# The PanoramIX Authoring Tool

## Version 0.1.4.92, February 26, 1998

## 4.3 Balance Exposures Dialog

In principle, a scene is captured, printed, scanned, and processed with equal exposures for all images used to capture the scene. In the case of computer generated images, it is very easy to enforce such uniformity of color intensity for all images. In the case of photographic images of real scenes, however, there a number of factors which can cause unequal exposures for individual images representing a panoramic scene. Both film cameras and digital cameras, as well as video cameras typically provide automatic exposure features in order to make each image as good as possible. If possible, this feature should be disabled. For some low-cost digital cameras, it may be impossible to disable this feature.

In addition, when film negatives are printed, the film processing laboratory typically attempts to optimize the exposure for each image individually. If possible, the film processing laboratory should be asked to avoid any exposure optimizations when making photographic prints. This is usually not possible.

Because of well-intentioned efforts to optimize the exposure of each image, adjacent images used to capture a panoramic scene may have large differences in exposure, especially of the camera is pointed towards a bright light source in one image. The panoramic editor provides two means of compensating for these exposure variations. One is an automatic exposure balancing. The second is manual exposure balancing. Either type of exposure balancing may be obtained by selecting the Balance Exposures entry in the Options menu. Making this selection opens the Balance Exposures dialog.

The Balance Exposures Dialog has the following parts:

1. Image Identification and Selection
2. Global Exposure Settings
3. Optimizing Exposures for Each Image

### 4.3.1 Image Identification and Selection

The currently selected image is indicated in the Balance Exposures dialog by its image number, image type (top, bottom, side, panorama, or sprite), and the name of the image file from which this image was obtained. One may step through successive images by clicking on the Next and Previous buttons, or by clicking on the spin control in the image number field. One may also jump directly to any image by entering the image number (starting from zero) in the image number field.

When balancing exposures for a panorama defined by a top image, a bottom image, and a set of side images, one should skip over the top and bottom images until after all of the side images have been balanced. The first side image should either be left unchanged, or its exposure should be adjusted manually. Do not apply automatic exposure balancing to the first side image.

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### 4.3.2 Global Exposure Settings

After opening the Balance Exposures dialog, one may choose an individual image to adjust, or one may select a global exposure setting function. The two global exposure setting functions are selected by the buttons labeled "Balance All" and "Reset All". The Reset All button resets all exposure factors to unity. This is the same as setting all exposure changes to zero. This change takes effect immediately and cannot be reversed. The projected image seen in the main window should show the effects of this change as soon as the panorama can be reconstructed.

The Balance All button causes the panorama editor to compare every side image except the first image to the preceding image (the one to the left). In addition, the top and bottom images are compared to all overlapping side images. The resulting exposure changes are applied to all of the images and the panorama is updated. This change takes effect immediately and cannot be reversed. The projected image seen in the main window should show the effects of this change as soon as the panorama is reconstructed.

The effects of the Balance All may be very good for some images, but very poor for other images. The reasons for this variation are not understood. This may reflect undetected bugs or flaws in the exposure balancing logic, or they may represent limitations to the ability to balance exposure based on overlapping blocks of pixels. Whatever the reason, if the results of the Balance All function are not satisfactory, the user can select each image individually and allow the exposure to be determined automatically or manually.

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### 4.3.3 Optimizing Exposures for Each Image

Exposure factors for individual images can be optimized automatically or manually.

To adjust an exposure factor manually, enter the exposure change value in the Exposure Change field for the current image. An exposure change of "p" percent is equivalent to an exposure factor of 1+(p/100). The value entered in the Exposure Change field does not take effect until the Apply button is selected. After the Apply button is selected, the panorama will be updated and the effects of the exposure change may be seen in the main window.

After setting the desired exposure level for the first side image, select each successive side image and try the automatic exposure factor for that image. This factor is determined by clicking on the Balance button. If this factor is good, continue to the next image. If the automatic exposure factor is too low or too high, alter the value in the Exposure Change field manually and click on the Apply button. Repeat this until a suitable exposure level is established for the current image, then advance to the next image by clicking on the Next button. After setting the exposure levels for all of the side images, go back and set the exposure factors for the top and bottom images.

The automatic exposure balancing function works by comparing the brightness of corresponding pixels in overlapping areas of adjacent side images. A block of pixels in the left side of a selected image is compared to the corresponding set of pixels on the right side of the adjacent image to the left. The ratio of intensities determined by these blocks of pixels is then used to adjust the intensity of the entire image. Exposure factors for the top and bottom images are determined by comparing the intensities of pixels in the top and bottom images to the corresponding pixels in side images.

To close the Balance Exposure dialog, select the OK button or the close (X) button in the title bar.

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Last modified February 27, 1998.
wluken@watson.ibm.com