Re: Shouldn't Cairo use/offer degrees rather than radians?

Has anyone figured out a good explanation for this? I haven't been able to figure it out. It looks like the problem is tied to only one particular precision point. I don't know why that is. If I change the size of the surface, radius, y-point or even pi I get the same problem point. If I run with valgrind I get a different repetitive problem point. Maybe on some hardware this doesn't happen? I am testing on a Intel 32 bit Atom CPU N270 1.60GHz × 2 with Ubuntu16.04. This is my latest to try to figure out what is happening. 

Eric


#include <cairo.h>
#include <stdio.h>
#include <math.h>

int main()
{
  printf("%s\n", cairo_version_string());
  cairo_surface_t *surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 30, 20);
  cairo_t *cr = cairo_create (surface);
  cairo_set_line_width (cr, 2);
  cairo_set_antialias (cr, CAIRO_ANTIALIAS_NONE);

  double test_number=0.0;
  double increment=0.1;
  int channels=4;
  int width=cairo_image_surface_get_width(surface);
  int height=cairo_image_surface_get_height(surface);
  int stride=cairo_format_stride_for_width (CAIRO_FORMAT_ARGB32, width);
  printf("width %i, height %i, stride %i\n", width, height, stride);
  unsigned char *s=cairo_image_surface_get_data(surface);
  unsigned char *pixels=s;
  //Safety for do loop.
  int safety=0;
  int counter=0;
  int i=0;
  int j=0;
  
  //Change the arc radius for each loop. Use i to change the test number for new search also.
  for(i=0;i<5;i++)
    {
      do
        {
          test_number+=increment;      
          cairo_set_source_rgb(cr, 0.0, 0.0, 0.0);
          cairo_paint(cr);
          cairo_set_source_rgb(cr, 0.0, 1.0, 0.0);
          //Test changing arc radius to see if this changes the numbers.
          cairo_arc(cr, 15, test_number, 5+i, 0, M_PI);
          cairo_stroke(cr);  
          cairo_surface_flush(surface); 

          //Look for the color pixels in the row.
          counter=0;
          for(j=0;j<width;j++)
            {
              if(pixels[i*stride+j*channels+1]!=0) counter++;
            } 
          //Refine precision if the row was all 0's. Backup and start looking again.
          if(counter==0)
            {
              //Go back one.
              test_number-=increment;
              increment=increment*0.1;
              printf("%i %i %i %.15f, %.15f\n", i, counter, safety, test_number, increment);
            }
         
          if(safety>100)
            {
              printf("Couldn't find partial arc.\n");
              break;
            }
          safety++;    
        }while(counter<1||counter>3);

      printf("Row %i %.15f ", i, test_number);
      for(j=0;j<width;j++)
        {
          printf("%i, ", pixels[i*stride+j*channels+1]);
        }
      printf("\n");

      safety=0;
      test_number=(double)i+1;
      increment=0.1;
    }

  cairo_destroy(cr);
  cairo_surface_destroy(surface);
  cairo_debug_reset_static_data();
  return 0;
}

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