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Aw: Re: Acquiring Dental RVG on Linux

draw four lines touching the circle, each pair at 90°

draw a line splitting each 90° in half (at 45° that is) going through the circle

the intersection of those two midlines should be the centre of the circle :-)

better: fit a square around the circle ...


> Gesendet: Dienstag, 05. Januar 2021 um 18:03 Uhr
> Von: "Sonali Warunjikar" <sonali.warunjikar@gmail.com>
> An: debian-med@lists.debian.org
> Betreff: 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
> [2]
> https://pdf.indiamart.com/impdf/22884448473/MY-16502350/dental-rvg-systems-kodak-5200.pdf

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