Cycles: Add new Sky Texture method including direct sunlight

This commit adds a new model to the Sky Texture node, which is based on a
method by Nishita et al. and works by basically simulating volumetric
scattering in the atmosphere.

By making some approximations (such as only considering single scattering),
we get a fairly simple and fast simulation code that takes into account
Rayleigh and Mie scattering as well as Ozone absorption.

This code is used to precompute a 512x128 texture which is then looked up
during render time, and is fast enough to allow real-time tweaking in the
viewport.

Due to the nature of the simulation, it exposes several parameters that
allow for lots of flexibility in choosing the look and matching real-world
conditions (such as Air/Dust/Ozone density and altitude).

Additionally, the same volumetric approach can be used to compute absorption
of the direct sunlight, so the model also supports adding direct sunlight.
This makes it significantly easier to set up Sun+Sky illumination where
the direction, intensity and color of the sun actually matches the sky.

In order to support properly sampling the direct sun component, the commit
also adds logic for sampling a specific area to the kernel light sampling
code. This is combined with portal and background map sampling using MIS.

This sampling logic works for the common case of having one Sky texture
going into the Background shader, but if a custom input to the Vector
node is used or if there are multiple Sky textures, it falls back to using
only background map sampling (while automatically setting the resolution to
4096x2048 if auto resolution is used).

More infos and preview can be found here:
https://docs.google.com/document/d/1gQta0ygFWXTrl5Pmvl_nZRgUw0mWg0FJeRuNKS36m08/view

Underlying model, implementation and documentation by Marco (@nacioss).
Improvements, cleanup and sun sampling by @lukasstockner.

Differential Revision: https://developer.blender.org/D7896

1 files changed, 95 insertions, 0 deletions

diff --git a/intern/cycles/render/image_sky.cpp b/intern/cycles/render/image_sky.cpp
new file mode 100644
index 00000000000..3e7b491f609
--- /dev/null
+++ b/intern/cycles/render/image_sky.cpp
@@ -0,0 +1,95 @@
+/*
+ * Copyright 2011-2020 Blender Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "render/image_sky.h"
+
+#include "util/util_image.h"
+#include "util/util_logging.h"
+#include "util/util_path.h"
+#include "util/util_sky_model.h"
+
+CCL_NAMESPACE_BEGIN
+
+SkyLoader::SkyLoader(
+    float sun_elevation, int altitude, float air_density, float dust_density, float ozone_density)
+    : sun_elevation(sun_elevation),
+      altitude(altitude),
+      air_density(air_density),
+      dust_density(dust_density),
+      ozone_density(ozone_density)
+{
+}
+
+SkyLoader::~SkyLoader(){};
+
+bool SkyLoader::load_metadata(ImageMetaData &metadata)
+{
+  metadata.width = 512;
+  metadata.height = 128;
+  metadata.channels = 3;
+  metadata.depth = 1;
+  metadata.type = IMAGE_DATA_TYPE_FLOAT4;
+  metadata.compress_as_srgb = false;
+  return true;
+}
+
+bool SkyLoader::load_pixels(const ImageMetaData &metadata,
+                            void *pixels,
+                            const size_t /*pixels_size*/,
+                            const bool /*associate_alpha*/)
+{
+  /* definitions */
+  int width = metadata.width;
+  int height = metadata.height;
+  float *pixel_data = (float *)pixels;
+  float altitude_f = (float)altitude;
+
+  /* precompute sky texture */
+  const int num_chunks = TaskScheduler::num_threads();
+  const int chunk_size = height / num_chunks;
+  TaskPool pool;
+  for (int chunk = 0; chunk < num_chunks; chunk++) {
+    const int chunk_start = chunk * chunk_size;
+    const int chunk_end = (chunk + 1 < num_chunks) ? (chunk + 1) * chunk_size : height;
+    pool.push(function_bind(&nishita_skymodel_precompute_texture,
+                            pixel_data,
+                            metadata.channels,
+                            chunk_start,
+                            chunk_end,
+                            width,
+                            height,
+                            sun_elevation,
+                            altitude_f,
+                            air_density,
+                            dust_density,
+                            ozone_density));
+  }
+  pool.wait_work();
+
+  return true;
+}
+
+string SkyLoader::name() const
+{
+  return "sky_nishita";
+}
+
+bool SkyLoader::equals(const ImageLoader & /*other*/) const
+{
+  return false;
+}
+
+CCL_NAMESPACE_END