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Flatbed Scanning Hints

3 channel Brightness & Contrast controlsEven when the scanning software includes an AUTO button/option for automatically setting the tonal range of the cropped picture area, we must understand the fundamental differences between Brightening a picture and altering the Contrast so that manual changes can be easily applied.

At this point note that if your cropping area on the scanning preview window includes image information outside the actual picture you are scanning then any feedback values or histograms will include the colour values of those extra areas also. In other words relying on your preview-scan image information or especially relying on any auto options, if the image is not tightly cropped, will usually result in very inaccurate output from the scanner.

Brightening. This option does not alter the tonal range of a picture, but merely lightens or darkens all pixels (unless adjusted so far that clipping occurs and picture information is lost).

Contrast: Adjusting the contrast of a scan or an existing bitmap will alter the tonal range of an image equally at both ends of the tonal range.

Single Brightness (may show 'Threshold')The picture above and at the left show two different ways the scanning software might present these controls. Most decent scanning software (more expensive) will offer individual channel adjustment for the more experienced operator that may wish to change each channel separately. However, when scanning line images a window with only one control will appear. This control might keep the name, 'Brightness, or change to 'Threshold'. Read the Threshold page in the GENERAL classroom for an explanation of how this important control works.
A graphic look at what the controls do:

Bright + and Bright -

Compared to our Normal greyscale below, it can be seen in the first box above that both the Shadows or dark areas of the picture AND the Highlights or light areas of the picture have been lightened by applying a positive Brightness value. By applying a negative Brightness value, the effect is the opposite by making the picture darker overall (second box above).

 Normal, Contrast + and Contrast -

Again referring to the Normal greyscale above, it can be seen how the adjustment of Contrast with a positive value will make the Shadows darker and the Highlights lighter. Alternatively, adjusting the Contrast with a negative value will lighten the Shadows and darken the Highlights (often referred to as flattening a picture).

Using the controls to adjust an image or preview-scan:

The horizontal line and vertical end points in the boxes below represent the tonal range of a scan or an existing bitmap. The box represents the maximum image range possible - RGB 0 black to 255 white. Follow these simple steps required to adjust a picture with both the Brightness and Contrast controls. Refer back to the pictures above if necessary.

Poor scan
Image too flat (short range) and out of balance

First, attempt an adjustment with the Brightness control to centre the tonal range within the maximum allowable range.

Brightness contol centers image data
Image too flat (short range), but midtones balanced

Next the image tonal range is adjusted by the Contrast control so that the tonal range changes (expanded in this case) to fit equally within the maximum range allowable without clipping.

Contrast Control expands image data to full range
Corrected Shadows, Midtones and Highlights

The last page describes the Gamma control which can be used after a bitmap is correctly adjusted to primarily alter the Midtones, if desired.

Clipping occurs when any original picture information is adjusted beyond the maximum values and is therefore lost, affecting usually important picture detail
i.e. The subtle shadow detail often seen in the dark areas in and under trees etc.
i.e. The subtle Highlight cloud detail that makes a bright sky interesting or dull.

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e-mail 1997 '98. Last Revised:  Friday, 31 October 2003 22:04