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Thread: QwtPlot issue under Win CE6

  1. #1
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    Default QwtPlot issue under Win CE6

    I have an application developed with Qwt 6.1.3 both for Windows 10 and Win CE6.
    Everything is ok with Windows 10, but under CE6 I see a strange behavior.

    When I plot data with setSamples()
    AND
    values for xData buffer are greater than 20.000.000 (more or less)
    AND
    the platform is Win CE6
    the plot is shown in a wrong way.

    What I see is "as if" the same value from xData buffer is used for 4 points:
    Qt Code:
    1. point #1 : x[0], y[0]
    2. point #2 : x[0], y[1]
    3. point #3 : x[0], y[2]
    4. point #4 : x[0], y[3]
    5. point #5 : x[4], y[4]
    6. point #6 : x[4], y[5]
    7. point #7 : x[4], y[6]
    8. point #8 : x[4], y[7]
    9. point #9 : x[8], y[8]
    10. point #10 : x[8], y[9]
    11. ...
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    Has someone seen something similar to this?
    Can you guess the reason?
    Is there a way to fix?

    THanks in advance

  2. #2
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    Default Re: QwtPlot issue under Win CE6

    Maybe something related to the significance of floating points ?

    Uwe

  3. #3
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    Default Re: QwtPlot issue under Win CE6

    Do you mean something related to the approximation of floating point numbers when stored in binary representation?
    I think that in CE6 double means IEEE-754 32-bit floating point (on Win 10 is 64-bit), but it seems to me that for 20.000.000 the approximation error is less than 1.0.

    Do you mean something different?

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    Default Re: QwtPlot issue under Win CE6

    No I meant exactly this - and the fact that you have 32-bit doubles points into this direction.

    I would use the debugger to find out, where things go wrong when translating from plot into paint device coordinates.
    Maybe start with checking QwtScaleMap::transform()

    HTH,
    Uwe

  5. The following user says thank you to Uwe for this useful post:

    vix (23rd November 2021)

  6. #5
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    Default Re: QwtPlot issue under Win CE6

    For numbers greater than 30.000.000,0 the error when storing them in IEEE-754 format is greather than 1,0.
    This is too much for my need and so I did it in a different way.
    I store the value for the first x element and I work with small delta values (1, 2, 3, ...).
    Then I use custom QwtDateScaleDraw to show the scale labels as I need.

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