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Clipping Maps

Creating a clipping map in Photoshop

This is far harder to write than to do, so please do not be put off by the length of this article.

First things first, what the heck is a clipping map, and why is one useful?  Remember our histogram?  It divides the picture into 256 segments from 0 (zero) which is black to 255 which is white. You can look at a single histogram for the whole picture (called the RGB histogram) or you can have one for each of the computer primary colours, Red, Green and Blue.

When Victor and Derek talk about an image being “Blown” - particularly sky around the sun - they mean the white is at 255 so is 255 right or is it really 260 or 290?  We cannot tell because 255 is the maximum number available and any amount above that is clipped to 255.  This has two issues; Detail is lost to the image (it is just pure white), and when printed, many printers simply leave 255 as just the paper white. I.e. there is no ink placed which creates artefacts on the print.

Similarly is 0 really 0 or is it -17 or -23? Again we don’t know because anything below zero is clipped to zero and the data is lost.  In JPG images this data is often lost beyond recovery but in RAW images it can be possible to recover this missing data, making both the screen image and the print a better quality.

So, starting back to front, the below image shows the relevant histograms, everything below 1 as bright blue, everything above 254 as bright red.  We can clearly see the Mother seagull is badly blown, as are parts of the chick, and parts of the jetty have the blacks blown out as well.  (I adjusted the image to highlight these, my original was not quite that bad :-))

Knowledge Image 1

Look at the histograms - you can see these back up this story with no gap between pure black and pure white and the graph.  The clipping map visually shows this so the tiny adjustments we will be making to correct this can be seen by increasing and decreasing splodges of red and blue highlights.

There are two layers to produce. We will start with the Blue layer (representing black).  Click on the “New fill or adjustment layer” icon (see the next image if you don’t know this one) and from the pop-up select the top item, Solid colour.  The colour picker will spring into life.  You can drag the centre bar up and down then drag the sample spot to the top right but I prefer to type in the colours Red = 0, Green = 0 and Blue = 255 = pure blue.  Click OK to that and you are probably looking at a solid blue screen.  Don’t panic, all will be revealed!

On the layers panel right-click on this new blue layer and pick the top option Blending options.  On the BOTTOM blend-if slider drag it all the way left until it is almost touching the other slider.  You are looking for it to say 1 or 2 as circled on my image.  As you drag it down more and more of your picture comes into view and, depending on your image, it may be totally visible with no blue (= no clipping).  That’s fine.  Click OK. I rename this one to “blacks” by double-clicking on its title and typing the new name.

Next, repeat those steps but use Red (to represent white). Shortcut; R=255, G=0, B=0. In the layer style drag the bottom LEFT slider all the way to the right until it says 254. Click OK. Rename the layer to Whites.

Finally, shift-click on the layer Blacks so both the whites and blacks layers are selected and press cntl-G (or Command-G on a Mac) to group those layers into one item.  Name this group Clipping.

Knowledge Image 2

Remember this group has to be at the TOP of all the layers (which it is now, but if you add more layers simply drag this group to the top again).  Finally, we are ready to correct our image.

Click the background layer then again click the “New fill or adjustment layer” icon.  This time select the sixth item down, Curves. To reduce the white clipping drag the top-right control (see image) down VERY SLIGHTLY.  Watch your image to see the clipping mask change. To reduce the black clipping drag the bottom-left control up very slightly.

Once you are done you can turn off the clipping map by clicking on the eyeball just to the left of that layer.  Clicking that eyeball again turns the layer back on.  This works for all the layers.

Knowledge Image 3

Footnote;  It is really easy to create an ‘action’ in Photoshop to do all the steps of creating the two layers, grouping them and renaming everything in one click. I have written that as a separate article.

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