C71 Brightness Scaling

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This is the most basic of image enhancement functions, and is the most frequently

Astronomical Image Processing for Windows

File Edt View Utilities Calibrate Measure Enhance Transform Color Multi-Image Preferences Window Help jnjx]

Histogram Shape

Input Output

Input. YîransfeiT Output Y Options Scalina Tyoe

{• Exponential C Sine wave

C Exponential C Cosine

C Rayleigh C Rooftop r Gaussian C Hyperbolic

C Tangent C Equalize Peak Skew

113.3755-Min

PV: 113.3755

Count 147 I Percent: 0.1775

113.3755-Min

PV: 113.3755

Histogram Shape

Input Output

Input. YîransfeiT Output Y Options Scalina Tyoe

{• Exponential C Sine wave

C Exponential C Cosine

C Rayleigh C Rooftop r Gaussian C Hyperbolic

C Tangent C Equalize Peak Skew

Count 147 I Percent: 0.1775

Figure C.9 Changing the value of pixels is the first and foremost technique used in astronomical image processing. Shown above, the Histogram Shaping tool modifies pixel values in the Eagle Nebula image to reveal both the dim glowing cloud of gas and the bright star cluster at its core.

used. AIP4Win uses brightness scaling operations when displaying data on your computer screen. Having the Auto Low/High Stretch capability selected causes the program to examine the range of pixel values in the image and assign the lowest value found to black and the highest one to white. All the other values are spread across the 254 available gray levels in between.

Step 1: Load an Image. To make the results of this tutorial more visible, we are going to set the default image stretch mode to Black/White Stretch. You do this by selecting the Black/White Stretch button on the Defaults tab of the Image Display Control. (You will need to load an image first, in order for this window to come up.) Make sure that the Black/White Stretch values are set to 0 and 65535. First load the image file "ml6.fts" from the Image Enhancement subdirectory of the Tutorial directory on the CD-ROM. When the image loads, click the Default B/W button on the Image Display Control.

You will see that the image is completely black, or only faintly displays the brighter stars and blooming trails, and gives no hint at all at what faint objects hide in it. In this view, the pixel value of zero is assigned to the gray level of zero, and the value of 65535 (the maximum value for a 16-bit CCD camera) is assigned the gray level of 255. If you right-click on the image, the Image Status window will open, displaying (among other things) the minimum and maximum pixel values present in it.

For now, leave the Black/WhiteStretch as the Default Display Mode, as it will make the effects of the various enhancement tools much more apparent.

Step 2: Invoke the Brightness Scaling Tool. Click on the Enhance \ Brightness Scaling... menu item to bring up the Scale Image window. Leave the Info window on the screen; you will be using it later.

The Brightness Scaling tool window has the following controls and displays:

• a Transfer Function display, which shows the histogram of the image, overlaid with the transfer function curve being applied.

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