Artwork reproduction and uneven illumination

[QT Luong]

One of the difficulties in reproducing flat artwork with a camera is to light the piece evenly. A few years ago I heard Bill Atkinson mention that it was very easy to overcome that difficulty when shooting with a Better Light back. You take an exposure of a white sheet, which records the illumination level pixel by pixel. Then you use a special function that is in the Better Light software to automatically correct any subsequent shots. This has the advantage of also correcting for any light fall-off from the lens.

I do not have a Better Light back, but I was wondering if the same technique could be applied with a film camera after scanning the reference image and the image to be corrected ? The problem is how to apply the correction. Questions:

(a) Is the correction done simply by dividing the pixel values of the image to be corrected by the pixel values of the reference image, or a normalized version of it ?

(b) Is there a way to do such an operation in Photoshop CS 3 Extended ?

(c) If not, is there a way to do that operation with imagemagick ?

[Greg Miller]

I have never done this or read about it. But I think this process would work.

1) Photograph a piece of white paper under the same lighting that will be used for the art
work.
2) In Photoshop invert the image (image | invert)
3) Turn the inverted image into a layer mask. The layer mask should be fully black in the center where the brightest luminosity is and and gradually lighten where the light falloff is.
4) shoot the artwork with the same lighting as #1
5) Use the layer mask from #3 as the layer mask for a curve layer on top of #4. The curve will be pulled to the left to lighten the image and the layer mask will cause the curve to only affect the areas of the art work affected by light falloff.
6) Adjust the layer mask if necessary my applying a curve to the layer mask (select the layer mask, then choose Image | Adjust | Curve).

[Greg Miller]

Now that I think about it, Photoshop has a vignetting tool. So if your light falloff is spherical you could use that. I don't have Photoshop in front of me but I believe that you will find in in Filters | Lens Correction.

[Mark Sampson]

That all sounds a lot more difficult than simply lighting the piece properly... which is not difficult, merely painstaking.

[Greg Lockrey]

What Mark Said. Another issue is that the white you might be using isn't truly white after all a perhaps add in a color cast with your mask. I mention this becaue I use a calibrated black-white-grey target that the white doesn't at all look like the white of papers.

[David A. Goldfarb]

I've got to agree with Mark and Greg. The only reason I could see for doing it the other way would be if the piece were in a space that was too small or in a position (say, with one edge up against a brightly colored wall) where it was impossible to light it properly.

[Greg Miller]

What Mark Said. Another issue is that the white you might be using isn't truly white after all a perhaps add in a color cast with your mask. I mention this becaue I use a calibrated black-white-grey target that the white doesn't at all look like the white of papers.

The mask would be active on a curve layer. So being pure white would not be factor. You could actually achieve the same result shooting any color surface, as long as he color is uniform.

[QT Luong]

Since posting this question, I did a bit of research on my own, as well as experimenting with digital images, and this single imagemagick command-line did the job very well:

convert to_be_corrected.tif \( reference.tif -colorspace Gray \) -fx '(u/v)*v.p{w/2,h/2}' corrected.tif

Explanation: convert the reference image to greyscale to prevent color casts, divide and normalize by the center pixel value of the reference image.

Advantages of this approach: (a) I don't need to buy lights and can set up anywhere (b) I can use directional lighting to emphasize texture

[jetcode]

One of the difficulties in reproducing flat artwork with a camera is to light the piece evenly. A few years ago I heard Bill Atkinson mention that it was very easy to overcome that difficulty when shooting with a Better Light back. You take an exposure of a white sheet, which records the illumination level pixel by pixel. Then you use a special function that is in the Better Light software to automatically correct any subsequent shots. This has the advantage of also correcting for any light fall-off from the lens.

I do not have a Better Light back, but I was wondering if the same technique could be applied with a film camera after scanning the reference image and the image to be corrected ? The problem is how to apply the correction. Questions:

(a) Is the correction done simply by dividing the pixel values of the image to be corrected by the pixel values of the reference image, or a normalized version of it ?

(b) Is there a way to do such an operation in Photoshop CS 3 Extended ?

(c) If not, is there a way to do that operation with imagemagick ?

This approach in effect creates a white reference layer (level and color) that is used to compute and correct subsequent images. Most likely some form of color correction must take place because white reference shifts affect color. Greg's approach will adjust levels but if there is any change in color based on illumination levels this approach will not work by itself as far as I can tell.

[Greg Lockrey]

The mask would be active on a curve layer. So being pure white would not be factor. You could actually achieve the same result shooting any color surface, as long as he color is uniform.

I see what you mean now. Pretty cool.

BTW I like your panoramics of the Hudson Valley.

[Henry Ambrose]

Since posting this question, I did a bit of research on my own, as well as experimenting with digital images, and this single imagemagick command-line did the job very well:

convert to_be_corrected.tif \( reference.tif -colorspace Gray \) -fx '(u/v)*v.p{w/2,h/2}' corrected.tif

Explanation: convert the reference image to greyscale to prevent color casts, divide and normalize by the center pixel value of the reference image.

Advantages of this approach: (a) I don't need to buy lights and can set up anywhere (b) I can use directional lighting to emphasize texture

I'd like to see the result after you've tried a few. I doubt that you will get anything like accurate color rendition but I could be wrong.

[dwhistance]

Sorry but I've come late to this thread. I believe that Bill Atkinson was referring to software called Equalight which is marketed by Robin Myers Imaging (http://www.rmimaging.com/). Robin Myers is one of the technical people at Betterlight.

The software is fairly cheap but works very well, doing exactly what is says it does (as long as you pay attention to setup). It should work just as well with scanned film as with a digital image as there is essentially no difference between them for the program. I am fairly certain that it is available as a trial if you want to see what it does.

I've been using it for a while now and am very pleased with it, however it has possibly made me a bit lazy with my lighting set-up.

David Whistance