Digital Reduction of Film Graininess


A photo of  NGC7023 by Tony Hallas (used by permission) was used as a reference for this image processing technique.  Here is the before and after results of a process developed for grain reduction using the tools in Picture Window:

Ngc7023ba.jpg (251041 bytes)

Here's the process:

1) Define a mask which will protect the light area and will allow further processing of the grainy low light areas.

     a) Load the image into PW

     b) Select the Mask Function Mbutton.gif (1004 bytes)

     c) In the Mask Window select the Light Curve Button Cbutton.gif (984 bytes)

     d) Select the Probe wpe1.jpg (828 bytes) and with the left mouse button pressed point to the image at the point where the grainyness ends and lighter area begins:

probe.jpg (76018 bytes)

   e) Now move the left bottom up-arrow to the grid line shown by the red vertical line and press the "Apply" button

masked.jpg (75462 bytes)

   f) Select the masked image, magnify to a 3:1 with the magbutton.gif (953 bytes) button (on main menu bar) and move the  image to a bright star area.  Hit the feather button fbutton.gif (963 bytes) , slide the amount to "1" and hit "Apply".  The idea here is to increase the mask to the edge of the bright stars.  High resolution images will likely require a higher amount of feathering to reach the edge.

feather.jpg (50322 bytes)

  g) Hit the "OK" button in the mask window.   You should now have the completed mask as partly shown below:

maskend.jpg (16493 bytes)

2) Now use the mask with a Gaussian Blur to remove the grain in the low light areas:

  a) Select the "Blur" function under the "Transformation" button on the main menu and reset the image back to a 1:1 using the minusbutton.gif (905 bytes) button.   Select "gaussian" in the blur window.  Point and Click the "Input Image" box and select the original color  image.  Point and Click the other box and select the untitled mask image.  Move the white slider bar to the far left and the black slider bar to the far right.            

  b)  Check the "Auto" box and enlarge the preview image to a 1:1 so that you can see the final result side by side with the original image.  For this example the "R"  was set to "1" and the Threshold was set to "28".  These two setting will vary from image to image.  I usually start with R at 1 and then vary the threshold up from 0 until the grain just disappears.  Too much of either with make the image look motted.

  gblur.jpg (115805 bytes)

c) Hit "OK" when you have what you want and the final image is ready to be saved.

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