Compositor: Constant folding

Currently there is no clear way to know if an operation is constant
(i.e. when all rendered pixels have same values). Operations may 
need to get constant input values before rendering to determine 
their resolution or areas of interest. This is the case of scale, rotate
and translate operations. Only "set operations" are  known as 
constant but many more are constant when all their inputs are so.
Such cases can be optimized by only rendering one pixel.

Current solution for tiled implementation is to get first pixel
from input. This works for root execution groups, others
need previous groups to be rendered.

On full frame implementation this is not possible, because buffers
are created on rendering to reduce peak memory and there is
no per pixel calls.

This patch evaluates all operations that are constant into primitive
operations (Value/Vector/Color) before determining resolutions.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D11490

6 files changed, 102 insertions, 21 deletions

diff --git a/source/blender/compositor/operations/COM_ConstantOperation.cc b/source/blender/compositor/operations/COM_ConstantOperation.cc
new file mode 100644
index 00000000000..f905edbde76
--- /dev/null
+++ b/source/blender/compositor/operations/COM_ConstantOperation.cc
@@ -0,0 +1,28 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * Copyright 2021, Blender Foundation.
+ */
+
+#include "COM_ConstantOperation.h"
+
+namespace blender::compositor {
+
+ConstantOperation::ConstantOperation()
+{
+  flags.is_constant_operation = true;
+}
+
+}  // namespace blender::compositor
diff --git a/source/blender/compositor/operations/COM_ConstantOperation.h b/source/blender/compositor/operations/COM_ConstantOperation.h
new file mode 100644
index 00000000000..2709efeebd8
--- /dev/null
+++ b/source/blender/compositor/operations/COM_ConstantOperation.h
@@ -0,0 +1,36 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * Copyright 2021, Blender Foundation.
+ */
+
+#pragma once
+
+#include "COM_NodeOperation.h"
+
+namespace blender::compositor {
+
+/**
+ * Base class for primitive constant operations (Color/Vector/Value). The rest of operations that
+ * can be constant are evaluated into primitives during constant folding.
+ */
+class ConstantOperation : public NodeOperation {
+ public:
+  ConstantOperation();
+
+  virtual const float *get_constant_elem() = 0;
+};
+
+}  // namespace blender::compositor
diff --git a/source/blender/compositor/operations/COM_SetColorOperation.h b/source/blender/compositor/operations/COM_SetColorOperation.h
index 6785a2c5ca3..f4c0948ee1b 100644
--- a/source/blender/compositor/operations/COM_SetColorOperation.h
+++ b/source/blender/compositor/operations/COM_SetColorOperation.h
@@ -18,7 +18,7 @@
 
 #pragma once
 
-#include "COM_NodeOperation.h"
+#include "COM_ConstantOperation.h"
 
 namespace blender::compositor {
 
@@ -26,7 +26,7 @@ namespace blender::compositor {
  * this program converts an input color to an output value.
  * it assumes we are in sRGB color space.
  */
-class SetColorOperation : public NodeOperation {
+class SetColorOperation : public ConstantOperation {
  private:
   float m_color[4];
 
@@ -36,6 +36,11 @@ class SetColorOperation : public NodeOperation {
    */
   SetColorOperation();
 
+  const float *get_constant_elem() override
+  {
+    return m_color;
+  }
+
   float getChannel1()
   {
     return this->m_color[0];
diff --git a/source/blender/compositor/operations/COM_SetValueOperation.h b/source/blender/compositor/operations/COM_SetValueOperation.h
index e346bc12ffb..f18d44d9554 100644
--- a/source/blender/compositor/operations/COM_SetValueOperation.h
+++ b/source/blender/compositor/operations/COM_SetValueOperation.h
@@ -18,7 +18,7 @@
 
 #pragma once
 
-#include "COM_NodeOperation.h"
+#include "COM_ConstantOperation.h"
 
 namespace blender::compositor {
 
@@ -26,7 +26,7 @@ namespace blender::compositor {
  * this program converts an input color to an output value.
  * it assumes we are in sRGB color space.
  */
-class SetValueOperation : public NodeOperation {
+class SetValueOperation : public ConstantOperation {
  private:
   float m_value;
 
@@ -36,6 +36,11 @@ class SetValueOperation : public NodeOperation {
    */
   SetValueOperation();
 
+  const float *get_constant_elem() override
+  {
+    return &m_value;
+  }
+
   float getValue()
   {
     return this->m_value;
diff --git a/source/blender/compositor/operations/COM_SetVectorOperation.cc b/source/blender/compositor/operations/COM_SetVectorOperation.cc
index 7152d5e61d4..7b8cf44048c 100644
--- a/source/blender/compositor/operations/COM_SetVectorOperation.cc
+++ b/source/blender/compositor/operations/COM_SetVectorOperation.cc
@@ -32,9 +32,9 @@ void SetVectorOperation::executePixelSampled(float output[4],
                                              float /*y*/,
                                              PixelSampler /*sampler*/)
 {
-  output[0] = this->m_x;
-  output[1] = this->m_y;
-  output[2] = this->m_z;
+  output[0] = vector_.x;
+  output[1] = vector_.y;
+  output[2] = vector_.z;
 }
 
 void SetVectorOperation::determineResolution(unsigned int resolution[2],
diff --git a/source/blender/compositor/operations/COM_SetVectorOperation.h b/source/blender/compositor/operations/COM_SetVectorOperation.h
index b444339fcb2..41fd06659d6 100644
--- a/source/blender/compositor/operations/COM_SetVectorOperation.h
+++ b/source/blender/compositor/operations/COM_SetVectorOperation.h
@@ -18,7 +18,7 @@
 
 #pragma once
 
-#include "COM_NodeOperation.h"
+#include "COM_ConstantOperation.h"
 
 namespace blender::compositor {
 
@@ -26,12 +26,14 @@ namespace blender::compositor {
  * this program converts an input color to an output value.
  * it assumes we are in sRGB color space.
  */
-class SetVectorOperation : public NodeOperation {
+class SetVectorOperation : public ConstantOperation {
  private:
-  float m_x;
-  float m_y;
-  float m_z;
-  float m_w;
+  struct {
+    float x;
+    float y;
+    float z;
+    float w;
+  } vector_;
 
  public:
   /**
@@ -39,37 +41,42 @@ class SetVectorOperation : public NodeOperation {
    */
   SetVectorOperation();
 
+  const float *get_constant_elem() override
+  {
+    return reinterpret_cast<float *>(&vector_);
+  }
+
   float getX()
   {
-    return this->m_x;
+    return vector_.x;
   }
   void setX(float value)
   {
-    this->m_x = value;
+    vector_.x = value;
   }
   float getY()
   {
-    return this->m_y;
+    return vector_.y;
   }
   void setY(float value)
   {
-    this->m_y = value;
+    vector_.y = value;
   }
   float getZ()
   {
-    return this->m_z;
+    return vector_.z;
   }
   void setZ(float value)
   {
-    this->m_z = value;
+    vector_.z = value;
   }
   float getW()
   {
-    return this->m_w;
+    return vector_.w;
   }
   void setW(float value)
   {
-    this->m_w = value;
+    vector_.w = value;
   }
 
   /**