Meade Image Processing Service Manual download pdf (Page 49)

Appendix A 45

Appendix A

Image Processing Basics

Image processing allows you to modify the appearance of an image by applying various types of filters, scaling options

or transformations. The simplest type of processing is linear scaling. With linear scaling, one pixel from the source

image is multiplied by a scale factor, then an offset term is added. The original pixel value is then replaced with the

resulting value. This process is repeated on each pixel in the image.

The contrast and brightness of the image can be controlled by varying the scale factor (contrast) and the offset value

(brightness). You should attempt to set the contrast to a value that allows the full range of the important parts of the

image to fall on the sloping section of the transfer curve (see the Scale Image function), then set the brightness so that

the minimum pixel values appear black or very dark gray. The following histogram and transfer graph illustrates the

optimal contrast and brightness settings to give a normal view of the image.

You may need to experiment to find the right settings for any given image. Sometimes it may be necessary to scale

different areas of the image using different scaling factors. This can be accomplished by using a technique called

masking. Masking allows you to separate various areas of an image based on a range of pixel values. Try the

following example which illustrates masking:

1. Load the VGR_IO_C.BMP image.

2. Copy the image into Buffer B.

3. Select the Image, Scale dialog box.

4. Select Linear scaling if not already set.

5. Move the right red slider to the third line from the left side of the graph. You should now see only the volcano portion of the

image.

6. Copy this image into Buffer A.

7. Again bring up the scaling dialog box, then select the Dynamic Scaling option. The resulting image should now show the

volcano with the desired contrast.

8. Copy this image back into Buffer A.

9. Select the Merge Buffers menu, then choose A plus B. This will add the volcano 'mask' to the original copy of the image.

The resulting image will now show the planet detail with the enhanced volcano.

To be strictly accurate, you should also create a mask of the normal image which excludes the volcano area before

copying the image into Buffer B. Try this on your own and compare the results.

Another useful image processing technique, convolution, allows you to enhance the appearance of an image by

'filtering' the image data by either smoothing (low-pass filter) or sharpening (high-pass filter) or combining parts of

both types of filters.

This is accomplished by passing a convolution matrix, or kernel, over the entire image and replacing the center pixel in

the resulting matrix with the scaled sum of all of the other values. For example, if you wanted to average each pixel

with its adjacent neighbors the following kernel could be used.

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Meade Image Processing Service Manual Page 49