Stretch

Figure 21. The Stretch command modifies the data contained within the image in order to improve contrast and brightness.

Color images from CCD cameras typically require a background level adjust- ment. This is accomplished by bringing the background level (or bias) in each color plane down to zero. Each of the Background Level values is subtracted from every image pixel in its color plane. Any pixel values that become nega- tive are forced to zero. The Auto button automatically determines the settings necessary to equalize the image background in all three color planes. The Reset button resets the background level subtraction to zero on all planes.

Scaling adjustment (entered as a percentage) allows you to compensate for transmittance differences between the filters used to acquire the three color planes. Values of 100% result in no change. The scaling percentages can be typed in or adjusted using the “spin” controls (small up and down click arrows to the right of the scaling numbers). The Preview Image is particularly help- ful in monitoring the results when using the spin controls. The Reset Scaling button resets to 100% on all three planes.

The Click On White Area to Set Scaling check box enables the operation of the mouse to set the scaling. Set the Background Level first (you can use Auto), then click on a white object (e.g. a neutral-colored star) in the image (not the Preview Image) with the mouse. The Scaling settings will automatically be adjusted to make the selected point appear white. If an area of the image

is known to be white (or gray), this is an easier way to determine the scaling factors, and can be used to instantly color balance the image.

The Stretch command (located in the Process menu, Figure 21) modifies the brightness and contrast, and optionally the range (maximum and minimum pixel values) of an image. Unlike the screen stretch settings in the Screen Stretch Window, which only affects the how an image appears on the com- puter screen, the Stretch command actually changes the image data pixel values in the memory buffer. In reference to the histogram, the Screen Stretch Window changes how the histogram is displayed on the computer screen, the Stretch command alters the histogram itself.

There are three elements which must be set: the Permanent Stretch Type, the Input Range, and the Output Range. You can use the Preview Image to see how changing these parameters will affect the resultant image (or click the Full Screen button to see the changes applied to your actual image). You can also open the Screen Stretch Window to see how changing these param- eters change the histogram.

For the Permanent Stretch Type, the Linear Only mode is useful for adjust- ing the range of pixels to match a particular file format. The Log mode is useful for compressing the dynamic range of the image, but can be rather harsh. The Gamma mode allows you to selectively emphasize bright or dim parts of the image by entering a suitable Gamma Value. A Gamma of 1 has no effect, less than 1 will emphasize faint details, while a value greater than 1 will emphasize bright details.

The Input Range can be set to Screen Stretch, which in Linear Mode pro- duces an output matching the current screen appearance. This is useful in producing final images for output to 8-bit image formats which have limited range. Note that the minimum and maximum values are always taken from the screen stretch settings for the original image. These can be adjusted dynami- cally using, for example, Quick Stretch. The Max Pixel selection sets the input range from 0 to the brightest pixel in the image. This prevents any image pixels from being saturated in the final result, but may produce images with low contrast. You can also manually set values with Manual Settings in a similar fashion to the Screen Stretch Window.

The Output Range is used when preparing a file to be saved in a format that has limited range. The Output Range maps minimum input to zero, and maxi- mum input to 255, 4095, or 65535 depending on the settings. Any values that exceed the limits are clipped. The Unlimited setting disables all limiting and is recommended when performing Gamma and Log stretches; it is appropriate when the image will be saved in floating point format.