Sony DCR-TRV39 - Manual (Page 206)

Hard Light
The pixels of the foreground image will be lightened if they are
50% brighter than the pixels of the background image. Black and
white pixels remain unchanged.
Darken
Displays only the parts of the foreground image that are darker
than the background image.
Lighten
Displays only the parts of the foreground image that are lighter
than the background image.
Difference
Subtracts the foreground color from the background color, or vice
versa, depending on which has the greater brightness value.
Transparency slider Drag the slider to specify the transparency of the picture.
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