[doc] Updating README.md and CHANGELOG.md

CHANGELOG.md

@@ -2,32 +2,158 @@
 
 ## Open PGL 0.7.0
 
-- New (Experimental) Features:
-  - Radiance Caching:
-  - Guided/Adjoint-driven Russian Roulette:
-    - Stochastical path termination does not only imp
-    - `openpgl::cpp::util::StandardThroughputBasedRussianRoulette`
-      `openpgl::cpp::util::GuidedRussianRoulette` and
-      `#include <openpgl/cpp/RussianRoulette.h>`
-  - Image-space guiding buffer:
-    - `openpgl::cpp::util::ImageSpaceGuidingBuffer`
-      `#include <openpgl/cpp/ImageSpaceGuidingBuffer.h>`
-      `OPENPGL_EF_IMAGE_SPACE_GUIDING_BUFFER`
-- Api changes:
-  - `SampleData`
-    - New flag enum `EDirectLight`
-  - `pgl_direction` `SampleData`
-  - `pgl_spectrum` `SampleData`
-  - `ZeroValueSampleData`
-  - `SampleStorage`
-    - `AddZeroValueSample` and `AddZeroValueSamples`
-    - `GetSizeZeroValueSurface` and `GetSizeZeroValueVolume`
+- New (**Experimental**) Features:
+
+  - Radiance Caching (RC):
+    - If RC is enabled, the guiding structure (i.e., `Field`) learns an
+      approximation of multiple radiance quantities (in linear RGB)
+      ,such as outgoing and incoming radiance, irradiance, fluence, and
+      in-scattered radiance. These quantities can be queried using the
+      `SurfaceSamplingDistribution` and `VolumeSamplingDistribution`
+      classes. RC support can be enabled using the
+      `OPENPGL_EF_RADIANCE_CACHES` CMake option. **Note:** Since the RC
+      quantities are Monte-Carlo estimates, zero-value samples
+      (`ZeroValueSampleData`) that are generated during
+      rendering/training have to be passed/stored in the `SampleStorage`
+      as well.
+  - Guided/Adjoint-driven Russian Roulette (GRR):
+    - The information stored in radiance caches can be used to optimize
+      stochastic path termination decisions (a.k.a. Russian roulette) to
+      avoid a significant increase in variance (i.e., noise) caused by
+      early terminations, which can occur when using standard
+      throughput-based RR strategies. We, therefore, added to example
+      implementation for guided
+      (`openpgl::cpp::util::GuidedRussianRoulette(...)`) and standard
+      (`openpgl::cpp::util::StandardThroughputBasedRussianRoulette(...)`)
+      RR, which can be found in the `openpgl/cpp/RussianRoulette.h`
+      header.
+  - Image-space guiding buffer (ISGB):
+    - The ISGB can be used to store and approximate per-pixel guiding
+      information (e.g., a pixel estimate used in guided Russian
+      roulette). The ISGB class
+      (`openpgl::cpp::util::ImageSpaceGuidingBuffer`) is defined in the
+      `openpgl/cpp/ImageSpaceGuidingBuffer.h` header file. The support
+      can be enabled using the `OPENPGL_EF_IMAGE_SPACE_GUIDING_BUFFER`
+      CMake option.
+
+- API changes:
+
+  - `pgl_direction`: A new **wrapper** type for directional data. When
+    using C++ `pgl_direction` can directly be assigned by and to
+    `pgl_vec3f`.
+  - `pgl_spectrum`: A new **wrapper** type for spetral (i.e., linear
+    RGB) data. When using C++ `pgl_spectrum` can directly be assigned by
+    and to `pgl_vec3f`.
+  - `SampleData`:
+    - New enum `EDirectLight` flag that identifies if the radiance
+      stored in this sample comes directly from an emitter (e.g.,
+      emissive surface, volume, or light source).
+    - `direction`: Changes the type `pgl_vec3f` to `pgl_direction`.
+  - `ZeroValueSampleData`: This new structure is a simplified and more
+    compact representation of the `SampleData` struct representing a
+    zero-value sample. It contains the following members:
+    - `position`: The position of the sample (type `pgl_point3f`).
+    - `direction`: The incoming direction of the sample (type
+      `pgl_direction`).
+    - `volume`: If the sample is a volume sample (type `bool`).
+  - `SampleStorage`: To add, query, and get the number of
+    `ZeroValueSampleData`, the following functions were added.
+    - `AddZeroValueSample` and `AddZeroValueSamples`: These functions
+      add one or multiple `ZeroValueSampleData`.
+    - `GetSizeZeroValueSurface` and `GetSizeZeroValueVolume`: These
+      functions return the number of collected/stored surface or volume
+      `Ze1roValueSampleData`.
+    - `GetZeroValueSampleSurface` and `GetZeroValueSampleVolume`: Return
+      a given `ZeroValueSampleData` from either the surface or volume
+      storage.
+
