Sharpening artefacts and MTF of monitors?

Discussion in 'Digital Photography' started by Ilya Zakharevich, Apr 20, 2005.

  1. I'm trying to understand visible effects of artefacts of sharpening with
    different viewing conditions. Suppose I have a sharpening matrix such
    that I can't see artefacts on monitor (with a very good CRT monitor,
    VS P815). However, when I blow pixels 200%, a careful examination
    exposes some artefacts. With magnification about 500% artefacts become
    very visible.

    Given this data, can I estimate how the image will be seen on LCD monitors
    (which have better MTF at high spacial frequencies)? Can I estimate
    on which pixel-per-inch value the printed image will have visible

    In short: what makes the artefact invisible without magnification: bad
    MTF of the CRT monitor (due to internal reflection in glass), or bad MTF
    of the eye? If the latter, then a print with about 120 pixels/inch or
    more should show no artefacts (pixel size on the monitor is 0.26mm).
    If the former, then one needs much higher pixels/inch value...


    Here is an example: take the image

    This is a resolution chart, the artefacts should be very visible. I run
    DSP software, and it shows the following MTF curve of the combined

    1 """"xxx__''''''''''''''''''''''''''''''''''''''''''''''''''''''|
    | ""x_ |
    | "x_ |
    | "x_ |
    | "x_ |
    | "_ |
    | "x_ |
    | x_ |
    | "x |
    | "x_ |
    | "_ |
    | "x_ |
    | "_ | |
    | "x | |
    | "x_ | |
    | x_ | |
    | "_ | |
    | "x |
    | |"x_ |
    | | x_ |
    | | "x_ |
    0 3000

    (vertical line is the Nyquist frequency of the sensor). One can easily
    see that sharpening with the matrix

    0 -0.375 0
    -0.375 2.5 -0.375
    0 -0.375 0

    makes the throughput MTF curve almost horizontal:

    1 """"""""""""""""xxxx____'''''''''''''''''''''''''''''''''''''''|
    | ""xx__ |
    | ""xx_ |
    | "xx_ |
    | "_ |
    | "x |
    | "x |
    | "_ |
    | x |
    | "_ | |
    | _| |
    | x |
    | |x |
    | | x |
    | | x |
    | | " |
    | | " |
    | | " |
    | | "_ |
    | | _ |
    0 3000

    Indeed, this sharpening has enormous "clearing" effect on the picture;
    the artefacts are as described above: invisible on CRT without
    magnification (initial image has no artefacts). It would be
    interesting to know what happens with other media...

    Ilya Zakharevich, Apr 20, 2005
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  2. Sharpening could work in this case. In some cases you could see ripples
    in that chart or the S/N ratio will get bad.
    Kevin McMurtrie, Apr 20, 2005
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  3. [A complimentary Cc of this posting was sent to
    Kevin McMurtrie
    Of course, S/N is decreased about 2.6 times by this sharpening. So it
    depends on what is the initial value of S/N. I did not measure S/N of
    the Adobe demosaicer; with the builtin demozaicer of the camera it is
    about 60 (for luma of 18% gray, at ISO50). Probably such a good RTF
    curve should result in larger noise than the lousy MTF curve of the
    builtin demozaicer... Still, it may happen that S/N is above 20
    (i.e., tolerable). Anyway, it is going to be a usual tradeoff between
    detail and noise; note that one can process different parts of the
    image with different settings.

    About ripples: with MTF curve such as above, I doubt that any artefact
    like "ripples" (whatever it is ;-) may be possible; there is a light
    halo about dark areas (visible only under ). However, I was
    seriously surprised about the quality of the image; having MTF above
    50% corner-to-corner for an equivalent of 38 lp/mm (for 35mm film)
    without visible artefacts gives very counterintuitive results...

    Ilya Zakharevich, Apr 20, 2005
  4. I wrote in article <d449su$1aih$>:

    Throughput MTF curve of lens+sensor+demosaicer+sharpening is
    BTW, I temporarily put the sharpened picture on

    It is 1.3M file (saved with "85% quality"; should have I chosen a
    better quality?).


