Hey,

thanks for this bug report. What an interesting bug. The reason is really weird, so if we check out the compositor settings right before the optical flow is rendered, we can see the forward and the backward settings and an additional node at the top. This node at the top is weird it shouldn't be there, as we are not interested in the colors in this particular render call.

So, if I add some code to remove it:

class _FlowRendererUtility():

    @staticmethod
    def output_vector_field(forward_flow: bool, backward_flow: bool, output_dir: str):
        """...."""
        tree = bpy.context.scene.node_tree
        links = tree.links

        # remove the output composite node
        output_node = tree.nodes.get("Composite")
        if output_node:
            tree.nodes.remove(output_node)

The node vanishes, but blender still rendered this color image. I have no clue why. I assume that blender has to render this data to disc, and we can not turn it off. The best fix is to name this file something different so that there is no overwriting happening even if someone changes the output format.

This needs to be replaced with: (I have no clue why it even says here "bwd_flow_")

        # this image is only rendered as blender provides no option to turn this off
        temp_flow_name = "img_flow_temp_ignore_me_"
        RendererUtility.render(temp_dir, temp_flow_name, None, load_keys=set())

@cornerfarmer Do you have an idea how to turn this off?

Best,
Max

This was fixed.

@themasterlink Hi, I have pulled the latest code but found that the backward flow is still wrong using the above code.

You are right, somehow this was missed. Now it should be fixed.