Alpha Operations

Alpha Operations

Status:

Maintained

Keyframes:

Yes

Source library:

frei0r

Source filter:

alpha0ps

Available:

On Master only:

No

Known bugs:

No

Description

This effect can shrink, grow, threshold and invert the alpha channel. It is mainly intended to improve keying edges. It can also display the alpha channel in various ways to enable quick assessment of the effect. It is cascadable and thus allows a soft shrink first followed by threshold which gives a slightly different result than a hard shrink[1].

Parameters

Parameter	Value	Description
Display	Selection	What to display (see below)
Display input alpha	Switch	Use input alpha for the display function above (check what alpha we are getting on the input)
Operation	Selection	Select the operation to be done on the alpha channel (see below)
Invert	Switch	Inverts the input alpha channel, transparent will become opaque and vice versa.
Threshold	Integer	This is only used for the Threshold operation
Shrink/Grow/Blur	Integer	Depending on the chosen operation: How far the shrinking/growing will extend, or the amount of blur

The following selection items are available:

Display

This is intended for monitoring during adjustment mostly. After adjusting the parameters, it should be left on Image, which lets the unchanged input image through - this effect is intended to change only the alpha channel.

Image	Image after the operation (default)
Alpha as gray	Displays the alpha channel as gray
Gray + red	Displays the alpha channel as gray, everything else as red
Selection on black	Displays the non-alpha channels on a black background
Selection on gray	Displays the non-alpha channels on a gray background
Selection on white	Displays the non-alpha channels on a white background
Selection on checkers	Displays the non-alpha channels on a checkered background

Operation

NO OP	No operation is performed
Shave	Tries to remove the “hairy” stuff, and also shrinks the selection a bit
Shrink hard	Shrinks the area of the alpha channel
Shrink soft	Shrinks the area of the alpha channel
Grow hard	Grows the area of the alpha channel
Grow soft	Grows the area of the alpha channel
Threshold	Increases or decreases the area of the alpha channel by the threshold
Blur	Blurs the area between the alpha and the other channels

The … hard operations introduce no new values to the alpha channel, so if you have a hard key (only 0 and 255) it will stay that way.

The … soft operations will introduce interpolated values, making the edge softer.

Note

The shave, shrink and grow operations are quite slow, because they do many conditional operations on each pixel.

Notes

This tutorial shows usage of Alpha Operations with Shrink Hard as well as the following effects: Chroma Key: Basic, Denoiser, and Key Spill Mop Up.

Note

This video is somewhat outdated. In newer versions of Kdenlive the Key Spill Mop Up effect is installed by default, and it is no longer required to use a composite transition. Nevertheless, the basic steps of chroma keying and key spill mop up are explained and are still valid.

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[1]

The description of this effect has been taken in parts from the readme file for the frei0r alpha0ps plugin. You find much more detailed information there.