While many CS2 capabilities are not available with HDR files, including layers and most of the tools, some filters and a few other functions can be used with 32-bit files. Personally I am not sure what adjustments I would wish to make at this point, given the lack of layers and the inability to directly see the full image at once because of its expanded DR. Perhaps 32-bit image processing will continue to be an area of innovation in future versions of Photoshop. For now we will simply move on to tone mapping the HDR image into a 16-bit file.

CS2 HDR Conversion

In some ways everything up to this point really has just been preparation. Now it is finally time to get a concrete result – a 16-bit image that can be edited, displayed or printed just like any conventional image. However, the tone mapped HDR image will show an interpretation of the original scene in a way that is difficult (perhaps even impractical) to achieve using other means involving comparable time, effort or cost.

Following the above actions, CS2 is sitting with the merged (and saved) HDR image open in the editor. To perform the tone mapping process, you trigger the HDR Conversion function by selecting the Image>Mode>16 Bits/Channel menu item. This brings up the following panel:

CS2 permits tone mapping an HDR image using four different methods, available in the drop-down list of this panel. The first three methods are not satisfactory, at least for the type of HDR images I work with. While easy to use, they may produce useful results only in limited circumstances. This is because they provide simplistic global effects, with few or no control points, for compressing the DR of the 32-bit image into the target bit depth.

As you select each method, the HDR image open in CS2 will show a preview of what the conversion will look like. Feel free to explore these methods on your own HDR images, but they will not be discussed further in this article.

The interesting conversion method is called “Local Adaptation” and this is what I will focus on. When you select this method from the drop-down list and also click the small arrow icon labeled Toning Curve and Histogram, the HDR Conversion panel now looks like this:

This provides a familiar looking curves control, with some differences from the standard curves adjustment tool. The Load and Save buttons perform a familiar function. A custom set of parameters can be set up for an image, saved (using the file name extension “.hdt”), and then reloaded as a starting point in future editing sessions. As in other editing panels, pressing the Alt key (Option key on a Mac) switches the Cancel button to Reset. Clicking Reset returns the entire panel to its default state, which means the Exposure and Gamma method with its default control values.

The panel provides no ability to select just the red, green or blue channels – the curve is applied to the entire image because HDR is about working with luminance, not color. Also, there are no black, gray or white point eyedroppers, nor any automatic functions or freehand curve drawing. The curve can be edited by moving the initial black and white end points, and by adding and dragging new points.

Editing points on the curve is one of the two main attractions of using this conversion method. It permits direct control over the translation of the HDR image’s wide DR into something that will fit within 16 (or 8) bits, and also present a desirable balance of tonality and contrast. Thus editing the curve here is done for the same reason as making a curve adjustment on a normal image.

The difference here is that the curve is affecting a much greater initial DR than normal (see the red tick marks of the histogram), and will redistribute contrast as well as actually compress the DR when you hit OK. To get the look you want it may be necessary to do a more than simply apply a bit of traditional “S-curve.” The greater the DR, the more work you will have to do to prevent CS2’s automatic functions from deciding for you how to compress the DR. You may have several tonal regions that you need to adjust in different ways, rather than simply focus on a simple shadows-midtones-highlights break down as with normal images.

Looking at the unfamiliar controls on this panel, first there is the Corner check-box below the curves control. Not available with the standard curves tool, this check-box controls whether the next point added to the curve creates a smooth change along the curve where it joins other existing points, or creates a sharp and angular join. This effect can be used to create a sharp transition in the tone of the image at certain levels, rather than a transition that is smoothed out over a broader luminance range. Again, because this curve potentially must compress the DR by a large margin, you may not have the luxury of making smooth tonal transitions in some cases. Having a sharp transition can protect tonalities that you have already adjusted from being skewed by further adjustments along other parts of the curve.

As with the normal curves tool, you can move the mouse cursor over the main image window, where the pointer turns into an eyedropper. Clicking and holding over a point in the image highlights, on the curve, where that image point falls. Dragging the eyedropper around on the image, the highlight point on the curve tracks the mouse movement. This lets you target areas of the image tone you may wish to fine tune on the curve.

The principles of adjusting the curve here are the same as when editing curves in a non-HDR image. Move the bottom left and top right end points closer to the tails of the histogram, to set the black and white points where image data starts to appear. Then use one or more points along the curve to adjust its slope. Likely you often will use several points, reflecting the fact that you have more than one area of tonal range with significant detail you wish to enhance. Make the slope of the curve steeper (more vertical) to increase contrast where important image detail lies. Make the slope shallower (more horizontal) to compress tone where little or less important image detail lies. Remember that the large DR present is going to get compressed into a smaller range as a result of this tone mapping exercise, so you may have to sacrifice contrast in some areas that have good detail in order to preserve detail in other areas.

The other main attraction of this panel besides editing the curve is using the Radius and Threshold controls to alter the degree of local contrast enhancement performed. As described much earlier, local contrast enhancement increases the contrast within very small regions of the image in order to enhance the appearance of detail there. Even if you spend considerable effort editing the curve, because it is a global effect across the image it is still possible to get areas of contrast that do not work well visually. By changing these numeric values, you can exercise a different form of control over the tonal quality of the conversion.

The Radius control specifies the number of pixels that the conversion function will consider to mean “local.” A small radius, such as the default value of 16 pixels, means that contrast enhancement is applied in very tight regions across the image. This will result in hard edged, sharply defined tonal shifts that occur on a small scale. The advantage is that this tends to emphasize fine detail. The downside is that it tends to look less natural and over-processed, something that is often levied as a criticism of tone mapped HDR images.

The range of the radius is from 1 – 250 pixels. If you set the radius too low, the result will look very flat across much of the field of view because the contrast enhancements have fallen below the threshold that your eye can readily pick up. If you set the radius too high, the result often will look more natural in tonality but may seem a little “plastic” and lacking in fine details that are enhanced by strong local contrast. To determine the “right” radius for a given image, move the control between extremes that are clearly “wrong” until you find a region of values that works.

The Threshold control also affects the local contrast enhancement, and sets the difference in luminance between adjacent pixels for them to be included or excluded from the current local region. Similar to the threshold value in the Unsharp Mask filter, this control relates to edge detection. This is a key part of the human visual system’s ability to perceive detail and apparent sharpness. Changing the threshold value affects how “contrasty” the resulting image will be.

The threshold can range from 0.1 to 4.0. If you set the threshold too low, once again the converted image may appear washed out and flat. A lot of the inherent detail will be softened because very slight tonal differences between pixels are enough to exclude some of them from the region being enhanced. However the results will appear smoother and more natural. Setting the threshold too high, conversely, certainly will emphasize detail and make everything stand out. But the results likely will be too stark. I often set the threshold somewhere between 0.5 and 1.0.

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