POSITION PASS MAGIC

USE FUSION’S ‘VOLUME MASK’ TOOL TO ISOLATE PARTS OF A RENDER WITH A MASK THAT MOVES PERFECTLY WITH THE CAMERA

i. Make sure your renderer is set to save a ‘World Position Pass’ AOV, often called just ‘Position.’ Once you have your rendered Position AOV, load it into Fusion and assign the R, G and B channels to the X, Y and Z Position channels in the loader’s Format tab.

ii. Now pipe that into the Volume Mask node and you’re ready to select the area of your image that you’d like to create a mask on. Set the Volume Mask node to a viewer. Now in the Volume Mask tool, drag the eyedropper tool to the viewer to select a location on the image. Under the hood, Fusion will select the appropriate values from your Position channel. This area you have selected will be the centre of your mask.

iii. You should now see the default mask that has been created. All that is left to do is adjust the size, softness and position of the mask. Make sure to check ‘Mask Only’ under the Color tab of the Volume Mask tool when you’ve got everything dialed in. In my example, I moved the position and scale of the mask to isolate the wooden borders of the front door windows so I could change the colour.

iv. You can apply this same workflow to create Volumetric Fog to add a sense of depth. Pipe the same ‘Position’ AOV that you’ve loaded in the previous steps into the ‘Volume Fog’ tool. Selection of the fog area is the same as what we did for selecting the mask area previously. For the fog, I’m going to soften the mask area by dragging the ‘Soft Edge’ slider.

v. I am also importing an FBX with the camera and scene lights from this render, because the Volume Fog tool can take a 3D scene input and simulate light scatter and absorption. This allows for many different looks to be created, all correctly masked and remaining in the correct position as the camera moves, because again, this is all calculated volumetrically. This tool also allows inclusion of 3D volumetric noise, via the Noise tab.

vi. This means you can create much more detail and have wind ‘blow’ the 3D noise detail in different directions by keyframing the translation of the volume, adding to the feeling of depth, as well as setting the ‘Seethe’ rate, which will cause the 3D noise to evolve over time. Matt Hoffman, Blufire Studios