Adobe Photoshop CS5 for Photographers - Martin Evening [180]
Exposure and Gamma
You can use the Exposure slider to compensate for the overall exposure brightness and the Gamma slider to (effectively) reduce or increase the contrast. These controls are rather basic, but they do allow you to create a usable conversion from the HDR image data.
Highlight Compression
The Highlight Compression simply compresses the highlights, preserving all the highlight detail. It can render good midtones and highlights at the expense of losing detail in the shadows.
Local Adaptation
The Local Adaptation method is designed to simulate the way our human eyes compensate for varying levels of brightness when viewing a scene. For example, when we are outdoors our eyes naturally adjust and compensate for the difference between the brightness of the sky and the brightness of the ground. The difference in relative brightness between these two areas accounts for the ‘global contrast’ in the scene. As our eyes concentrate on one particular area, the contrast we observe in, say, the clouds in the sky or the grass on the ground, is contrast that is perceived at a localized level. The optimum settings to use in an HDR conversion will therefore depend on the image content. In Figure 7.4 we have a photograph of a scene that has a high dynamic range. The global contrast would be the contrast between the palm tree seen in silhouette against the brightly lit buildings in the background, while the localized contrast would be the detail contrast within both the bright and dark regions of the picture (magnified here).
Figure 7.4 Here is an example of a subject that has a wide dynamic range and strong global contrast.
Equalize Histogram
The Equalize Histogram option attempts to map the extreme highlight and shadow points to the normal contrast range of a low dynamic range Photoshop image, but this will usually make for a rather blunt conversion.
The Radius slider in the Local Adaptation HDR Toning dialog Edge Glow section (Figure 7.5) is said to control the size of the glow effect, but I prefer to think of this as a ‘global contrast’ control. Basically, the tone mapping process lightens the shadows relative to the highlights and the tone mapping is filtered via a soft edge mask. Increasing the Radius amount widens the halos. At a low setting you'll see an image in which there may be a full tonal range from the shadows to the highlights, but the image looks rather flat. As you increase the Radius this widens the halos, which softens the underlying mask and this is what creates the impression of a normal global contrast image. You can then use the Strength slider to determine how strong you want the effect to be. At a zero Strength setting the picture will again look rather flat. As you increase the Strength amount, you'll see more contrast in the halos that are generated around the high contrast edges in the image. The Glow Strength slider can therefore be used to soften or strengthen the Radius effect, but you do need to watch for ugly haloes around the high contrast edges.
Figure 7.5 The Local Adaptation tone mapping method (also displaying the Tone Curve and Histogram options).
When the Gamma slider is dragged all the way to the left, there is no tone compression between the shadows and highlights. As you drag the other way to the right, this compresses the shadows and highlights together. The Exposure slider can then be used to compensate for the overall exposure brightness. Note that this slider adjustment can have a strong impact as it is applied after the tone mapping stage rather than before.
The Detail slider works a bit like the Clarity slider that's found in Camera Raw and you can basically enhance the localized contrast by adding more Detail. The Shadow and Highlight sliders are fine-tuning adjustments. These