Clipping when drawing dots using cairo - c

Question

I've got a simple Cairo program that attempts to draw a diagonal consisting of dots in a 600x600 PNG. However, the output appears to be truncated whenever I attempt to render all of the dots using a single call to cairo_stroke().
Specifically, consider the following program:
#include <cairo/cairo.h>
int main(int argc, char **argv)
{
cairo_surface_t *surface =
cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 300, 300);
cairo_t *cr = cairo_create(surface);
cairo_set_line_cap(cr, CAIRO_LINE_CAP_ROUND);
cairo_set_source_rgb(cr, 0, 0, 0);
cairo_set_line_width(cr, 5);
for (double x = 0.0; x <= 300; x += 10) {
cairo_move_to(cr, x, x);
cairo_close_path(cr);
cairo_stroke(cr);
}
cairo_surface_write_to_png(surface, "output.png");
cairo_destroy(cr);
cairo_surface_destroy(surface);
return 0;
}
which generates the following correct output:
If I move the
cairo_stroke(cr);
outside of the for loop, then the following incorrect output is generated instead:
.
Can someone else explain why the second attempt fails? I suspect I must be doing something wrong here...

Answer 1

#include <cairo/cairo.h>
int main(int argc, char **argv)
{
cairo_surface_t *surface =
cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 300, 300);
cairo_t *cr = cairo_create(surface);
cairo_set_line_cap(cr, CAIRO_LINE_CAP_ROUND);
cairo_set_source_rgb(cr, 0, 0, 0);
cairo_set_line_width(cr, 5);
for (double x = 0.0; x <= 300; x += 10) {
cairo_move_to(cr, x, x);
cairo_close_path(cr);
}
cairo_stroke(cr); /* moved here */
cairo_surface_write_to_png(surface, "output.png");
cairo_destroy(cr);
cairo_surface_destroy(surface);
return 0;
}
Moving cairo_stroke(cr) outside the for-loop seems to generate equal output with Cairo version 1.10.1.
Compiled as: gcc test.c `pkg-config --libs --cflags gtk+-2.0` -std=c99 -lcairo