Thanks for the replies.  I've uploaded the original image, along with
three modified images that are satisfactory at various distances along
my imaginary gradient.


contrast applied with increasing upper levels parameter and minimal
gamma (no gray)::

You can see from the trend that there does exist a set of parameters
for the threshold function that gives acceptable contrast for every
given point, and that this one parameter (the upper level) increases
monotonically as it moves along the "vector field".

Regarding the suggestions: I think I've played with levels enough
without results.  On the other hand, I don't know about the layer mask
approach.  From my limited abilities, I can only see that layer masks
modulate the opacity of the resulting image after having applied some
filter to a duplicate layer, whereas I want to modulate the input
parameters of a "filter" (threshold) to the original layer.

Sorry if I'm making this more complicated than necessary, and thanks
for reading.


On 3/31/12, Owen <> wrote:
>> Hello,
>> I would like to obtain a black and white image of a photograph of a
>> book page.
>> My attempt to do this is by running the threshold function over the
>> entire image.  This appears to give the desired result, but only
>> locally.  It appears that the optimum parameters of the threshold
>> function vary by position (i.e., according to some scalar field), with
>> one extreme in the center of the glare and then radiating outward.  So
>> just running the threshold function over the entire image with
>> constant parameters always gives sub-optimum contrast for 75% of the
>> image.  Will I have to find a way to run the threshold function with
>> non-constant parameters, or is there some other tool to do the job?
> Without seeing the image, one can only suggest you try converting the
> image to Indexed and 2 colors.
> You could also leave it as rgb and play with the curves, maybe one of
> the colors can be filtered out.
> If you get a good B & W image, you might then want to run it through
> the unsharp mask to improve the text.
>  --
> Owen
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