How to Use the History Brush Tool in Photoshop CS6
In Adobe Photoshop CS6, you can use the History Brush tool to apply an image area from a different state or snapshot to your current state. Use this tool to restore a portion of an image to an earlier state, while leaving the rest of the modified image alone. The History Brush has an advantage over the Eraser tool because the History Brush allows access to many blend modes.
In the History panel, click in the far-left column of the state or snapshot that you want to use as the source for the History Brush tool.
A brush icon appears in the column, indicating that Photoshop will use this state as the source for the History Brush tool.
Select the History Brush tool in the Tools panel.
You can also press Y to select the tool.
On the Options bar, select any other brush options that you want to use — such as Brush size and type, Mode, Opacity and Flow percentages, and Airbrush.
Select your desired layer in the Layers panel and, in the History panel, select the state that you want to paint back to.
Drag with the History Brush tool to paint over the portion of the image you want to restore.
Photoshop paints over the image in the layer with the image from the state you specified as the source in Step 1.
The original faces are painted by using a 10-15% Opacity setting over the Water Paper filtered image.
Sometimes, Photoshop won’t let you go back. You may see a No symbol (a slashed circle) when you try to use the Eraser with the Erase to History option, the History Brush tool, or the Fill with History command. Your current image must be the same file size (have the same number of pixels) as the state you’re trying to go back to.
Such actions as cropping, trimming, using the Image Size or Canvas Size commands, or rotating any amount other than 180 degrees can prevent you from going back to a previous state. However, if you happen to have a square image, you can still use the Eraser with the Erase to History option if you rotate that image in 90-degree increments.