Hi,
Sorry, only just got back to this thread.
OK, using the curves.
It's kind of hard to explain, so you might be able to work it out just by fiddling with them.
First off, lets work with a greyscale image (so we only have to worry about 1 channel).
Get your image open in photoshop (convert it to greyscale if its colour).[/li]
So we don't destroy the original image, we will do this with an adjustment layer. So make an adjustment layer (Layer -> New adjustment layer -> Curves). Click OK.[/li]
You should see a panel like this
Look at the White to Black bar underneath the graph. Make sure it's as show (with white to the left). If it's not, click the arrows in the middle of the bar.[/li]
Now an explanation of what the curve means.
The vertical axis of the graph represents the level of grey within the image. The horizontal represents the level of grey after the curve has been applied.
So at the bottom left of the line we have 0% input = 0% output and at the top right of the line 100% input = 100% output, like so...
Now click in the middle of this line
Following the logic, at 50% input we have 50% output. It's a straight line after all.
Now if we drag the 50% point down...
We have 50% input but we are telling photoshop to make it 30% output. You can enter the required values in the little boxes if you want.
The curve, or more accurately, the gradation curve, effects the image it is applied to. Most importantly it effects the image smoothly based on the curve. So when we make 50% into 40%, it means that at 70% we around 53%. Even more important is that the darkest (100%) areas are still 100% and the lightest (0%) areas are still 0%. This retains a nice level of contrast in the image and avoids it looking "flat".
Ok, so using this technique we can "drop the background" out of an image by moving the 0% output point of the line across so that, say, at 25% input we have 0% output.
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This has the effect of making any point of the image that is 25% appear as 0%.
You can place more than 1 point on the curve and drag it to set multiple points. This allows you to accurately control how the image will look. Lighter in some areas, darker in others.
If you completely reverse the direction of the line, then you effectively inverse the image.
Hope that helps a bit. Play with it and you will quickly understand it.
Bearing all that in mind. It might
NOT be the solution for you! It's only going to work if there are no areas in your image that are of a similar tone to the background. I am sure you can see why.
You may be better off using the pen tool.
Draw an accurate clipping path
Use that path to make a selection, perhaps feather it slightly
Then either delete the background, or better still apply a masking layer to it.
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