    P.S. I calculated MTF to the cut-off frequency of the lens (f/4):

    1 """"""xx_''''''''''''''''''''''''''''''''''''''''''''''''''''''|
    | "x_ |
    | "_ |
    | x |
    | x |
    | x |
    | _ | |
    | | |
    | x| |
    | | |
    | x |
    | : |
    | |: |
    | |x |
    | |: |
    | | : |
    | | " |
    | | |
    | | " __ _
    0 -----------------------x-----__xxxxxxxxxxx_-----------x"--"xxx"-
    | | "__x" "x_ _" |
    0 8000

    (the vertical line is at Nyquist of the sensor, anything to the right
    results in aliasing; but the data too far to the right is probably not
    very reliable; I doubt that the second "dip" in the graph is actually
    there). It is increadibly good; very little aliasing and very high
    MTF in the main area; anyone having an idea how Adobe does it?
    Ilya Zakharevich, Apr 20, 2005
  5. [A complimentary Cc of this posting was sent to
    Kevin McMurtrie
    I measured the S/N ratio after this sharpening. It looks like S/N of
    18% gray should be 36. This is the S/N ratio of luma; thus S/N ratio
    of luminosity is 16; so it coincides with S/N ratio of Velvia 50 (as
    given on Roger Clark's web site).

    Ilya Zakharevich, Apr 20, 2005
  6. But the broadband signal-to-noise ratio may not reflect how the image is
    perceived. You need to know the narrowband SNR, and weight that with the
    visual systems frequency response (as is done with audio and weighted SNR
    measurements). This will be particularly important in comparing different
    media (e.g. film and digital).

    David J Taylor, Apr 21, 2005
  7. [A complimentary Cc of this posting was sent to
    David J Taylor
    Thanks, this may be relevant in some other situation. But given that
    MRF of the whole workflow is provided in the initial message, and is
    "almost horizontal", weighting will not change things much on the
    digital side.

    On the film side - I know no data about narrow band SNR of Velvia 50;
    do you? And the MTF curve of Velvia 50 + reasonably good fixed focal
    length lens is going to be "similarly good" when you compensate for 4x
    difference in sensor size, so the broadband S/N should be viable

    Anyway, it may be that your suggestions will change the numbers about
    25%; but I expect the change to be in the same direction for digital
    and film. Given that I suspect very much [*] the data for film noise
    on Roger's site, such a correction is not very important.


    [*] E.g., his S/N=16 for 18% gray on Velvia 50

    does not specify the window size into the film; he compares with a
    camera which has 8M pixels, and 8.2 micron cells, so there may be
    different interpretations of "equivalent" window size into the film.
    In a private communication he says that the window size is actually
    6.3 microns (equivalent of 24M pixels).

    Actually, comparing noise of 24MP scan with noise of 8MP digital
    sensor may be not that crazy, given his "translation rules" from scan
    resolution to digital sensor resolution (in other papers on his site).
    However, it is clear that one can reduce noise of 24MP scan by
    post-processing without lowering the resolution much (comparing to 8MP
    scan); I do not know whether this is taken into account.

    Additionally, it is not specified whether S/N=16 is for noise of
    density, or noise of "initial luminance". Given high contrast of
    slide film (gamma = 1/1.5), the latter should be about 66% of the
    former... Roger still did not answer my email about this issue.
    Ilya Zakharevich, Apr 21, 2005
  8. I'm not so sure about "almost horizontal MTF". For any sampled-data
    system there needs to be an antialias filter present prior to the sampling
    to avoid artefacts. The frequency response shape of this filter varies
    considerably between cameras (e.g. some cameras have none just to boost
    the system high-frequency response). Depending on the distance of the eye
    from the viewed image, the maximum sensitivity of the eye might fall in
    this rather variable MTF range.
    No, I have no experience of measuring film in this way, my experience is
    entirely with electronic imaging systems (visible, thermal etc.). I do
    suspect that the different amplitude linearity, MTF and noise sprecta is
    what distinguishes "digital" from "film", and that some people will prefer
    one to the other. Perhaps a lifetime of looking at film images makes the
    brian adjust to thinking them "normal".