+- API changes (`OPENPGL_EF_RADIANCE_CACHES=ON`): When the RC feature is
+  enabled, additional functions and members are available for the
+  following structures:
+
+  - `SurfaceSamplingDistribution`:
+    - `IncomingRadiance`: The incoming radiance estimate arriving at the
+      current cache position from a specific direction.
+    - `OutgoingRadiance`: The outgoing radiance at the current cache
+      position to a specific direction.
+    - `Irradiance`: The irradiance at the current cache position and for
+      a given surface normal.
+  - `VolumeSamplingDistribution`:
+    - `IncomingRadiance`: The incoming radiance estimate arriving at the
+      current cache position from a specific direction.
+    - `OutgoingRadiance`: The outgoing radiance at the current cache
+      position to a specific direction.
+    - `InscatteredRadiance`: The in-scattered radiance at the current
+      cache position to a specific direction and for a given HG mean
+      cosine.
+    - `Fluence`: The volume fluence at the current cache position.
+  - `SampleData`:
+    - `radianceIn`: The incoming radiance arriving at the sample
+      position from `direction` (type `pgl_spectrum`).
+    - `radianceInMISWeight`: The MIS weight of the `radianceIn` if the
+      source of it is a light source, if not it is `1.0` (type `float`).
+    - `directionOut`: The outgoing direction of the sample (type
+      `pgl_direction`).
+    - `radianceOut`: The outgoing radiance estimate of the sample (type
+      `pgl_direction`).
+
+  `ZeroValueSampleData`: - `directionOut`: The outgoing direction of the
+  sample (type `pgl_direction`).
+
+- API changes (`OPENPGL_EF_IMAGE_SPACE_GUIDING_BUFFER=ON`): When the
+  ISGB feature is enabled, additional functions and members are
+  available for the following structures:
+
+  - `ImageSpaceGuidingBuffer`: This is the main structure for storing
+    image-space, per-pixel guiding information approximated from pixel
+    samples. -`AddSample`: Add a pixel sample of type
+    `ImageSpaceGuidingBuffer::Sample` to the buffer.
+    - `Update`: Updates the image-space guiding
+      information/approximations from the previously collected samples
+      (e.g., denoises the pixel contribution estimates using OIDN). For
+      efficiency reasons, it makes sense not to update the buffer after
+      every rendering progression but in an exponential fashion (e.g.,
+      at progression `2^0`,`2^1`,…,`2^N`).
+    - `IsReady`: If the ISGB is ready (i.e., at least one `Update` step
+      was performed).
+    - `GetPixelContributionEstimate`: Returns the pixel contibution
+      estimate for a given pixel, which can be used, for example, for
+      guided RR.
+    - `Reset`: Resets the ISGB.
+  - `ImageSpaceGuidingBuffer::Sample`: This structure is used to store
+    information about a per-pixel sample that is passed to the ISGB.
+    - `contribution`: The contribution estimate of the pixel value of a
+      given sample (type `pgl_vec3f`).
+    - `albedo`: The albedo of the surface or the volume at the first
+      scattering event (type `pgl_vec3f`).
+    - `normal`: The normal at the first surface scattering event or the
+      ray dairection towards the camers if the first event is a volume
+      event (type `pgl_vec3f`).
+    - `flags`: Bit encoded information about the sample (e.g., if the
+      first scattering event is a volume event `Sample::EVolumeEvent`).
+
 - Optimizations:
-  - Compression for spectral and directional:
-    - `OPENPGL_DIRECTION_COMPRESSION`
-    - `OPENPGL_RADIANCE_COMPRESSION`
+
+  - Compression for spectral and directional: To reduce the size of the
+    `SampleData` and `ZeroValueSampleData` data types it is possible to
+    enable 32-Bit compression, which is mainly adviced when enabling the
+    RC feature via `OPENPGL_EF_RADIANCE_CACHES=ON`.
+    - `OPENPGL_DIRECTION_COMPRESSION`: Enables 32-Bit compression for
+      `pgl_direction`.
+    - `OPENPGL_RADIANCE_COMPRESSION`: Enables 32-Bit compression for
+      `pgl_spectrum`.
+
 - Bugfixes:
-  - Numerical accuaracy problem
+
+  - Numerical accuracy problem during sampling when using parametric
+    mixtures.
+
+- Platform support:
+
+  - Added support for Windows on ARM (by [Anthony
+    Roberts](https://github.com/anthony-linaro)
+    [PR17](https://github.com/RenderKit/openpgl/pull/17)). **Note:**
+    Requires using LLVM and `clang-cl.exe` as C and C++ compiler.
 
 ## Open PGL 0.6.0