This tutorial is all about the relationship between Photoshop Curves and how all non-dissolve, per-channel Photoshop blend modes are equivalent to some kind of curve.
It has often been said that there is always more than one way to achieve an identical result in Photoshop. The trick of course is to work out which method is best for you or which is the most efficient.
As far as some people are concerned, Photoshop holds a certain mystique where it is assumed that a more complex approach will produce a more refined result. But very often the underlying math is the same as that used when applying a simpler approach. This is not to say that all tone adjustment methods can be reduced to a single curve, but the steps shown here are designed to illustrate the following three points:
1. It is always more efficient to use adjustment layer blend modes to ‘Screen’ or ‘Multiply’ an image rather than to duplicate the Background layer
2. An arbitrary map curve setting can be converted to a Photoshop Raw file format and edited in Photoshop.
3. By using this method to edit Curve settings, some cumulative image adjustments can be distilled into a single curve setting.
Adjustment layer blend modes
Figure 1 shows an unedited image where the sky requires darkening. The method we are going to use here is to use a Multiply blend mode to darken the overall image and then apply a black to white linear gradient to mask the bottom half of the picture. And in order to further darken the sky, we are going to duplicate the blend mode layer to produce a more pronounced darkening.
There are two ways you can do this. The most often taught method is to simply duplicate the Background layer and change the layer blend mode to Multiply. This will work, but when you save the image you will end up with a file which in this case will be three times the original file size!
A better alternative is to add an adjustment layer (any will do) without actually altering the settings and change the blend mode. This will achieve the exact same result as duplicating the Background layer but will only marginally increase the file size. So all you need to do is add an adjustment layer, let’s say a Curves adjustment layer, and click OK when the dialog appears to close it without editing the curve in any way. Then simply change the layer blend mode as before. It’s that simple.
Figure 3. Here is the end result when using either of the methods described above. The sky only is darkened because the linear gradient hides the adjustment to prevent it affecting the lower half of the picture.
If you refer back to Figure 2 you will notice how I duplicated the Multiply layer and adjusted the layer opacity of this second layer to 60%. By altering the layer opacity and duplicating layers, you can have full control over the intensity of the ‘Multiply’ effect.
Convert a curves setting to a Photoshop image file
OK, now for the complex part. This next section reveals a little-known trick, in which you can convert a curves setting to a Photoshop Raw format image file, edit the image in Photoshop and save it back as a Curve setting. At this point I should mention that it was Russell Williams at Adobe who first showed me this tip and who helped clarify some of the points made here about the similarity between blend modes and Photoshop curves settings. Just remember that the steps discussed in this next section all relate to what can be summarized as the ‘non-dissolve, per-channel blend modes’ and excludes those other blend mode formulas like Hue, Color and Saturation that don’t operate on the individual channels.
Now, I have described all the steps here in detail so that those of you who are inclined to experiment are able to follow the exact same steps that I used. I appreciate that not everyone will be interested in these details, so if you like you can skip to the end of this article and download some of the settings I created using this method and put them to use.
Steps to edit a curve setting
Step 1. I needed to create a gradient image that could be edited in Photoshop. One way to do this would be to create a new grayscale image that is exactly 1 x 256 pixels in size and draw a horizontal linear gradient going from black to white. But the gradient steps must be precise, where each pixel represents a discrete pixel value going from 0–255. The most accurate way to achieve this is to open the Curves dialog and click on the pencil icon to switch the Curves dialog to ‘arbitrary map’ mode. It is important that no edits are made to the curve. Next, I went to the dialog fly-out menu and chose Save Preset…
Step 2. I then saved this preset to a desired location and clicked ‘Save’ to create the Cruves arbitrary map (.amp) setting shown here.
Step 3. I then wanted to convert the setting into a Photoshop Raw file. To do this I renamed the setting file, changing the file extension from ‘.amp’ to ‘.raw’.
Step 4. Now we can open the renamed raw file in Photoshop. You can use Bridge or the system navigation dialog to locate the file and then click Open.
Step 5. This next bit is quite critical. The Photoshop Raw Options dialog will appear next and here I entered the dimensions for the file we were about to open: 256 pixels for the width and 1 pixel for the height. In the Channels section it was important that I set the count to 1, which meant that we would be opening this file as a grayscale image. After I clicked OK, another dialog appeared telling me that the specified image is smaller than the source file. I clicked OK to open anyway.
Step 6. At this stage the missing profile dialog may appear (depending on how you have the color management preferences configured) and it is important here to NOT color manage the file we are about to open.
Step 7. At last! We have managed to open the saved curve setting as a Photoshop image.
Step 8. Now for the part where we edit the image file in Photoshop. In this example I added an adjustment layer and changed the blend mode to Multiply @ 100% layer opacity and then flattened the image to permanently edit the pixels in the file I had just opened. This is the stage where you can experiment using other blend modes, a combination of blend modes, or any other tone editing techniques that will work when applied to a grayscale image.
Step 9. I saved the edited file to the Photoshop Curves presets folder. This can be accessed by going to the Photoshop application folder and locating the Curves folder inside the Presets folder. You will note here that I saved the file using the same Photoshop Raw file format, but I changed the extension to .amp when saving (in the ‘Save As’ section the file is saved as ‘Multiply.amp’). When I clicked Save, the Photoshop Raw Options dialog appeared and I simply clicked OK.
Step 10. Now it is time to put the edited curve setting to use. I opened a new image, added a Curves adjustment layer and chose Load Preset… from the Curves dialog fly-out menu.
