Is there a way to convert a colour HDRI added to the Skydome light to B&W (parameter or connect a node to colour correct)?
Pipe the HDRI into an Arnold Color Correct node and set saturation to 0.
Then link the Color Correct node to the Skydome.
Thanks for the tip. I am new to Houdini so it took a while for me to figure out how to set it up. This seems to be the way:
(1) In Skydome set Light tab-Color Type: Shader and click on Open floating operator icon (cross with arrow) on right of Color Shader parameter and select the Arnold_vopnet item under arnold_skydome_light1-shopnet.
(2) Click on Jump to operator icon next to it (arrow icon) to navigate to arnold_vopnet node. Double click on the node and connect an Image node to the OUT_light (rgba to color). Select the HDRI for Texture tab-Filename.
(3) Connect a Color Correct node in between these nodes (rgba to input; rgba to color). In color_correct1 node set Saturation: 0
Color Correct with Is Luminance enabled
rgba to float with Mode = Luminance
both convert to greyscale using a formula like those shown here: https://en.wikipedia.org/wiki/Grayscale
But Arnold will do the conversion using the right coefficients for the working color space