Fix for #36468, "Buffer Groups" option changes compositing output.

Problem is that the read/write buffer operations only work with actual
image inputs. If a singular value is used as group input no actual
buffer will be created, the write operation does not schedule any chunks
and the ReadBufferOperation subsequently returns zero
(MemoryBuffer::read).

The fix uses the (0,0) resolution to detect single value input of the
WriteBufferOperation. The actual resolution is then clamped to (1,1) to
ensure we have a single pixel to store the value in. A m_single_value
flag is also set, so we can reliably distinguish this from genuine image
resolutions without having to check m_width/m_height later on.

The ReadBufferOperation copies this flag from the associated
WriteBufferOperation and if set will always return the single value from
pixel (0,0).

1 files changed, 15 insertions, 2 deletions

diff --git a/source/blender/compositor/operations/COM_ReadBufferOperation.cpp b/source/blender/compositor/operations/COM_ReadBufferOperation.cpp
index 03d41edda64..22820375831 100644
--- a/source/blender/compositor/operations/COM_ReadBufferOperation.cpp
+++ b/source/blender/compositor/operations/COM_ReadBufferOperation.cpp
@@ -27,6 +27,7 @@
 ReadBufferOperation::ReadBufferOperation() : NodeOperation()
 {
 	this->addOutputSocket(COM_DT_COLOR);
+	this->m_single_value = false;
 	this->m_offset = 0;
 	this->m_buffer = NULL;
 }
@@ -47,11 +48,17 @@ void ReadBufferOperation::determineResolution(unsigned int resolution[2], unsign
 		if (this->m_memoryProxy->getExecutor()) {
 			this->m_memoryProxy->getExecutor()->setResolution(resolution);
 		}
+
+		m_single_value = operation->isSingleValue();
 	}
 }
 void ReadBufferOperation::executePixel(float output[4], float x, float y, PixelSampler sampler)
 {
-	if (sampler == COM_PS_NEAREST) {
+	if (m_single_value) {
+		/* write buffer has a single value stored at (0,0) */
+		m_buffer->read(output, 0, 0);
+	}
+	else if (sampler == COM_PS_NEAREST) {
 		m_buffer->read(output, x, y);
 	}
 	else {
@@ -61,7 +68,13 @@ void ReadBufferOperation::executePixel(float output[4], float x, float y, PixelS
 
 void ReadBufferOperation::executePixel(float output[4], float x, float y, float dx, float dy, PixelSampler sampler)
 {
-	m_buffer->readEWA(output, x, y, dx, dy, sampler);
+	if (m_single_value) {
+		/* write buffer has a single value stored at (0,0) */
+		m_buffer->read(output, 0, 0);
+	}
+	else {
+		m_buffer->readEWA(output, x, y, dx, dy, sampler);
+	}
 }
 
 bool ReadBufferOperation::determineDependingAreaOfInterest(rcti *input, ReadBufferOperation *readOperation, rcti *output)