Scanned Slides: Filtering non-uniform edge discolorations

"Acecliffy" schreef in bericht

MY SITUATION: I am scanning color slides that are 20 years old. They have edge discoloration, meaning that the sides are somewhat darker (perhaps blueish) than the center of the slide. This is not a problem with my slide scanner; rather, it is inherent in the slides. I don’t know why they have this problem, since I am doing this job for my major professor.

So… here is an example of what I am talking aoubt. There is a link to a sample image here:
http://web.ics.purdue.edu/~racz/images/Image012.jpg

The concrete on the left should have roughly the same color as the concrete in the center, but it has a blue tint (I believe, maybe it is a mixture).

MY QUESTION: Is there a filter to adjust/perform color corrections for non-uniformity such as this, where a hue/saturation/balance is non-uniform across the image? It may be a linear gradient, or perhaps even a circular/eliptical gradient?

Thanks in advance.

No problem: in PS, make a rectengular selection with a big feather, invert the selection and correct with curves or any other means you like.

Rutger

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http://www.flickr.com/photos/zwaarddrager/

+ remember that you are looking at a slide… a transparency… color fade in the jpg you posted is almost indiscernible. As for the color cast, Gimp has numerous plug-ins which will correct this.

Acecliffy wrote:

MY SITUATION: I am scanning color slides that are 20 years old. They have edge discoloration, meaning that the sides are somewhat darker (perhaps blueish) than the center of the slide. This is not a problem with my slide scanner; rather, it is inherent in the slides. I don’t know why they have this problem, since I am doing this job for my major professor.

So… here is an example of what I am talking aoubt. There is a link to a sample image here:
http://web.ics.purdue.edu/~racz/images/Image012.jpg

The concrete on the left should have roughly the same color as the concrete in the center, but it has a blue tint (I believe, maybe it is a mixture).

MY QUESTION: Is there a filter to adjust/perform color corrections for non-uniformity such as this, where a hue/saturation/balance is non-uniform across the image? It may be a linear gradient, or perhaps even a circular/eliptical gradient?

Thanks in advance.

"Acecliffy" wrote in message

MY SITUATION: I am scanning color slides that are 20 years old. They have edge discoloration, meaning that the sides are somewhat darker (perhaps blueish) than the center of the slide. This is not a problem with my slide scanner; rather, it is inherent in the slides. I don’t know why they have this problem, since I am doing this job for my major professor.

So… here is an example of what I am talking aoubt. There is a link to a sample image here:
http://web.ics.purdue.edu/~racz/images/Image012.jpg

The concrete on the left should have roughly the same color as the concrete in the center, but it has a blue tint (I believe, maybe it is a mixture).

MY QUESTION: Is there a filter to adjust/perform color corrections for non-uniformity such as this, where a hue/saturation/balance is non-uniform across the image? It may be a linear gradient, or perhaps even a circular/eliptical gradient?

Try this
Copy the layer and adjust the saturation of the copied layer to lower the blue component, then apply a layer mask to this layer and try a radial gradient fill on the mask to mask out the overcorrected centre of the top layer.

MH

Acecliffy wrote:

MY SITUATION: I am scanning color slides that are 20 years old. They have edge discoloration, meaning that the sides are somewhat darker (perhaps blueish) than the center of the slide. This is not a problem with my slide scanner; rather, it is inherent in the slides. I don’t know why they have this problem, since I am doing this job for my major professor.

So… here is an example of what I am talking aoubt. There is a link to a sample image here:
http://web.ics.purdue.edu/~racz/images/Image012.jpg

The concrete on the left should have roughly the same color as the concrete in the center, but it has a blue tint (I believe, maybe it is a mixture).

MY QUESTION: Is there a filter to adjust/perform color corrections for non-uniformity such as this, where a hue/saturation/balance is non-uniform across the image? It may be a linear gradient, or perhaps even a circular/eliptical gradient?

Thanks in advance.

The problem here is that you need to remove the shadow on the edges, without effecting the rest of the image’s structure. We need a layer that represents the low-frequency hue/saturation/value information of the image.

Here is my hack.

1. Duplicate the layer.
2. Hit the duplicate layer with a large Gaussian Blur. I chose a radius of 300, as your image was 600 pixels tall. This will wipe out all higher frequency structure, leaving a layer that only contains the non-uniform hue/saturation/balance information.
3. I then set the Mode of the top layer to Grain Extract.

This results in an image of even hue and value. However the image does seem to have become somewhat desaturated, and increased brightness. This could be fixed by:
4. Merging the visible layers, and applying Colour/Contrast, Levels or Hue/Saturation adjustments.
5. Desaturating the top layer before you merge the visible layers, except that this doesn’t get rid of the blueness in the concrete, and still results in an increase in overall brightness.

A final option would be to decrease the brightness of the image BEFORE applying steps 1/2/3, or adjust the top layer’s brightness/contrast after step 3 but before step 4.

Hope this helps?

Joal Heagney