Johannes Hanika, Marc Droske, and Luca Fascione. 2015. Comput. Graph. Forum 34, 4 (July 2015), 87–97. https://jo.dreggn.org/home/2015_mnee.pdf [2] Manifold exploration: a Markov Chain Monte Carlo technique for rendering scenes with difficult specular transport Wenzel Jakob and Steve Marschner. 2012. ACM Trans. Graph. 31, 4, Article 58 (July 2012), 13 pages. https://www.cs.cornell.edu/projects/manifolds-sg12/ [3] The Natural-Constraint Representation of the Path Space for Efficient Light Transport Simulation. Anton S. Kaplanyan, Johannes Hanika, and Carsten Dachsbacher. 2014. ACM Trans. Graph. 33, 4, Article 102 (July 2014), 13 pages. https://cg.ivd.kit.edu/english/HSLT.php The code for this samping technique was inserted at the light sampling stage (direct lighting). If the walk is successful, it turns off path regularization using a specialized flag in the path state (PATH_MNEE_SUCCESS). This flag tells the integrator not to blur the brdf roughness further down the path (in a child ray created from BSDF sampling). In addition, using a cascading mechanism of flag values, we cull connections to caustic lights for this and children rays, which should be resolved through MNEE. This mechanism also cancels the MIS bsdf counter part at the casutic receiver depth, in essence leaving MNEE as the only sampling technique from receivers through refractive casters to caustic lights. This choice might not be optimal when the light gets large wrt to the receiver, though this is usually not when you want to use MNEE. This connection culling strategy removes a fair amount of fireflies, at the cost of introducing a slight bias. Because of the selective nature of the culling mechanism, reflective caustics still benefit from the native path regularization, which further removes fireflies on other surfaces (bouncing light off casters). Differential Revision: https://developer.blender.org/D13533
2022-03-16Cleanup: remove some unused Cycles GPU codeBrecht Van Lommel
To make porting to other architectures easier, clarifying that this does not need to be supported. The unused parallel_reduce implementation assumed warp size 32, but is easy to update if we ever need it in the future.
2022-02-11Cycles: use SPDX license headersBrecht Van Lommel
* Replace license text in headers with SPDX identifiers. * Remove specific license info from outdated readme.txt, instead leave details to the source files. * Add list of SPDX license identifiers used, and corresponding license texts. * Update copyright dates while we're at it. Ref D14069, T95597
2022-02-11File headers: SPDX License migrationCampbell Barton
Use a shorter/simpler license convention, stops the header taking so much space. Follow the SPDX license specification: https://spdx.org/licenses - C/C++/objc/objc++ - Python - Shell Scripts - CMake, GNUmakefile While most of the source tree has been included - `./extern/` was left out. - `./intern/cycles` & `./intern/atomic` are also excluded because they use different header conventions. doc/license/SPDX-license-identifiers.txt has been added to list SPDX all used identifiers. See P2788 for the script that automated these edits. Reviewed By: brecht, mont29, sergey Ref D14069
2021-12-16Cycles: pointcloud renderingBrecht Van Lommel
This add support for rendering of the point cloud object in Blender, as a native geometry type in Cycles that is more memory and time efficient than instancing sphere meshes. This can be useful for rendering sand, water splashes, particles, motion graphics, etc. Points are currently always rendered as spheres, with backface culling. More shapes are likely to be added later, but this is the most important one and can be customized with shaders. For CPU rendering the Embree primitive is used, for GPU there is our own intersection code. Motion blur is suppored. Volumes inside points are not currently supported. Implemented with help from: * Kévin Dietrich: Alembic procedural integration * Patrick Mourse: OptiX integration * Josh Whelchel: update for cycles-x changes Ref T92573 Differential Revision: https://developer.blender.org/D9887
2021-11-29Cycles: MetalRT support (kernel side)Michael Jones
