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Grokking The Gimp
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7.5.2 Color and Brightness Matching

You can see that there is significant brightness variation from layer to layer in Figure  7.35. This is normal for photos taken with most consumer digital and regular film cameras. These cameras typically auto-expose scenes according to average lighting conditions and do not offer user controls for exposure. For the image in layer A of Figure  7.35 the light, coming from the windows is very bright which, due to the average light metering of the camera, causes the features of the room's interior to be underexposed. Thus, the room features in this layer are quite a bit darker than in the other layers. Otherwise, the brightness of the other layers are more or less consistent with each other.

In addition to the brightness mismatch between layers you can also see from Figure  7.35 that there is a color balance mismatch from layer to layer. The combination of color and brightness variations means that the layer boundaries are plainly visible instead of presenting a smooth and imperceptible transition across the panorama.

The strategy for correcting the differences in brightness and color is to use the Curves tool. The idea is to match color at boundaries between layers using a method similar to that described in Section  6.2.2. The method measures pixel values on both sides of a layer boundary using the Color Picker  tool. The Curves tool is then used to match the values. This procedure corrects for both color and brightness mismatch simultaneously.

Matching the color and brightness of two layers has a chain reaction effect in a panorama project. Matching layer B to its neighbor A, means that subsequently layer C must be matched to B, and so on. Thus, some care must be taken to avoid blowing out the available tonal range. Typically, the wisest decision is to choose the layer of average brightness and to match the other layers working away from this one. However, for this panorama project, it is layer E that is chosen as the reference because its lighting for the room seems the most natural. The work flow, then, is from the leftmost layer to the rightmost, from layer E to layer A.

Starting with the boundary between layers E and D, a pixel value was measured on the white wall just above the wood wainscoting. The measured values are 177R 183G 194B to the left of the boundary and 153R 156G 171B to the right. Using this information, the Curves tool is used on layer D to match the pixel values measured in D to those of layer E. Representative pixels are then measured across the boundary between layer D and layer C. Here, the measured pixel values are located at the midway point between the hanging picture and the ceiling molding. The values are found to be 179R 175G 185B to the left of the boundary and 112R 119G 139B to the right. The Curves tool is employed again, this time on layer C, matching C's pixel values to those of D's.

Continuing with the boundary between layers C and B, the measured pixel values at a point midway between the mantle and the molding are 201R 197G 211B to the left and 101R 99G 112B to the right. The final boundary is between layers B and A. Here the pixels are measured at the midpoint between the plant and the molding. The values found are 199R 198G 208B and 86R 75G 81B. The Curves tool is applied for each of these boundaries, as it was for the first two.

The results of the color and brightness matching operations are shown in Figure  7.37.

  
Figure 7.37: Initial Color and Brightness Matching
Figure 7.37

The overall color and brightness of the images in the layers are now much more consistent. However, there continues to be sufficient mismatch between the layers to perceive the layer boundaries. Fortunately, this visual defect can be corrected using a layer mask  blending technique described in the next section.

Grokking The Gimp
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  Published under the terms of the Open Publication License Design by Interspire