The brushes palette for every paint tool also contains a
popup menu of varying brush modes. These modes modify how a tool will
integrate with existing color on the canvas.
When you draw something on the canvas, Photoshop
modifies the existing pixels in the background according to the logic of
painting. Thus if you paint a streak of red over a streak of blue, the
pixels in the streak of blue will be changed to pixels of purple.
Thus, you can think of painting in Photoshop as involving three
values: the base value, the blend value, and the result value. Brush modes modify the
way this blending works. Specifically, modes specify the way in which
the blend and base values will interact to create a result value.
Let's look at each of the modes...
The Normal mode applies the full blend value. In the case of
a painting tool, the blend color will completely coat the base color. in the case of an
edit tool, the edit value will completely override the existing color.
The Threshold mode is specific for Bitmapped and Indexed Color
images. Essentially, it assures that the color value of the tool is applied
according to the closest available color in the color map.
The Dissolve mode, which is only available for painting tools.
randomly scatters the blend color to give a rough textured
non anti-aliased brush stroke.
The Behind and Clear modes are available in layered
images. Essentially, they modify whether the blend color is applied to the
foreground or background layer. When the behind mode is selected for example, a color will be
applied behind the layer showing through only int he blank or transparent areas.
The Multiply mode multiplies the brightness
of the base color and the blend to create a darker tone. The opposite
effect can be achieved by using Screen
Overlay works like screen and multiply
depending on the value of the base color. Specifically, the hues
of the base color will move towards the blend color. Soft light works like
overlay with less intensity while Hard light is more intense.
Darken darkens the base color using the blend color as a basis
and Lighten lightens the base color by using the lightest value of the blend color.
Difference compares the brightness values of the base and blend colors and creates
a result value by subtracting the smaller from larger values.
Hue replaces the hue of the base with the hue of the
blend. Saturation does the same for saturation and Color does the same for both hue and saturation.
Luminosity works the same way for the lightness value.