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hand-blending high dynamic range (hdr) images using luminosity masks



Blending the Light Tones

Figures 1, 2, and 3 show my original exposures.

Figure 1—Underexposed
Figure 1

Figure 2—Normal
Figure 2

Figure 3—Overexposed
Figure 3

I arranged them in a layer stack as shown in Figure 4.

Figure 4
Figure 4

The first order of business is to blend the lighter tones from the Underexposed image into those of the Normal exposure. In order to make this happen, a tone mask is needed for the Underexposed layer that reveals the light tones and masks the dark tones. That would be one of the Lights-series masks: Expanded Lights through Super Lights. Since there is often an element of experimentation required to figure out which mask will work best with luminosity painting, it's a good idea to just make all the masks in the series—Lights-series in this case—and let them sit on the Channels palette (Figure 5) for easy access.

Figure 5
Figure 5

Both the Light Lights (Figure 6) and Bright Lights (Figure 7) seemed to have potential for matching the correct tones needed in this situation; they are very light in the brightest tones and black in the darker tones. This means either might be useful in revealing just those tones that need to be visible on the Underexposed layer. In the end I chose to use the Bright Lights. While it may look like the Light Lights reveals the blue sky better and would thus work better, my experience is that a more restrictive mask often works best for luminosity painting. As the paint is applied repeatedly through the selection created by the mask, enough also passes through the darker areas, such as the blue sky areas of the Bright Lights mask, to have the proper effect.

Figure 6—Light Lights mask
Figure 6

Figure 7—Bright Lights mask
Figure 7

In order to paint, a canvas is needed to paint on. In this case, the canvas is going to be a black layer mask on the Underexposed layer. To make this mask, Alt+click (Mac: Opt+click) on the Add-layer-mask button at the bottom of the Layers palette. (red circle Figure 8). The menu command Layer > Layer Mask > Hide All can also be used. When completed, there should be a black layer mask next to the image on the Underexposed layer (yellow square Figure 8).

Figure 8
Figure 8

With the layer mask in place, load the selection of the Bright Lights by doing a Ctrl+click (Mac: Cmd+Click) on the mask itself on the Channels palette. Be careful not to click in the box to the left of the mask which turns the eyeball on and causes some problems. Then hide the marching ants by typing Ctrl+H (Mac: Cmd+H). This command only turns off the ants, the Bright Lights selection that was just loaded is still active and selecting the appropriate tones. Painting requires constant visual assessment, so turning off the ants means they won't interfere with making judgments about where and how much to paint.

Make sure the painting brackets surround the layer mask as in Figure 8 on the Underexposed layer. If not, click once on the layer mask to move the brackets to the mask.

Select the Brush tool (type the letter B). From the drop down menu on the options bar for this tool, choose an appropriate diameter for the brush and a low-percentage hardness so that the brush is well-feathered. Set Mode to normal, Opacity to 100%, and Flow to 100% (Figure 9). I've started using 100% opacity on all my initial luminosity painting strokes since it often takes high-percentage opacity to force enough paint through the selection onto the canvas to make a visible difference. If it looks like 100% provides too great an effect, then I undo the step and lower the opacity. However, more often than not, I need multiple strokes at 100% to build up to the desired effect, especially if I'm using a restrictive luminosity selection.

Figure 9
Figure 9

Since the layer mask is black, white paint needs to be applied to reveal what's in the image, so make sure the foreground color is set to white. (Figure 10). If not, type D to restore the default colors and/or type X to reverse the foreground and background colors. The white square should be on top before starting to paint.

Figure 10
Figure 10

Now it's simply a matter of painting over the sky area on the layer mask until the desired amount of properly exposed light tones is revealed. The black mask starts turning white when selected pixels are painted. Because painting occurs through a mask (Bright Lights), the paint is applied in proportion the brightness of the pixels in that mask. The lighter the pixel, the more paint the pixel receives. So the white that emerges on the layer mask is not solid. It is perfectly textured and feathered to match the tones from which it was created, which are the tones in the Underexposed image. This feathering in the painted-on layer mask reveals pixels in the Underexposed image in a way that insures they blend well with the Normal image below.

Keep in mind that there are many options while painting. If the effect is too strong, simply undo the brush stroke—Ctrl+Z (Mac: Cmd+Z), lower the brush's opacity, and paint again. It's also possible to reverse the revealing effect of the painted-on mask by changing the color of the paint to black (type the letter X) and painting the areas that need to be concealed. Don't forget that if the first luminosity mask choice doesn't seem to work, it's very easy to discard the layer mask, load a new luminosity selection from the masks on the Channels palette, and paint through the alternate selection. Lastly, it's also possible to lower the effect of the parts of the image revealed by the painted-on mask by decreasing the layer's opacity. That's what was done here. While the final paint job revealed the right pixels in the Underexposed layer, the sky became a little too dark in the process. I lowered the Underexposed layer's opacity to 65% to allow some of the lightness in the Normal layer below to show through to mitigate this.

Figure 11 shows what the layer mask looked like when painting was finished. It's obviously quite different than the Bright Lights mask shown in Figure 7 above. By using multiple brush strokes through the Bright Lights mask, luminosity painting essentially provided a method to customize the Bright Lights mask for the Underexposed image.

Figure 11
Figure 11

Figure 12 shows the results. The painting procedure does a nice job of revealing the light areas of the Underexposed image in a way that helps them blend well with the normal exposure (though the normal exposure obviously still needs blending work also to make it less dark).

Figure 12
Figure 12

It's important to note that because the selection for the Bright Lights is totally black in the area where the arch is, there's no fear or worry that the painting will accidentally reveal the severely underexposed arch on the Underexposed layer. The active selection being painted through makes sure this can't happen. So painting can be fast and sloppy and haphazard and still turn out well. Painting through a luminosity mask selection insures that only the right pixels get painted so that only the right pixels will be blended from the Underexposed layer. Anything too dark and not part of the selection—like the arch—gets no paint at all and stays concealed.