Convert color range selection to "Clean, Hard edge" selection?

Question

I am trying to create forest cover bitmap from aerial photos and getting stymied by something in Photoshop. Here is process and issues:

1. load a geotiff image. Convert background to layer and delete non-data areas(ie. areas outside the aerial photo bounds that are just solid color background)

2. load a color range and inspect it for minor adjustments. Hit Ok to accept the selection.

3. Now, with a rough selection of forested areas, I turn on "Edit in quick mask mode"

4. In quick mask mode I "paint" out areas that were miscategorized in the initial color range selection.

5. Turn off quick mask mode and have my final desired selection.

6. Here is where the problems start: at this point I create a new, empty layer with the selection still active and attempt to fill the selection with solid black using either the edit-fill command, or the paint bucket tool set to 100 opacity and 100 tolerance. What I get is a mixed greyscale fill in the selection areas, and, there is an unwanted linear gradient from left to right so that the rightmost 25% of the image looks like its opacity has been turned down about 50%.

What the heck am I missing?

Is there a clean, simple way to get from my completed selection in step 5 to a solid black fill in all the selection, with hard edges?

THanks,

Tom

davescm · Accepted Answer

There is something strange in your screenshot.

After you have made your selection and then you switch to quick mask mode then there should be no marching ants on your screen. Turning off the RGB channels then leaves a greyscale with a range from full white to full black . Yours does not have that range.

You then apply the threshold to that image.

I didn't have a aerial photo so I used this picture of a lamb in a field to demonstrate (selecting some of the greens with color range).

1. Initial picture:

2. Select using color range to select certain shades of the green - marching ants visible

3. Edit in quick mask mode - red overlay becomes visible

4. Turn off RGB channels in channel panel - just Quick mask channel visible in grayscale - no marching ants

5. Image - Adjustments -Threshold applied (note the full range from black at the left to white at the right

6. RGB channels back on

7. Click in toolbar to exit quick mask mode - Marching ants return but this time with hard edges

changes to

Dave

davescm · Answer

It sounds like your selection is not entirely black or white but has shades of grey within it. These don't show with the marching ants and are not clear with the red overlay.

You should be able to the shades if after your step 4 you go to the channels panel and click the eye symbols so that only the quick mask channel is visible on your screen as a grayscale image.

With this grayscale image visible, you can now either use :

a. Image adjustments threshold - to turn the mask completely black or white

or

b. Set a brush to Mode overlay and 50% opacity. Now paint over the dark areas of the image with black and the light areas with white white to make the black areas blacker and white areas whiter. This is a common technique to sharpen up a mask.

Now switch back on the visibility of the RGB channels and continue with your other steps.

Dave

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