Re: A "slanted edge" analysis program

On Wed, 28 Sep 2005 15:28:36 +0200, "Lorenzo J. Lucchini"
<ljlbox@xxxxxxxxxx> wrote:

>> I've only been skimming this thread and not paying too much attention
>> because the MTF of my scanner is what it is and there's nothing I can
>> do about it... So, with that in mind...
>
>Well, but I don't plan to stop here. What I'd mostly like to obtain from
>all this is a way to "sharpen" my scans using a function that is
>tailored to my scanner's specifics (rather than just "unsharp mask as I
>see fit").
>
>So, you see, there is a practical application of the measurements I can
>obtain, it's not just about knowing how poor the scanner's resolution is.

I agree with that in principle. However, in practical terms I think
that starting with an 8-bit image there's only so much accuracy you
can achieve.

I strongly suspect (but don't know for a fact) that you will not be
able to show a demonstrable difference between any custom sharpening
and just applying unsharp mask at 8-bit depth.

I think you can improve the sharpness considerably more (even at 8-bit
depth) by simply aligning individual channels to each other.

>And why do you say I'm measuring the "objective values" of the pixels
>instead of their "perceptual values"? I'm mostly trying to measure
>resolution, in the form of the MTF.

Because it's all based on those gray pixels which are created because
the scanner can't resolve that border area. So it's much better to
read the actual values of those gray pixels rather than take either an
average or luminance value.

If the three RGB channels are not perfectly aligned (and they never
are!) then combining them in any way will introduce a level of
inaccuracy (fuzziness). In case of luminance that inaccuracy will also
have a green bias, while the average will be more even - which is why
I said that your original idea to use average seems like the "lesser
evil" when compared to the skewed and green-biased luminance values.

>So you see that I'm *already* doing measurements that are inherently
>"perceptual". So why not be coherent and keep this in mind throughout
>the process?

Because perception is subjective. When there is no other way, then
yes, use perception. But since you already have the values of those
gray pixels it just seem much more accurate to use those values.

>> Actually, what I would do is measure each channel *separately*!
>
>... I'm doing this already.
>The "gray" channel is measured *in addition* to the other three
>channels, and is merely a convenience.

That's good. So displaying individual results should be easy.

Don.
.