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Calculate your way to better images

Added on Tuesday 11th of January 2011 07:38 am EST


Photoshop CS4

Operating System(s):

Macintosh, Microsoft Windows


Photoshop's Calculations command hasn't been a real attention-getter. Nonetheless, it still holds interesting possibilities for enhancing images both technically and creatively. On the more technical side, there are many different image problems related to correction for print. One that isn't commonly addressed is color depth. At some point, you may receive an image that has good color but still appears a little flat and lacks depth or richness. Such is our sample image shown in Figure A. We have nice colors in the walls and table, but the image has poor contrast and detail.

Figure A: Images without depth can be enhanced by adding more detail to the Black channel.



One way to enhance an image such as this is to create a new Black channel that contains more detail and contrast. You can do this easily by using the Calculations command to blend the existing Black channel with one of the other color channels that has more detail in it, thus creating a new channel with the desired attributes. Then, you can paste the new channel into the old Black channel. This may seem complicated, but we'll take you through the steps we used in correcting the coffeehouse image, and you'll see it's a fairly easy correction to make. We'll also show you how to use this command to creatively enhance your images, as we did with the butterfly images shown on the cover.


How it works

The Calculations command works by manipulating pixel values with blending modes, just like the blending modes in the Layers palette. Each pixel in an image channel can have a brightness value from 0 to 255. By adjusting these values, a new and improved channel can be created. You can work with either CMYK or RGB images. In this article, we'll focus on CMYK.

Because the Calculations operation combines corresponding pixel values, you must use images that have the same pixel dimensions and resolution. The results of the combination can be saved out as a new channel, a new image, or a selection.

Analyzing the image

Open a CMYK image you want to work on and then click on the Channels palette. You can see our four color channels in Figure B. It's quite apparent how low contrast the Black channel is. By looking at the other channels, we determined that the Cyan channel would be the best choice to combine with our Black channel. It has more detail as well as similar tonality to the Black channel. We could copy the channels into different layers and combine them with the blending modes, but we'll make this combination far more easily with the Calculations command.

Figure B: Our Black channel has low contrast and lacks detail. We'll boost its detail by combining it with the Cyan channel.




Using Calculations

Now that you know a little about how Calculations works and which channels you want to work with, choose Image > Calculations. In the resulting dialog box, as shown in Figure C, select the Preview check box so you can see the results of your combinations in the image window. Then, at the bottom of the dialog box, select New Channel from the Result pop-up menu to specify that you want the resulting blend to be placed in a new channel in the active image. Now, choose the Source 1 image and channel from the respective pop-up menus. You can choose from any open image (of the same size) and any layer or channel within that image. In this case, select the Cyan channel.

Figure C: Choose your source and target channels in the Calculations dialog box.




Now, set the Source 2 image and channel. In this case, we chose the Black channel because that's the one we want to enhance. In the next section down, you'll see the Blending pop-up menu. You're probably already familiar with most of its options since they also exist in the Layers palette. However, two of the settings are different—Add and Subtract. For more information about these settings, see the sidebar "To add or subtract." Experiment with the different modes until you find one that creates the desired effect. For our example, we found the Hard Light setting does the trick. To fine-tune the effect, you can also adjust the Opacity setting. This lightens the opacity of the image specifie...


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