This patch adds MetalRT support to Cycles kernel code. It is mostly additive in nature or confined to Metal-specific code, however there are a few areas where this interacts with other code: - MetalRT closely follows the Optix implementation, and in some cases (notably handling of transforms) it makes sense to extend Optix special-casing to MetalRT. For these generalisations we now have `__KERNEL_GPU_RAYTRACING__` instead of `__KERNEL_OPTIX__`. - MetalRT doesn't support primitive offsetting (as with `primitiveIndexOffset` in Optix), so we define and populate a new kernel texture, `__object_prim_offset`, containing per-object primitive / curve-segment offsets. This is referenced and applied in MetalRT intersection handlers. - Two new BVH layout enum values have been added: `BVH_LAYOUT_METAL` and `BVH_LAYOUT_MULTI_METAL_EMBREE` for XPU mode). Some host-side enum case handling has been updated where it is trivial to do so. Ref T92212 Reviewed By: brecht Maniphest Tasks: T92212 Differential Revision: https://developer.blender.org/D13353
2021-11-19Cleanup: fix typos in comments and docsBrecht Van Lommel
Contributed by luzpaz. Differential Revision: https://developer.blender.org/D10447
2021-11-18Cycles: declare constants at program scope on MetalMichael Jones
MSL requires that constant address space literals be declared at program scope. This patch moves the `blackbody_table_r/g/b` and `cie_colour_match` constants into separate files so they can be declared at the appropriate scope. Ref T92212 Differential Revision: https://developer.blender.org/D13241
2021-11-18Merge branch 'blender-v3.0-release'Brecht Van Lommel
2021-11-18Fix T93045: Cycles HIP not rendering OpenVDB volumesBrecht Van Lommel
Build HIP kernels with NanoVDB, and patch NanoVDB to work with HIP. This is a header only library so no rebuild is needed. The changes are being submitted upstream to openvdb, so this patch should be temporary. Thanks Thomas for help testing this.
2021-11-17Fix Cycles CUDA/HIP compiler error after recent changesBrecht Van Lommel
2021-11-17Cycles: add build option to enable a debugging feature for MISSebastian Herholz
This patch adds a CMake option "WITH_CYCLES_DEBUG" which builds cycles with a feature that allows debugging/selecting the direct-light sampling strategy. The same option may later be used to add other debugging features that could affect performance in release builds. The three options are: * Forward path tracing (e.g., via BSDF or phase function) * Next-event estimation * Multiple importance sampling combination of the previous two methods Such a feature is useful for debugging light different sampling, evaluation, and pdf methods (e.g., for light sources and BSDFs). Differential Revision: https://developer.blender.org/D13152
2021-11-10Fix T92985: CUDA errors with Cycles film convert kernelsPatrick Mours
rB3a4c8f406a3a3bf0627477c6183a594fa707a6e2 changed the macros that create the film convert kernel entry points, but in the process accidentally changed the parameter definition to one of those (which caused CUDA launch and misaligned address errors) and changed the implementation as well. This restores the correct implementation from before. In addition, the `ccl_gpu_kernel_threads` macro did not work as intended and caused the generated launch bounds to end up with an incorrect input for the second parameter (it was set to "thread_num_registers", rather than the result of the block number calculation). I'm not entirely sure why, as the macro definition looked sound to me. Decided to simply go with two separate macros instead, to simplify and solve this. Also changed how state is captured with the `ccl_gpu_kernel_lambda` macro slightly, to avoid a compiler warning (expression has no effect) that otherwise occurred. Maniphest Tasks: T92985 Differential Revision: https://developer.blender.org/D13175
2021-11-10Cycles: Adapt shared kernel/device/gpu layer for MSLMichael Jones
This patch adapts the shared kernel entrypoints so that they can be compiled as MSL (Metal Shading Language). Where possible, the adaptations avoid changes in common code. In MSL, kernel function inputs are explicitly bound to resources. In the case of argument buffers, we declare a struct containing the kernel arguments, accessible via device pointer. This differs from CUDA and HIP where kernel function arguments are declared as traditional C-style function parameters. This patch adapts the entrypoints declared in kernel.h so that they can be translated via a new `ccl_gpu_kernel_signature` macro into the required parameter struct + kernel entrypoint pairing for MSL. MSL buffer attribution must be applied to function parameters or non-static class data members. To allow universal access to the integrator state, kernel data, and texture fetch adapters, we wrap all of the shared kernel code in a `MetalKernelContext` class. This is achieved by bracketing the appropriate kernel headers with "context_begin.h" and "context_end.h" on Metal. When calling deeper into the kernel code, we