Compositor: Fix node preview when input resolution is not known

The final render will use scene resolution in this case.

For example, when Color Input is plugger to preview and composite output
nodes, final render will flood-fill the final image which is a size of
scene resolution with this color. Before this fix the node preview was
empty. After this fix the node preview will be flood-filled with the
color.

Fixes T78586

Differential Revision: https://developer.blender.org/D8263

1 files changed, 15 insertions, 2 deletions

diff --git a/source/blender/compositor/operations/COM_PreviewOperation.cpp b/source/blender/compositor/operations/COM_PreviewOperation.cpp
index 07bf534cd74..43d20271141 100644
--- a/source/blender/compositor/operations/COM_PreviewOperation.cpp
+++ b/source/blender/compositor/operations/COM_PreviewOperation.cpp
@@ -126,8 +126,21 @@ void PreviewOperation::determineResolution(unsigned int resolution[2],
                                            unsigned int preferredResolution[2])
 {
   NodeOperation::determineResolution(resolution, preferredResolution);
-  int width = resolution[0];
-  int height = resolution[1];
+
+  /* If resolution is 0 there are two possible scenarios:
+   * - Either node is not connected at all
+   * - It is connected to input which doesn't have own resolution (i.e. color input).
+   *
+   * In the former case we rely on the execution system to not evaluate this node.
+   *
+   * For the latter case we use 1 pixel preview, so that it's possible to see preview color in the
+   * preview. This is how final F12 render will behave (flood-fill final frame with the color).
+   *
+   * Having things consistent in terms that node preview is scaled down F12 render is a very
+   * natural thing to do. */
+  int width = max_ii(1, resolution[0]);
+  int height = max_ii(1, resolution[1]);
+
   this->m_divider = 0.0f;
   if (width > height) {
     this->m_divider = (float)COM_PREVIEW_SIZE / (width);