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Smoother Blends

Cutting and pasting images in Photoshop can be a laborious process. If the joins are to appear seamless, a lot of time has to be spent perfecting the outline masks. I work as a professional photographer and use Photoshop to produce photo montages for clients as well as my own picture library shots. It is best to start planning a montage at the shooting stage - photographing a subject against a plain backdrop will make it easier later to create a mask on the computer. Other factors to consider are perspective and how the shadows will match. If there are several separate elements to combine, they should ideally all be photographed with matching camera viewpoints and lighting.

The most common problems, concern matting and the way the edges of a pasted element give away the fact that a picture has been retouched. People often leave the edges of a pasted element too sharp, so it looks like a cut and paste job, whereas in reality a photographic edge should be much smoother. The remove white/black matte commands are a bit crude and I prefer the flexibility of the Defringe command. Instead of defringing you could also try shrinking and feathering the outline selection (which I describe here), convert the selection to Quick Mask mode and refine by pushing out the edges from within using the paint and smudge tools.

The original images were shot against light, plain coloured backdrop. Instead of spending ages perfecting the outline masks I took a few short cuts by making full use of Photoshop's layers and the different overlay modes.

1) In this example the background is altered by inserting a new backdrop on top of the original image, rather than pasting the original as a cut-out. The advantage being that you retain all the subtle detail of the hair outline. Begin by making an outline mask as described in the last example - just concentrate on getting the hair outline right, then use the paths tool to draw around the shape of the neck and body. Convert the path to a selection and add to the mask.

2) As before the mask should have a soft edge. Working in Quick Mask mode, blur around the outline formed with the paths tool to introduce a gentle feathering.

3) Time to add the backdrop which is placed as a new layer above the background layer. This is a shot I took of a crumbling wall, the colours match well, so there is no need to adjust this image. To reveal the model, add a layer mask, load the mask as a selection (Option-Command-4), inverse the selection and fill with black (Press 'D' to restore default foreground / background colours followed by Option-Delete).

4) Change the overlay mode to Multiply. You'll find now the blend will look much smoother, this is because the brightness values of the pixels in the backdrop layer are now reacting with the brightness values of the pixels below. Selecting Darken mode will also produce good results. Refine the layer mask with brush/smudge/focus tools if the blend doesn't look right yet.

5) The shading from the previous background inter-acts well with the new backdrop. This can be enhanced by gently burning in with the toning tool on the left of the picture and dodging on the right with a very large brush size set to a low pressure setting.

6) The image is nearly complete - all that remains is to sharpen and carry out a few final checks. Unsharp masking is best carried out at the end of a retouching session and there are ways you can improve the quality of the filter sharpening. Do a test first - adjust the Amount setting but also try varying the Radius and Threshold values to see which looks best. Undo the filter and repeat, but this time only activate the Red and Green channels only (shift click to select them) - this is because the Blue channel tends to produce more noise than the other channels after sharpening.

7) When you open the Unsharp Masking dialogue box the values will be the same as before when you carried out the test. Divide the Amount value by a quarter and apply the filter four times. The three images above show left: no filter, centre: standard Unsharp masking, right: the gradual method with the blue channel turned off.

8) Unsharp masking can produce unwanted artefacts. Above is an example showing the effect of sharpening the model with and without the use of a feathered outline selection. The outline mask is loaded as a selection, shrunk by choosing contract 2 pixels from the Select/ Modify sub-menu followed by Feather (2 pixels). Sharpening will occur only within the selection. The effect is subtle - note how the jagged pixels I've circled disappear in the right-hand example.

Rasterised text and graphics will suffer in appearance if sharpened. It is usually a good idea to keep these on a separate layer and sharpen the image layers only.

Extract originally published in
MacUser 1996. Copyright: Martin Evening

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