must reference the context class (e.g. `context.integrator_init_from_camera`). This extra prefixing is performed by a set of defines in "context_end.h". These will require explicit maintenance if entrypoints change. We invite discussion on more maintainable ways to enforce correctness. Lambda expressions are not supported on MSL, so a new `ccl_gpu_kernel_lambda` macro generates an inline function object and optionally capturing any required state. This yields the same behaviour. This approach is applied to all parallel_... implementations which are templated by operation. The lambda expressions in the film_convert... kernels don't adapt cleanly to use function objects. However, these entrypoints can be macro-generated more concisely to avoid lambda expressions entirely, instead relying on constant folding to handle the pixel/channel conversions. A separate implementation of `gpu_parallel_active_index_array` is provided for Metal to workaround some subtle differences in SIMD width, and also to encapsulate some required thread parameters which must be declared as explicit entrypoint function parameters. Ref T92212 Reviewed By: brecht Maniphest Tasks: T92212 Differential Revision: https://developer.blender.org/D13109
2021-10-26Fix Cycles runtime GPU kernel compilation after recent refactorBrecht Van Lommel
2021-10-26Cycles: remove prefix from source code file namesBrecht Van Lommel
Remove prefix of filenames that is the same as the folder name. This used to help when #includes were using individual files, but now they are always relative to the cycles root directory and so the prefixes are redundant. For patches and branches, git merge and rebase should be able to detect the renames and move over code to the right file.
2021-10-26Cycles: changes to source code folders structureBrecht Van Lommel
* Split render/ into scene/ and session/. The scene/ folder now contains the scene and its nodes. The session/ folder contains the render session and associated data structures like drivers and render buffers. * Move top level kernel headers into new folders kernel/camera/, kernel/film/, kernel/light/, kernel/sample/, kernel/util/ * Move integrator related kernel headers into kernel/integrator/ * Move OSL shaders from kernel/shaders/ to kernel/osl/shaders/ For patches and branches, git merge and rebase should be able to detect the renames and move over code to the right file.
2021-10-22Fix Cycles HIP binaries always recompilingBrecht Van Lommel
2021-10-22Cycles: various fixes for HIP and compilation of HIP binariesSayak Biswas
* Additional structs added to the hipew loader for device props * Adds hipRTC functions to the loader for future usage * Enables CPU+GPU usage for HIP * Cleanup to the adaptive kernel compilation process * Fix for kernel compilation failures with HIP with latest master Ref T92393, D12958
2021-10-19Cycles: decouple shadow paths from main path on GPUBrecht Van Lommel
The motivation for this is twofold. It improves performance (5-10% on most benchmark scenes), and will help to bring back transparency support for the ambient occlusion pass. * Duplicate some members from the main path state in the shadow path state. * Add shadow paths incrementally to the array similar to what we do for the shadow catchers. * For the scheduling, allow running shade surface and shade volume kernels as long as there is enough space in the shadow paths array. If not, execute shadow kernels until it is empty. * Add IntegratorShadowState and ConstIntegratorShadowState typedefs that can be different between CPU and GPU. For GPU both main and shadow paths juse have an integer for SoA access. Bt with CPU it's a different pointer type so we get type safety checks in code shared between CPU and GPU. * For CPU, add a separate IntegratorShadowStateCPU struct embedded in IntegratorShadowState. * Update various functions to take the shadow state, and make SVM take either type of state using templates. Differential Revision: https://developer.blender.org/D12889
2021-10-11Cycles: restore Christensen-Burley SSSBrecht Van Lommel
There is not enough time before the release to improve Random Walk to handle all cases this was used for, so restore it for now. Since there is no more path splitting in cycles-x, this can increase noise in non-flat areas for the sample number of samples, though fewer rays will be traced also. This is fundamentally a trade-off we made in the new design and why Random Walk is a better fit. However the importance resampling we do now does help to reduce noise. Differential Revision: https://developer.blender.org/D12800