You can adjust the blending curves of each surface separately. The blending curve is similar to the Keyer luminance blending curve, but you can adjust it per surface.

When you create a matte for the front clip, a matte for the back clip is automatically created to specify which part of the back clip is used for the composite. By default, the back matte is the inverse of the front matte.

You can adjust the luminance of the front matte and back matte separately in the Action blending curve. For example, increase the luminance of the back matte so that more of the back clip shows through at the edges of the key. This creates a better blend at the edges.

The following calculation is applied to each pixel of the image to create the composite. The calculation is applied in three passes, one each for the R, G, and B values of the front and back images, and the pixel is given the resulting R, G, and B values.

Result = F * FrontLUT + B * BackLUT

where:

• F = the R, G, and B values of the front image
• B = the R, G, and B values of the back image
• FrontLUT is the front matte pixel value, re-mapped according to any luminance curve change made in the blending curve. The value is expressed as a decimal, where, for example:
  • in 8-bit mode, 0 = 0, 127.5 = 0.5, and 255 = 1
  • in 12-bit mode, 0 = 0, 2047.5 = 0.5, and 4095 = 1
  • 16-bit floating point images, the values are represented on a logarithmic scale between 0 and 1.
• BackLUT is the back matte pixel value, re-mapped according to any luminance curve change made in the blending curve. The value is expressed as a decimal, as is the FrontLUT.

To adjust the blending curve:

1. In the Surface menu, click the Blending tab.

   The Blending Curve menu appears.

   
   

   (a) Front matte curve  (b) Back matte curve  (c) Matte box  (d) Blend Curves option box (e) Keyframe option box (f) Reset Selection box

2. Select Result view from the View box. This allows you to view a particular image as you adjust the curve.
3. To adjust the luminance curve for the front matte, select Front from the Matte box. To adjust the back matte curve, select Back. Alternatively, click a curve to select it.

   
   

   (a) 255 (White)  (b) Output (remapping of luminance values)  (c) 0 (Black)  (d) 0 (Black)  (e) Input (current luminance values)  (f) 255 (White)  

   In Move edit mode, click a point to display its tangent handle and drag the handle to adjust the curve. Use other modes in the Edit Mode box (Add, Delete, or Break, for example) to further adjust the curve, adding or deleting points, or breaking tangent handles as needed.

4. Use the options in the Keyframe option box to Set, Delete, or Reset keyframes. If Auto Key is enabled, a keyframe is added automatically when you adjust the blending curve.
5. Use the Blend Curves option box to switch between the surface blend curves and the keyer blend curves. This gives you a good comparison to luminance curve work you have already done in the Keyer. When Use Keyer Blend Curves is selected, all of the other Action blend curve settings are unavailable.