About Drawing Modes

Drawing Modes offer powerful ways of combining two colours or images to create interesting effects. It's important to understand the colour model when working with drawing modes: a colour is represented with four channels: red, green, blue and alpha. "Alpha" represents the opacity of the colour; opacity is the opposite of transparency. For example, a pixel that is 75% opaque, is 25% transparent.

If you consider only the alpha channel of all pixels in an image, you have the shape of the image. Some drawing modes affect only the red, green, and blue channels of the target image; some affect only the alpha channel (and hence are used to manipulate the shape of the image); some affect both.

In the icon representing each draw mode, the blue box is the "old" or "destination" image; the red circle is the "new" or "source" image.

cookiecut

Drawmode cookiecut old retains all the colours of the old image, but imposes the shape of the new image. In other words, cookiecut old copies the alpha channel of the new image to the old image, without changing colours.

Drawmode cookiecut new does the opposite: it keeps the shape of the old image and the colours of the new image.

over

under

over, retain alpha

Drawmode over, retain alpha draws the new colour or image "over" the old one, but does not change the shape of the old image ("retain alpha").

This is the most common way to apply gradients, so the gradient draws only on pixels that are already drawn, and doesn't change any transparent pixels.

replace