Pixel size or pixel pitch?

Started 3 months ago | Discussions thread
alanr0 Senior Member • Posts: 1,833
Re: The Simple Model
1

Jack Hogan wrote:

alanr0 wrote:

Jack,

As I explained previously, the lip is inevitable when you take the DFT of your PSF and there is significant spectral energy at frequencies above Nyquist. The higher frequencies are aliased down, distorting the shape below Nyquist, and creating your lip.

I don't know how you can avoid this if you sample the PSF at the pixel pitch.

Ok, thanks Alan, so I think we all agree that the lip is due to aliasing and it can be modeled by leaving the delta lattice in the formulas. If someone can come up with a way to model it without the delta lattice I'll be glad to take the deltas out.

Jack,

My concern with your MTFsys is that it predicts the sampled spectrum for a point source aligned with the pixel grid, but will be inaccurate for many other subjects.

Rounding the values on your graph, a unit amplitude sinusoidal subject at 0.48 cycles/pixel would have captured amplitude 0.06. A higher frequency 0.52 cycles/pixel sinusoid would have an amplitude only 0.02, aliased down to 0.48 cycles/pixel after sampling. Your MTFsys predicts an output of 0.08 at both 0.48 and 0.52 cycles/pixel.

Here is how I would simulate the process:

  • Simulate subject intensity at a multiple of the pixel resolution.
  • Convolve subject with (lens + pixel) PSF.
  • Sample low pass filtered image at pixel spacing.

This removes some of your constraints on target alignment and symmetry, and can show how fractional pixel shifts between target and sample grid impact the image, and change the nature of the aliasing.

If you restrict yourself to images sampled at the pixel pitch there is no way to know whether or how much aliasing is present without further information about the spectral content of the subject.

I appreciate you may have constraints on your model I am not aware of, but this is the approach I would take.

Good luck.

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Alan Robinson

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