    David J Taylor, Apr 22, 2005
  9. [A complimentary Cc of this posting was sent to
    David J Taylor
    Did not you see the curve in the initial posting? Or are you using
    proportional width fonts in your Usenet reading (which is a no-no-no
    ;-)? Then you need to cut&paste the graph to an editor with
    monospaced font (e.g. Courier).
    Any photo camera has such a filter builtin; it is called a lens. ;-)
    Of course, for interchangeable-lens cameras one needs a way to
    compensate for difference in lens quality. (This is probably the
    reason why dSLR have such pitiful MTFs...)
    Hmm, a very interesting idea. I need to think about it more...

    Ilya Zakharevich, Apr 22, 2005
  10. Ilya Zakharevich wrote:
    No, I was responding about the MTF of typical camera equipment.
    Well, relying on the lens as the sole anti-alias filter is not adequate,
    at least not for interchangable lens cameras. [I have always felt that
    the Nikon 5700 produces such good results because its lens and sensor are
    "well-matched", probably that the lens is carefully designed not to excite
    artefacts in the sensor by keeping the MTF under control.] It's the
    "pitiful MTFs" coupled with the much better signal-to-noise ratio which
    produce a characteristically different image between film and digital.
    It's then subjective as to which is "better"!

    David J Taylor, Apr 22, 2005
  11. [A complimentary Cc of this posting was sent to
    David J Taylor
    Well, what I meant is the MTF of a particular pipeline through a
    particular camera. Actually, my initial calculations of "sharpening
    which compensates the MTF curve" were faulty (I used linear
    appriximation to trig functions while they were well accessible
    directly); the corrected result has following properties:

    a) MTF within 0.15% of 1 up to 0.4 of Nyquist frequency; 0.99% at
    0.5 of Nyquist; 0.95 at 0.6 of Nyquist; 75% at 80% of Nyquist (this
    is close to center; but the radial and tangential data at corner are
    very similar to that at center);

    b) No visual artefacts (for Gibbs phenomenon);

    c) Visual noise is slightly increased by this sharpening, but this
    is hardly noticable (the noise *is* increased at high frequency, but
    probably the eye is not sensible in this region);
    Well, the fact that DSLR give *very* similar MTF with prime lens
    (presumably at f/8) as non-interchangeble lens cameras with 8x zooms
    at f/4 says enough....

    Ilya Zakharevich, Apr 25, 2005
  12. [A complimentary Cc of this posting was NOT [per weedlist] sent to
    Ilya Zakharevich
    As I said in another message on this thread, the image above is
    oversharpened at low frequencies (due to an error on my part: I used a
    linear approximation at one point of calculations). Sorry. The image
    is now removed.

    A corrected image is on

    It is obtained in the following way:

    a) Take the Adobe-demosaiced image cited in the original page; it
    already has most features of MTF curve corrected;

    b) Apply this convolution matrix (divided by 16):

    # 0 0 0.6 0 0
    # 0 1.2 -7.12 1.2 0
    # 0.6 -7.12 37.28 -7.12 0.6
    # 0 1.2 -7.12 1.2 0
    # 0 0 0.6 0 0

    c) Correct the wrong (?) gamma (I think the Adobe image has gamma=3
    instead of 2.2; do not know why; I just compared it with
    in-camera produced JPEG).
    New data:

    1.016 """"""""""xx_''''''''''''''''''''''''''''''''''''''''''''''''''|
    | "_ |
    | x |
    | x |
    | _ |
    | |
    | x |
    | |
    | " | |
    | | |
    | "| |
    | | |
    | x |
    | | |
    | |x |
    | | |
    | | x |
    | | |
    | | " _|
    0 ,,,,,,,,,,,,,,,,,,,,,,|,,_,,,,,,,,__________x___,,,,,,,,,,,_"","
    | | x _x" "x_ _" |
    0 7350

    As usual, the vertical line is at Nyquist frequency of the sensor.

    Ilya Zakharevich, Apr 29, 2005
  13. [A complimentary Cc of this posting was NOT [per weedlist] sent to
    Ilya Zakharevich
    .... and I forgot to write what I want from *you*. Can somebody look
    at the image on a good quality LCD monitor and check whether artefacts
    are visible without blowing up the pixels? (I mean with 1 image pixel
    mapped to 1 screen pixel.)

