Switching the Brush Mode between Normal and Overlay
Switching the Brush tool to Overlay blend mode is very handy for refining and cleaning up layer masks.
When in Overlay blend mode a white brush will affect lighter pixels (those that are lighter than 50% grey) but will not affect darker pixels, similarly when in Overlay mode a black brush will affect darker pixels (those that are darker than 50% grey) but will not affect lighter pixels.
With the Brush tool active use these shortcuts to switch the Brush blend mode between Overlay and Normal;
Shift+Option(Alt)+N – switch to Normal mode
Shift+Option(Alt)+O – switch to Overlay mode
Understand the Size of the Brush Cursor
When in normal mode (that is the Preferences / Cursors / Painting Cursors setting is set to Normal Brush Tip) be aware that when using a soft brush the brush tool will affect areas outside of the circle you see when you paint. When painting with a hardness of 0%, so as soft a brush as you can get, the edge of the circle represents the point where the brush feathering reaches 50% opacity.
If you select Full Size Brush Tip the circle shows the whole area that can be affected by the brush. That is the edge of the circle shows where the opacity of the brush feathering reaches 0%.
Obviously when using a hard brush, hardness at 100%, there two settings make no difference.
When the [Caps Lock] key is on the brush cursor will change for the selected cursor to the cross-hair, release [Caps Lock] to change it back.
Remember you can use the square bracket keys [ and ] to reduce and increase brush size, and holding down the [Shift] key while using them will reduce and increase the brush hardness.
The Difference Between Opacity and Fill
Brush Opacity defines the maximum amount of paint that can be laid down in one brush stroke (mouse click).
Fill defines the rate at which paint is laid down as you paint.
With the brush tool activated you can use the number keys to change the Opacity setting 0 = 100%, 1 = 10%, 2 = 20%, …. etc. If you want a more precise setting tap two number keys in quick succession, for example tapping the 4 quickly followed by the 5 will give you 45% opacity. Hold down the [Shift] key and use the same number key techniques to change the Flow setting.
The Behind and Clear Blend Modes
There are two blend modes that are available only on the tool blend mode drop-down on the Options Bar and not in the Layer blend mode. These are Behind and Clear. These define how the brush responds to transparent pixels on the layer you are working on.
If the transparency of the current layer is locked these options will be greyed out on the drop-down menu.
The Behind blend mode will cause the brush to only paint on transparent pixels on the layer.
The Clear blend mode will make all the painted pixels transparent.