Adjusting the Front and Matte Luminance Curves

When you create a matte for the front clip, the Modular Keyer automatically creates a matte for the back clip 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 Matte Curves menu. 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.

To learn how to set the matte curves to create an additive key, see Creating an Additive Key for CG Images.

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 Matte Curves menu. 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
• BackLUT is the back matte pixel value, re-mapped according to any luminance curve change made in the Matte Curves menu. The value is expressed as a decimal, as for the FrontLUT.

To adjust the luminance curves:

1. In Move mode, click the Result node in the pipeline. Alternatively, if you added a Matte Curves node elsewhere on the pipeline, click the Matte Curves node.

   The Matte Curves menu appears.

   
   

   (a) Result box  (b) Front matte curve  (c) Matte box  (d) View box  (e) Black matte curve  

2. Select Result view from the View box. This allows you to use the Result box to view a particular image as you adjust the curve.
3. From the Result box, select the image you want to view as you adjust the curves.

   Select:	To view:
   Result	The composite clip.
   Matte	The front matte. You cannot see changes made to the back matte when this view is selected.
   Bmatte	The back matte. You cannot see changes made to the front matte when this view is selected.
   Comp	The composite with a colour background. The default colour is white. To select a different colour, click the colour swatch below the Edit Mode box. The colour picker appears. See Picking Colours.

4. 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, Break, for example) to further adjust the curve, adding or deleting points, or breaking tangent handles as needed. For more information on using edit modes, see Animation.

Creating an Additive Key for CG Images

Computer graphics clips with a pre-multiplied alpha channel in which the area to be keyed out is black should only use the back matte. This is because the front curve calculation has already been applied to the front image.

To use only the back matte, re-map the entire front matte curve to white (255). This causes the curves calculation to ignore the front matte and use the entire front image. The curves calculation maps all the black areas of the front clip as transparent.

To create an additive key, adjust the front matte curve using the Matte Curves menu in the Result node.

To create an additive key:

1. Pull the key and perfect it.
2. Select the Result node.

   The Matte Curves menu appears.

3. Select the front matte curve from the Matte box.
4. Drag the vertex at the beginning of the curve from the lower-left corner of the graph to the upper-left corner of the graph.

   
   

   (a) Front matte curve  

   All pixels in the front matte are mapped to white (255 in 8-bit mode, 4095 in 12-bit mode). The black pixels on the front clip are calculated as transparent.

Processing Options

You can process the composite clip, the front matte, the back matte, or the composite using a coloured background instead of the back clip.

To process the composite:

1. Display the pipeline and select the Result node to display the Matte Curves menu.
2. Select the type of clip you want to process in the Result box.

   
   

   Select:	To process:
   Result	The composite clip.
   Matte	The front matte.
   Bmatte	The back matte.
   Comp	The composite with a colour background. The default colour is white. To select a different colour, click the colour swatch to the left of the Result box. The colour picker appears. See Picking Colours.

3. Click Process in the Modular Keyer menu.
   NoteWhen accessing the Modular Keyer from Action, the Process button is not available; you must return to Action and process it there.