Archive:Kdenlive/Manual/Effects/Alpha manipulation/Alpha shapes

Alpha Shapes

This is the Frei0r filter alphaspot

see also Frei0r-alphaspot readme file

Use this in combination with a composite transition to place areas of transparency onto an overlaying clip such that the underlying clip shows through in places defined by geometric shapes. By default the area of transparency is outside the shape that is is drawn. Inside the shape is an area of opacity where the overlaying clip is visible.

Shape Options

This controls the shape of the area of opacity that the effect will create.

Shape options are Rectangle, Elipse, Triangle Diamond.

Tilt

This controls the angle the shape appears on the screen

Position X and Y

This defines the position of the shape on the screen

Size X and Y

Defines the size of the shape

Transition Width

Defines the width of a border on the shape where the transparency grades from inside to outside the shape

Operations

Operations define what is to happen when you have more than one Alpha effect on the clip.

Operations are Write On Clear, Max, Min, Add, Subtract.

Write On Clear = existing alpha mask is overwritten
Max = maximum( existing alpha mask, mask generated by this filter)
Min = minimum(existing alpha mask, mask generated by this filter)
Add = existing alpha mask + mask generated by this filter
Subtract = existing alpha mask - mask generated by this filter

See the worked examples below to understand what these operations do.

Min and Max and Operations - Worked examples

These examples are performed on a simple composite transition with a video file on Video track 1 and a colour clip (yellow) on Video track 2 Alpha shapes effect draws areas of opacity onto the image. The addition of this filter (with the default settings of Min = 0 and Max =1000) makes the whole frame transparent except for an area of opaqueness where the top image can be seen.

The Max and Min controls adjust the opacity of the image inside and outside of the shape respectively. A setting of 1000 is 100% opaque. A setting of zero is 0 % Opaque (ie 100% Transparent).

Max control

The Max control regulates how opaque it is inside the shape. At Max= 1000 it is completly opaque inside the shape and none of the background image shows through

At Max = 500 it is semi transparent inside the shape and you can see the background bleeding through.

At Max = 0 inside the shape is completely transparent - the same as the rest of the foreground image - and you can see all background.

Min Control

The Min control adjusts how opaque it is outside the shape. When Min = 0 outside the shape is completely transparent (opacity of zero) and at Min = 500 we see some of the foreground appear outside the shape.

At Min = 1000 the opacity outside the shape is 100% and none of the background appears.

Combining Alpha Shapes - Operations

In this example I have a Triangle Alpha shape defined as shown and this is at the top of the effect stack with operation write on clear

Which appears like this on its own

And I have rectangle alpha shape as shown which is at the bottom of the effect stack. Note the Max = 500 - ie 50% opacicy inside the rectangle

Which appears like this when on its own.

In the images below I demonstrate the effect of different alpha operations on the rectangle alpha shape.

Write on Clear - existing alpha mask is overwritten

add = existing alpha mask + mask generated by this filter.

Note that areas with 1000 + 500 opacity would be 150% opaque. But you cant get 150% opaque so they look the same as the 100% opaque areas

subtract = existing alpha mask - mask generated by this filter

Note that areas with 0 - 500 opacity would be minus 50% opaque. But you cant get -50% opaque so they look the same as the 0% opaque areas

max = maximum( <existing alpha mask> , <mask generated by this filter>)

min = minimum( <existing alpha mask> , <mask generated by this filter>)