Step11. I located the recently saved Multiply.amp setting and clicked ‘Load’.
Step 12. This will load the curve setting for the multiply curve and append the setting to the list of presets that can be accessed via the Curves dialog Preset menu. As you can see below in Figure 4, one can use the steps described so far to add more presets to this list.
Figure 4. Here is a screen shot showing the Curves dialog where I had created and added curve settings for more Photoshop blend modes.
Figure 5. Here is a repeat view of the Curves dialog that was shown at the beginning of this article in which I have overlaid the curves used to define the Screen, Multiply, Soft Light and Overlay blend mode curves.
Putting theories to the test
There was a time when my instincts led me to believe that the Photoshop blend modes, in particular Screen and Multiply, performed some kind of special magic that made them superior to normal curves. I used to assume (incorrectly) that they somehow added levels to an image rather than simply stretched the existing levels in a picture, which is what Levels or Curves adjustments do. So one of the things this exercise allowed me to do was to directly compare blend mode created curves with basic blend mode adjustments.
As I said earlier, you can use the above steps to create your own set of blend mode curves, or if you want to cut out all the hard work, just download the ZIP file which contains a set of blend mode curves for you to experiment with.
In one test I did I took an image and duplicated it. For one version I added an adjustment layer, set the blend mode to say, Multiply and flattened the image. I then took the other version of the image, applied a Multiply curve and again flattened it. The two images should look identical. But not only that, they will statistically be identical too. One way to prove this is to overlay one of the images on top of the other in registration and set the blend mode to Difference. If the blended image is solid black, then you know that they are exactly the same. Now there is one point I will concede here. If you carry this out on an image that is in 8-bit per channel mode, the results will be identical. If you carry this out with an image that is in 16-bit per channel mode, the results will look the same but there will be a very slight difference in the statistical analysis. I raised this observation with Russell Williams, who explained that a curve map setting will specify an exact 256 values for the curve mapping and when this is applied to an image with more than 256 levels (i.e. a 16-bit image) it will have to interpolate the values that fall between those 256 points. Note here that the ‘interpolation’ here refers to the interpolation of the curve mapping for the in between curve values and this is true for any curves map mode type adjustment that is applied to a 16-bit image.
The other theory I wanted to test was whether blend mode adjustments will preserve more levels. The answer to this is no. A blend mode adjustment does exactly the same thing as a Levels or Curves adjustment, it simply maps the levels to new positions. Where clipping occurs some levels will be lost, but no levels will ever get added.
Blend modes use formulas to calculate the tone mapping and this it is argued, produces a smoother curve mapping when editing 16-bit images compared to using curve maps. This does appear to be true. For example, if you create a gradient test image in 16-bit, posterize it and compare the two different methods for adjusting the tones and then analyze the two histograms, the levels appear to shift more smoothly when you use the blend mode method of adjustment. So is the blend mode method still better? Possibly, although the difference between the two methods at 16-bit is so slight that it is hard to say which one is actually ‘better’. I would be interested to know what others think on this subject. As Russell Williams explained, the difference can be so negligible that the key conclusion would be to select whichever method was the most easy to use. This leads me to the final section where as an added exercise, I wanted to look at the possibility of merging multiple curves adjustments into one.
Merging cumulative adjustments into a single curve
At the beginning of this article I showed how two Multiply adjustments could be used to darken the sky in a beach seascape photograph. Now the chief advantage of working with adjustment layers is that you can mask the adjustment layer effect and the settings will remain editable. But when you add one adjustment layer on top of the other and flatten the image the image is effectively adjusted as if you had applied each of the curves adjustments sequentially. The downside of this is that the levels will get adjusted in sequence, one adjustment as a time, which is not the most efficient way to edit the tone levels in an image if you wish to preserve as much image detail as possible.
So for this final exercise I opened the standard Photoshop Raw linear gradient image and applied the same sequence of blend mode adjustment layers as were used in Figure 2 and applied these to the image, as shown in Figure 6. You will note that the uppermost adjustment layer has been set to 60% opacity, to match the settings used in Figure 2. I then flattened the image and saved the flattened file as a custom curve setting, naming it: ‘Multiply_100-60.amp’
I then opened up the original beach scene photograph and applied this custom curve setting. Figure 7 shows the layers palette with the single Curves adjustment layer in place. Figure 8 shows the curve shape appearance of the custom curve. And finally, Figure 9, shows the end result, which is identical to the one at the beginning of this article that was created using a combination of two adjustment layers.
This main point of the exercises and steps shown here has been to provide some insight into the way Photoshop calculates its blend mode tone adjustments and how at one level you can say these have the same effect on an image as applying a normal curve adjustment.
Of course from a practical point of view it is not always going to be feasible to jump through all the hoops described here in order to create a ‘merged curves’ adjustment. But if for example you had a favorite tone mapping adjustment that involved say adding a gradient map adjustment followed by a curves adjustment to add a kick to the shadows followed by a layer blend options tweak, you could consider using the steps described in the last example to create a single curve setting that would let you achieve the same result in much less time.
But apart from that I did also want to demonstrate that in theory as well in practice, it could be possible for Photoshop to have a ‘merge adjustments’ feature in which certain types of multi-layered tone adjustments could be calculated as a single Curves adjustment.
Download the curve settings
If the above techniques interest you then you can download the curve settings as a ZIP file that I created for this article. The unstuffed folder will contain a Screen, Multiply, Soft Light and Overlay curve setting as well as the original Photoshop Raw file gradient image.