    The artefacts which I can see with high magnification are:

    a) slight white "halo" about dark areas;

    b) very slight "light interior" in dark strokes. E.g., take number
    1; divide the vertical line into 3 parts: left, center and right.
    The center is slightly lighter than the left and right parts.

    Maybe somebody can see it even on a CRT? (I can't on mine.)

    [I would like to ask about prints, but can't, since they cost. My
    LJ1100 prints without artefacts visible; but given that it is only
    600dpi, this is hardly telling about anything.]

    Ilya Zakharevich, May 2, 2005
  14. Ilya Zakharevich wrote:
    I see that Konica Minolta /still/ haven't sorted out the nasty artefacts
    on fine edges - e.g. the horizontal resolution chart from about 12 onwards
    near the centre of the image. This defect has been on the A1, A2 and
    A200. Don't they care about it?

    David J Taylor, May 2, 2005
  15. [A complimentary Cc of this posting was sent to
    David J Taylor
    It is hard to understand what you are talking about here.

    First of all, I do not see what KM has to do with this: there is no
    KM firmware/software in the digital pipeline. The raw image was
    demosaiced by Adobe, then postprocessed by me.

    Second, can you be more specific? "Horizontal resolution" means the
    chart with vertical lines, right? I can't see anything wrong with
    them; actually, same applies to horizonal lines.

    So: what should I pay attention to?

    Ilya Zakharevich, May 2, 2005
  16. Well, if you can't see the defects, you have no need to worry about them.
    I am referring to the horizontal lines near the centre-right of the image
    labelled 18 (on the left) to 6 on the right. Just to the right of the
    central zone-plate spot. There is a "dirtiness" in the lines, leading to
    initial spots around the 18 resolution label. Perhaps there isn't enough

    It was picture defects like this which caused me to return my Minolta A2
    for a refund.

    David J Taylor, May 2, 2005
  17. [A complimentary Cc of this posting was sent to
    David J Taylor
    This threads is exactly about things not seen which may appear in
    different situations/viewing conditions. So thanks for pointing out
    some of what you can see.
    Can you see something like this on vertical lines?

    I blew the image out 300% and do not see anything which I can consider
    a "defect". There is some modulation in horizontal direction in the
    "gray area" between white and black (e.g., on the 5th line from top
    over "12" mark). Or do you mean, e.g., the 3rd white line from top
    over "6"?

    Ilya Zakharevich, May 2, 2005


    Yes, it's similar on the verttical lines, although perhaps not quite as
    intense. I'm viewing the image at 1:1 magnification in Internet Explorer.
    Measuring the pixel locations from top left, the artefacts are in the
    region (X:1800, Y:1215). It could be integer overflow or underflow.


    David J Taylor, May 3, 2005
  19. [A complimentary Cc of this posting was sent to
    David J Taylor
    What is probably more important: what monitor? LCD/CRT, dot pitch?
    OK, I will try to investigate this region. ;-)

    Meanwhile, I'm pretty sure the original Adobe picture should have
    none; see

    Can you see any defect (except the low contrast [and wrong gamma ;-]) there?

    I'm pretty sure that the artefacts must be added by the sharpening *I*
    did. The convolution matrix was calculated by me, but I do not have
    the source code for the software which applies it; on the other hand,
    I do not think it can do the convolution "wrong". So it must be due
    to the convolution matrix.

    Ilya Zakharevich, May 4, 2005
  20. I'm viewing on a 19 inch LCD at 1280 x 1024 resolution, but the artefacts
    show equally well if I open the image with Paint Shop Pro 9 and zoom

    I have noticed that the MTF of this monitor is much better than the 19
    inch CRT it replaced, resulting in images which were previously correct
    now appearing too sharp. My previous definition of "correct" was to have
    no perceptible overshoot on black/white transitions (well dark grey/light
    grey transitions - ones which didn't saturate).

    The original:

    shows just the same artefacts, both when viewed with Internet Explorer,
    Windows XP Preview, IrfanView, or Paint Shop Pro 9. I can't comment on
    the gamma of the original as I don't know what the grey levels were (there
    is no calibrated greyscale that I can see).

    David J Taylor, May 4, 2005
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