A good use of the History Brush is to
E apply an effect to an image in localised areas without the need to construct a mask or selection first. Here's our starting image to which we want to apply some special effects. First of all we resized it to make it a bit smaller.
Typing [Ctrl/Command]+[I] inverts the image,turning the shades and colours to their opposites.This will be be the basis for the effect we want to achieve using the History Brush, and it has been saved as a state in the History palette.
B 3 | We want to get the image back as is was I I so that we can selectively invert it, but undoing the last step also clears the state from the History,which is no good.Instead we mark the uninverted state as a History source. Choose Edit > Fill and set the Fill mode to History,et voila, back to normal but with History intact.
ENow we can change the History source to the Inverted state, select the History tool and a fancy brush and paint.Wherever we paint the pixels in the image will be inverted. Obviously you can use this technique with any effect,filter or adjustment you like. ^
Size does matter
To paint with History states the source state must be the same size as the current document.if you resized an image you will not be able to use any states prior to that as a source for the History Brush.
In order to paint with History states,the source state must also be the same colour space.If you changed this you will not be able to use any states prior to that point as a source for the History Brush.
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