From 1ac2d2dcb6d1a18e86d86b8d03f46f879a43f33c Mon Sep 17 00:00:00 2001
From: Brecht Van Lommel <brecht@blender.org>
Date: Thu, 20 Jan 2022 22:31:21 +0100
Subject: [PATCH] Fix T93711: Cycles diffuse/glossy baking does not write alpha

With the change to use render passes internally the alpha channel got lost.
Add support for these render passes to output an alpha channel for baking.
---
 intern/cycles/integrator/pass_accessor.cpp |  7 +++++--
 intern/cycles/kernel/film/read.h           | 15 +++++++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/intern/cycles/integrator/pass_accessor.cpp b/intern/cycles/integrator/pass_accessor.cpp
index 4479442df56..9fa5aab9ea9 100644
--- a/intern/cycles/integrator/pass_accessor.cpp
+++ b/intern/cycles/integrator/pass_accessor.cpp
@@ -141,6 +141,7 @@ bool PassAccessor::get_render_tile_pixels(const RenderBuffers *render_buffers,
   const PassType type = pass_access_info_.type;
   const PassMode mode = pass_access_info_.mode;
   const PassInfo pass_info = Pass::get_info(type, pass_access_info_.include_albedo);
+  int num_written_components = pass_info.num_components;
 
   if (pass_info.num_components == 1) {
     /* Single channel passes. */
@@ -188,8 +189,10 @@ bool PassAccessor::get_render_tile_pixels(const RenderBuffers *render_buffers,
     else if ((pass_info.divide_type != PASS_NONE || pass_info.direct_type != PASS_NONE ||
               pass_info.indirect_type != PASS_NONE) &&
              mode != PassMode::DENOISED) {
-      /* RGB lighting passes that need to divide out color and/or sum direct and indirect. */
+      /* RGB lighting passes that need to divide out color and/or sum direct and indirect.
+       * These can also optionally write alpha like the combined pass. */
       get_pass_light_path(render_buffers, buffer_params, destination);
+      num_written_components = 4;
     }
     else {
       /* Passes that need no special computation, or denoised passes that already
@@ -215,7 +218,7 @@ bool PassAccessor::get_render_tile_pixels(const RenderBuffers *render_buffers,
     }
   }
 
-  pad_pixels(buffer_params, destination, pass_info.num_components);
+  pad_pixels(buffer_params, destination, num_written_components);
 
   return true;
 }
diff --git a/intern/cycles/kernel/film/read.h b/intern/cycles/kernel/film/read.h
index 18a593a75b1..ba895fd8909 100644
--- a/intern/cycles/kernel/film/read.h
+++ b/intern/cycles/kernel/film/read.h
@@ -214,6 +214,21 @@ ccl_device_inline void film_get_pass_pixel_light_path(
   pixel[0] = f.x;
   pixel[1] = f.y;
   pixel[2] = f.z;
+
+  /* Optional alpha channel. */
+  if (kfilm_convert->num_components >= 4) {
+    if (kfilm_convert->pass_combined != PASS_UNUSED) {
+      float scale, scale_exposure;
+      film_get_scale_and_scale_exposure(kfilm_convert, buffer, &scale, &scale_exposure);
+
+      ccl_global const float *in_combined = buffer + kfilm_convert->pass_combined;
+      const float alpha = in_combined[3] * scale;
+      pixel[3] = film_transparency_to_alpha(alpha);
+    }
+    else {
+      pixel[3] = 1.0f;
+    }
+  }
 }
 
 ccl_device_inline void film_get_pass_pixel_float3(ccl_global const KernelFilmConvert *ccl_restrict