Fusion Transparency Control
Today, I'm going to share three important ways to use the Brightness Contrast node to control transparency on the Fusion page of DaVinci Resolve. The brightness contrast node is one of the fastest nodes in fusion to process.
Animating Overall Node Output Transparency
A lot of tools in Fusion have a built-in "blend" slider to control the opacity of layers (like a foreground layer), but some do not. For example, the Time Speed tool does not have one when pulling a freeze frame. In these cases, the Brightness/Contrast node is your best friend.
Press Shift + Space and type "BC" to add a Brightness/Contrast node to your node tree.
Load the node into the viewer and go to the Inspector. Enable the "A" (Alpha) slider.
Drag the Gain slider down to zero. Because Gain functions as a mathematical multiplication, you are essentially multiplying all channels by zero.
You can easily animate this transparency over time. Keep it at 1.0, set a keyframe, move a few frames forward, and drop it to 0.0 to completely clear out the composition for specific frames downstream.
Stenciling Out Unwanted Elements
If you want to remove a specific part of your image—for example, getting rid of the stars from an American flag graphic—you can cut a stencil using this node.
Draw a shape around the unwanted elements using a Polygon tool, and press Shift + O to close the path. Note: It is usually best to do this unattached and downstream from your Media In and Loader nodes so you don't permanently cut away original source pixels.
Add a new Brightness/Contrast tool from the toolbar.
Feed your video source and your newly created Polygon mask into the Brightness/Contrast node.
Just like before, turn on the Alpha channel and drag the Gain down to zero. You've successfully cut a stencil!
Bonus Tip: If you need a quick and dirty way to fill in the stenciled hole with the surrounding edge colors, add a Clean Plate tool (Shift + Space). Connect your mask into its garbage matte input to seamlessly in-fill the area.
Rotoscoping and Isolating a Subject
What if you want the exact opposite of a stencil? If you want to isolate an entire subject (like rotoscoping the whole flag) to lay it back on top of your original footage later on, the Brightness/Contrast node is perfect for this as well.
Grab the Polygon tool and create a precise shape around your entire subject.
Invert the polygon shape before applying it to the Brightness/Contrast mask input. (If you were doing a stencil, you would leave it un-inverted).
Enable the Alpha channel and drop the Gain to zero. You have now successfully isolated your subject!
Alternative Settings Workflow: If you don't want to manually use the Alpha and Gain sliders, you can go to the Settings tab of the Brightness/Contrast node (or many other nodes). You'll find options called "Multiply by Mask" and "Apply Mask Inverted". While these achieve the same result, utilizing the main Gain slider is often preferred because it keeps all your core adjustments visible on the main controls page, making it easier if another artist jumps into your comp.
