Advanced Green Screen Despill Masterclass
Follow along with the blackmagic raw sony FX3 footage by joining the cutting club here today.
Mastering the Delta Keyer in DaVinci Resolve Fusion
Welcome to an advanced green screen masterclass where we dive deep into the Delta Keyer inside the Fusion page of DaVinci Resolve Studio 20. You'll learn sophisticated techniques, like creating a custom Despill map, that are rarely taught elsewhere, ensuring your edges match your new background perfectly.
We're working with high-quality, raw Sony FX3 Blackmagic raw footage of a dog on a poorly lit green screen, demonstrating how these advanced methods can tackle challenging material. You can download the same footage to follow along by joining the cutting club at creativevideotips.com.
Initial Setup and Color Management
In Fusion, the recommended workflow is placing your clip directly on the timeline. When you park the playhead over it and click the Fusion button, you ensure you're working at the clip's native source resolution and can retain source timecode when rendering from the Deliver page.
Color Management and Linear Space This tutorial operates with color management disabled (DaVinci YRGB). This allows us to perform a linear conversion, which is crucial for advanced compositing like using Channel Booleans, ensuring all the math works out properly.
Log to Linear Conversion: Since we're starting with log footage, the first step is converting it to the linear space. We use the Cineon Log tool (Shift + Space, then search "Cineon Log") to remove the transfer curve (log to linear) and select the correct log type (e.g., Sony S-Log3 for FX3 footage). This is preferred over a Color Space Transform to preserve the original color gamut and primaries.
View LUT Setup: To correct the viewer for the linear space, click the View LUT (waffle) button in the viewer and select "Edit". Set the Source Space (e.g., Sony S-Gamut 3 Cine) and the Output (e.g., sRGB for your display), making sure to click Add Gamma. (Pro Tip: Right-click the View LUT button, go to Settings, and "Save Default" to save this setting) .
Linear to Log Conversion for Output: To return the image to the original log space for grading in the Color page, copy the Cineon Log tool and place it just before the Media Out node. Change the switch on this node from "lin to log". This setup creates a middle working space in linear for compositing operations.
Noise Reduction (Denoise)
Green screen footage, especially raw, often contains noise. Noise reduction should be performed in a log space (before the log-to-linear conversion).
Add a Noise Reduction tool after the Media In node. (Neat Video Reduce Noise V6 is great, but the default Resolve noise reduction is also excellent) .
Prepare a noise profile by selecting a featureless area and letting the tool analyze the noise. Denoising the green screen is essential for a good keying result.
Advanced Custom Despill Map
The Despill process is critical for clean edges. Gray Despill is generally undesirable, as it can make edges look "dead" or out of place in the composite. A custom Despill allows you to match the edges to a specific foreground or background color.
Initial Keying for Despill: Add a Delta Keyer after the de-noise and log-to-linear conversion.
Use the eyedropper in the Delta Keyer's inspector to drag-sample a color from the green screen. Hold Ctrl/Command while dragging to sample a larger, averaged area. This automatically applies internal Despill to the image.
Creating the Spill Map (Subtract): We want to separate the Despill information for custom color grading.
Add a Channel Booleans node and set the Operation to Subtract.
Connect the original green footage (after log-to-linear) to the Background input of the Subtract node.
Connect the output of the Delta Keyer (the despilled image) to the Foreground input of the Subtract node.
Creating the Luminance Map (Desaturation): The Subtract node output is the spill map, but it's full of color.
Add a Brightness Contrast (BC) node after the Subtract node.
Take the Saturation down to zero to create a grayscale luminance map.
Custom Color Grading the Despill (Multiply/Divide): Instead of simple gain, we use Channel Booleans to apply precise, color-picked gain values.
Add a second Channel Booleans node and set the operation to Multiply.
Add a third Channel Booleans node and set the operation to Divide.
The formula for the custom color gain is (User Sample Color / Keyer Sample Desaturated Color)
User Sample Color: Add a Background node (solid color) and use the color picker to sample a target color (e.g., the dog's golden fur or the background sky).
Keyer Sample Color (Green): Add a second Background node, rename it "Green," and use Expressions to link its R, G, and B color values to the Delta Keyer's
ScreenColorvalues. Desat the green and use this to divide and find the color grading remainder.
Reintroducing Luminance (Add):
Add a fourth Channel Booleans node and set the operation to Add.
Connect the output of the Multiply node (custom Despill color) to the Foreground input of the Add node.
Connect the output of the Delta Keyer (initial despilled image) to the Background input of the Add node. This adds the custom color Despill back onto the image, matching the original green screen's brightness.
Pulling the Key: Edge and Core Mattes
The Despill and the final alpha channel (the matte) are pulled separately for greater control.
Edge Matte: We use a second Delta Keyer for the fine edges.
Connect the de-noised linear footage to the new Delta Keyer.
Connect the Clean Plate to the Delta Keyer's purple input. The clean plate (a frame of the background without the subject) is the Delta Keyer's most powerful feature, as it smooths out background irregularities and significantly improves fine detail.
View the Alpha channel (tap 'A') and adjust the Balance slider in the Delta Keyer. Moving it slightly left often improves the matte without complex cleanup. This keyer provides your fine hair/edge detail.
Core Matte: The edge matte will have holes in the subject's center (the "core"). We create a hard core matte to fill this in.
Connect the Edge Matte's output to a Bitmap (BMP) node.
In the Bitmap node, use the Threshold slider to harden the key, making the solid white areas opaque (a value of 1). (You can check for speckles by "gamma slamming" with the View LUT Gain/Gamma sliders) .
Add an Erode/Dilate node after the Bitmap. Set the filter mode to Circle and use the slider to shrink (erode) the core matte inward. This prevents the hard core from eating into the fine edges of the soft matte.
Add a slight Blur after the Erode/Dilate to smooth the edges of the core matte.
Combining the Mattes:
Add a Matte Control node.
Connect the Edge Matte (Delta Keyer output) to the Background input.
Connect the Core Matte (Blur output) to the Solid Matte input. This combines the best parts of both keys: the soft edge detail and the opaque core. This is your final alpha channel.
Final Composite
Pre-Multiply: Before compositing, you must pre-multiply the custom Despill color image with the combined alpha matte.
Add a second Matte Control node.
Connect the output of the Add node from the custom despill to the Yellow Image Input.
Connect the Final Alpha Matte (output of the combined Matte Control) to the Green Foreground Input.
In the Inspector, check the Post-Multiply box and choose to combine the alpha to copy the alpha into the matte control node. This cuts out the image using the matte and removes the "weird checker underlay" by multiplying the color with the alpha channel.
Merge:
Drag the output of the Pre-multiplied image (the dog) and drop it onto the final background image node. This automatically creates a Merge node with the dog as the foreground.
Connect the Merge node to the log-to-linear conversion tool to prepare for the Color page.
Cleanup and Finish:
Insert a Transform (XF) node after the Pre-Multiply (cutout dog) to reposition and resize the subject over the background.
To better integrate the background, insert a Defocus node after the log-to-linear conversion of the background plate, reducing some of the sharpness and adding subtle bokeh. (Remember to hold Ctrl/Command while dragging sliders for fine adjustments) .
This process results in a comp with clean, detailed edges and a color-matched despill, providing an excellent starting point for final light wrap, grain, and color grading on the Color page.
