I was recently amusing myself trying to repair an old photo, and when I tried to use the healing brush tool (HBT) I realised I didn't really understand what it did. I did some research to discover just what it did do, and these notes show what I found.

The HBT is similar to the clone stamp tool (CST), but with a subtle difference.

Both copy material from a reference point (which can be another point on the image you are working on, or even on another image) and place it at a target point. You can set the size, shape and properties of the brush used to copy. You can also set whether the reference point is ‘aligned’, meaning that once you have selected a reference point and an initial target point, the source area is always offset from the mouse position by the same amount until you select a new reference point, or not aligned, in which case the same source area is used each time you click to make a new copy. If you want to copy a substantial area ‘aligned’ has the advantage that you can repair a large area by repeatedly clicking the mouse. Then if you make an error, you can step back and cancel it without losing everything you have done already. On the other hand if you are repairing blemishes it is often better not to use ‘aligned’, especially if there is only a small area with similar characteristics to copy, as you can then use the same reference point for each blemish.

The primary difference between the two tools is that the CST simply replaces the target area with whatever is in the reference area, whereas HBT blends the detail in the target area as you move away from the boundaries and copies details from the reference area. If there is no detail in the reference area nothing is copied, so that the tool simply blurs the target area. This applies even if the reference area is a different colour.

Figure 1

Example of patterned target area and plain reference area

This is demonstrated in Figure 1. I have used a fairly large diameter brush to copy each of the reference points (marked by black crosses on the right of the figure) to the corresponding points on the left. The reference points have different colours, but do not contain any pattern. (In all the figures the markers + were on a different layer when I did the test, so that they were not copied). In all cases there is no pattern in the reference area, and the contents of the target area have simply been blurred by averaging the colour within it, obliterating the text. Where letters have crossed the boundary of the target areas these have bled into the target areas as coloured stains.

In the third case the target area is on the red-blue boundary. The discontinuity has been blurred, so that the colour blends from red above to blue below.

Note that in all these cases the colours of the reference areas have no effect on the target areas.

 

Figure 2

Pattern in reference area, not in target area
In Fig 2 the reference areas contain text, whereas the target areas are plain. The detail in the reference area has been copied into the target area, but blurred into the original contents around the margins to avoid a sharp discontinuity. As before the reference areas are marked by black crosses, which were added subsequently.

Figure 3

Reference area differently coloured from target area
 

In Fig 3 the first and third reference areas are centered around a cross on a contrasting background. The cross is copied, together with the background colour immediately surrounding it. Where the target area is placed on the red blue discontinuity these colours are blended, as before. The second reference area is on the yellow black discontinuity, but contains no other texture (in this case the red marker was added subsequently).

 

Figure 4

Comparison of HBT and CST.

In Fig 4 I have taken a small piece of text from a recipe, and used first the HBT to copy the target area (circled in green) to the area circled in red, then the CST to copy it to the area circled in blue. It can be seen that in the former case the copied section is blurred into the original, whereas in the latter it has a sharp boundary.

 

Figure 5

Comparison of HBT and CST to repair image.

In Fig 5 I have taken a small section of the image accompanying this recipe, and used each tool to remove two small crumbs from the tablecloth. This task was was made more difficult because the random wrinkling of the tablecloth and the limited depth of focus in the photo meant that there was no directly equivalent area to copy. In each case I have copied the corresponding part of the stripe above the blemishes over them. With the HBT this has produced a barely noticeable blurred area, whereas with the CST both corrections have a noticeable sharp edge.

This freedom from sharp discontinuities is probably the main virtue of the HBT, but it comes at a cost, in that the blurring action can seriously distort any sharp discontinuities in the target area. If, for example, you are working in a light coloured area adjacent to a dark border, and you accidentally include a bit of the border in the target area, it will bleed into the target area as a very obvious dark smudge. The Photoshop users manual states that this can be avoided by selecting the area you want to work on before you start to copy.

The HBT is probably better for repairing blemishes, whereas the CST is better for copying substantial areas -- for example if you wish to give your girlfriend two heads! -- or areas with sharp discontinuities.

Both tools suffer from a substantial disadvantage in that it is not possible to position the target area exactly, and there is no way to adjust the position of the copied material once you have placed it. Where it is important to place the copied material accurately it may be better to select it, then use copy and paste to put a copy in the new position. This procedure is more cumbersome, but has the great advantage that you can adjust the position of the copied material after you have placed it, and if necessary you can even rotate, distort or trim it to get a better match.