[cairo] is skew supposed to move/shift top/left x?

Mark Olesen markjolesen at gmail.com
Thu Feb 11 04:38:30 UTC 2016

When skewing in the x direction, top x seems to move to the right a bit.I
have attached a png. The red rectangle is not skewed, while the black is
skewed at 45. The skewed (black) rectangle looks shifted to the right a few
pixels. That is the top is not aligned. I would think there should be a
diagonal line cutting through the top left hitting the bottom right corner.
>From my past history, I am probably doing something wrong. Thanks in

#include <cairo.h>
#include <math.h>
#include <string.h>
#include <stdlib.h>

Lets take that C = math.cos(A), S = math.sin(A), T = math.tan(A)
X-skew by A    --    mtrx = cairo.Matrix(1,0,T,1,0,0)
Y-skew by A    --    mtrx = cairo.Matrix(1,T,0,1,0,0)

main (int argc, char *argv[])
  cairo_surface_t* cs;
  cairo_t *cr;
  double angle;
  double skewx;
  cairo_matrix_t matrix;

  cs= cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 130.,100.);
  cr= cairo_create(cs);

  cairo_set_source_rgb(cr, 1., 0., 0.);
  cairo_rectangle(cr, 10., 10., 50., 50.);

  angle= 45.;
  skewx= tan(angle * M_PI / 180.);
      1.0, 0.0,
      skewx, 1.0,
      0.0, 0.0);
  cairo_transform(cr, &matrix);
  cairo_rectangle(cr, 10., 10., 50., 50.);
  cairo_set_source_rgb(cr, 0., 0., 0.);

  cairo_surface_write_to_png(cs, "skew.png");


  return 0;
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