Adjusting Luminance Using Curves

When you create a matte for the front clip, the Keyer automatically creates a matte for the background to specify which part of the background 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 Curves controls. For example, you can increase the luminance of the back matte so that more of the back clip shows through at the edges of the key. This can create a better blend at the edges of the key.

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 x FrontLUT + B x 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, remapped according to any luminance curve change made in the Curves controls. The value is expressed as a decimal, where, for example, in 8-bit mode, 0 = 0, 127.5 = 0.5 and 255 = 1.

BackLUT is the back matte pixel value, remapped according to any luminance curve change made in the Curves controls. The value is expressed as a decimal.

For example, if a pixel has a value of 127.5 (1/2 the value of pure white) in the front matte and in the back matte, then the value of each color channel in the result pixel is equal to 1/2 of its value in the front pixel plus 1/2 of its value in the back pixel.

Use the matte curves to change these ratios.

If you want more of the back image to show through, drag the back curve up to a higher value, and drag the front curve down by a similar amount.

Note: Increasing one curve without decreasing the other will result in a glow around the edge of the key. If you decrease both curves, you will get a dark outline around the key.

To adjust the matte curves:

1. Click Curves to show the Curves controls.
2. Use the Output options to select the output you want to view as you adjust the curves. To see the back matte, set the output to Matte and enable Back Matte.
3. To adjust the luminance curve for the front matte, click the Front curve label in the Curves controls. To adjust the back matte curve, click the Back curve label.

The selected curve becomes white.

4. Click on a point to display its tangent handle and drag the handle to adjust the curve. To break a tangent handle, press Ctrl while dragging. Press Ctrl again to unbreak the handle. Click on the curve to add a control point.