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Re: Acquiring Dental RVG on Linux

On Sat, Jan 02, 2021 at 05:24:15PM +0530, Sonali Warunjikar wrote:
> If I imagine line pair to be equivalent of 2 pixels, the pixel width isn't
> matching with that. Essentially lp/mm is appearing coarser than pixel
> width and probably hence called 'true (measured) resolution' and may be
> they aren't expected to match.
> I tried both: 1. using pixel width compute pixels per meter 2. using
> lp/mm, assuming 1 lp is equivalent to 2 pixels compute pixels per meter.
> The latter is coming closer to the reality, but not sure it's accurate
> enough as yet. May be there was a gap between the finger and the sensor
> when trial x ray was taken as our attention was on usb interface rather
> than lengths (getting computed length smaller than the actual one of the
> portion of the finger). I need to go and gather more x rays carefully (may
> be using a coin to make the boundaries more accurate) before confirming
> which measure is accurate or whether something else is required or is just
> a fixed calibration factor is needed.

[1] is the image produced by a coin kept right on the sensor whose
diameter is officially 21.93mm (appears 22mm to us by accuracy with which
we can measure at home).

The spec[2] says: pixel size = 19 micrometer, 'true (measured) resolution
= 16 linepairs/mm.

The png encoder requires pixels/metre
by pixel size it comes out to be: 52631
by lines per mm it is (explained above): 32000

I measured the count of pixels along diameter line using gimp, by drawing
a line that to a naked eye looks like a diameter. It comes to be 1172.

So pixels/m should be diameterpixels * 1000/21.93 = 53442.77. This is
closer to the one derived from pixel size, but quite far from linepair/mm.

Can someone suggest any better way (preferably some tool) to measure the
diameter of the circle in image [1] to improve the accuracy?

[1] http://mayuresh.sdfeu.org/coinxray.png

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