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Re: [fltk.coredev] Linux drawing in widget error: error in cairo_matrix_invert

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Re: Linux drawing in widget error: error in cairo_matrix_invert Gonzalo Garramuño 13:32 May 21  
 

El 21/5/23 a las 16:07, Manolo escribió:
The example program calls function fl_scale(double x, double y) with null x and y.
Correct, as a side effect of not initializing the values.
These values are not in the range of meaningful values for a scaling operation. I see that other platforms accept that in the context of the example program,
but I see that as a happy border effect an FLTK app should not rely on.

Why does the example program call fl_scale(0, 0) in the first place?

The example program is a (simplified) vectorscope, which measures the color saturation in a wheel.  Black, grays and white are in the 0,0 coordinate, while all other hues are rotated and scaled based on their saturation level (closer to the outbound squares).


About how the Cairo backend of FLTK draw points :
it draws a filled rectangle of size 1 unit as shown by this code fragment
  void Fl_Cairo_Graphics_Driver::point(int x, int y) {
    rectf(x, y, 1, 1);
  }

Right, but that 1,1 gets transformed by the matrix so it never draws one pixel.

I've switched to calculate the matrix transform myself instead of calling the cairo transformations and then calling rectf(x,y,1,1); with the calculated positions.

--
Gonzalo Garramuño
ggarra13@gmail.com

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