Create Pixel Mask

The Create Pixel Mask command creates a pixel mask, which is a list containing the (column,row) coordinates of pixels. The pixel mask is used by the Apply Pixel Mask method to repair defective points at the mask coordinates. To create the pixel mask, defects are identified in a source image by testing for pixel values greater than and/or less than specified values. The identified pixels are saved to a pixel mask file which is then loaded by the Apply Pixel Mask command. You can also edit this pixel mask file (or any pixel mask file) using the Edit Pixel Mask command.

• The Create Pixel Mask command is executed from the Process > Cosmetics menu.

Overview of the Merhod

A pixel mask contains x,y pairs of pixels. The purpose of this command is to automatically identify pixels having extreme values and save them into a pixel mask file. Later, that pixel mask file may be edited to remove or correct coordinates or to add coordinates that were not identified by the present command. This command creates the pixel mask from a source image, which must be chosen from one of the displayed images. To identify pixels for inclusion, their values either must be very high or very low compared with "normal" values occurring in the source image. You can use this command to identify values that are high, low, or both, and may be anywhere in the image or within a rectangular region of interest.

To use this technique to create a pixel mask, you need a source image containing high or low pixels that are defective. Usually a dark frame or a flat field frame is used for this purpose, since the dark frame shows hot pixels well above the normal distribution of values and a high-signal flat field frame reveals "cold" pixels far below the normal distribution of values. The threshold values can be determined before opening this dialog or afterwards by inspection of the source image. This dialog does not take command of the screen, so you can determine the threshold values while the dialog is open by roaming the cursor over the image or by doing image statistics or other analyses.

The result of this command is a collection of pixel coordinates saved as a Pixel Mask file (in the dialog, the pixel mask file name is shown scrolled left so that the most significant part of the file name is visible). The recommended file extension for this type of file is pxm, as in Pixel-mask.pxm, although txt may be used, or any other extension may also be used. However, it is best to stay with pxm. After running this command, you can view and edit the pixel mask file using the Edit Pixel Mask command. The pixel mask is separately applied to other images using the Apply Pixel Mask method in the Calibrate Images tool.

Detecting Hot Pixels

Select a dark frame in the Image field and check the Greater than option. This will identify pixels that are greater in value, or "hotter", than the threshold value. Enter a threshold value that is below the values of the hot pixels but is above the values of the "good" pixels in the dark frame.

Detecting Cold or "Dead" Pixels

Select a bright flat field frame in the Image field and check the Less than option. Enter a threshold value that is above the values of the cold or dead pixels but below the values of the typical "good" pixels.

Related Topics

Repairing Artifacts and Cosmetic Defects, Image Calibration, Apply Pixel Mask, Pixel Mask Editor, Blemish Mask Editor, Clean Image Set