[cairo] Question about extents and transformations

[cecashon at aol.com]

A path in user space can change it's bounding box size with a transform. A scale or rotation can change the size of a bounding box.

With your code, test the output by drawing it. That is one of the cool things about drawing. If you are working on a tough drawing you can often see mistakes that you might miss otherwise.

Eric

//gcc -Wall bounding_box1.c -o bounding_box1 `pkg-config --cflags --libs cairo` -lm

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

int main()
  {
    double width=500.0;
    double height=500.0;
    double x1=0.0;
    double y1=0.0;
    double x2=0.0;
    double y2=0.0;
    cairo_surface_t *surface=cairo_image_surface_create(CAIRO_FORMAT_ARGB32, width, height);
    cairo_t *cr=cairo_create(surface);
   
    //Paint the background.
    cairo_set_source_rgba(cr, 1.0, 1.0, 1.0, 1.0);
    cairo_paint(cr);

    //Translate shape.
    cairo_save(cr);
    cairo_set_source_rgba(cr, 1.0, 0.0, 0.0, 1.0);
    cairo_translate(cr, width/4.0, height/4.0);
    cairo_move_to(cr, 100.0, 0.0);
    cairo_curve_to(cr, 50.0, -50.0, -50.0, -50.0, -100.0, 0.0);
    cairo_curve_to(cr, -50.0, 50.0, 50.0, 50.0, 100.0, 0.0);
    cairo_path_extents(cr, &x1, &y1, &x2, &y2);
    cairo_fill(cr);
    cairo_set_source_rgba(cr, 0.0, 0.0, 0.0, 1.0);
    cairo_rectangle(cr, x1, y1, x2-x1, y2-y1);
    cairo_stroke(cr);
    cairo_restore(cr);

    //Translate and scale shape.
    cairo_save(cr);
    cairo_set_source_rgba(cr, 0.0, 1.0, 0.0, 1.0);
    cairo_translate(cr, 3.0*width/4.0, height/4.0);
    cairo_scale(cr, 0.75, 0.75);
    cairo_move_to(cr, 100.0, 0.0);
    cairo_curve_to(cr, 50.0, -50.0, -50.0, -50.0, -100.0, 0.0);
    cairo_curve_to(cr, -50.0, 50.0, 50.0, 50.0, 100.0, 0.0);
    cairo_path_extents(cr, &x1, &y1, &x2, &y2);
    cairo_fill(cr);
    cairo_set_source_rgba(cr, 0.0, 0.0, 0.0, 1.0);
    cairo_rectangle(cr, x1, y1, x2-x1, y2-y1);
    cairo_stroke(cr);
    cairo_restore(cr);

    //Translate and rotate shape.
    cairo_save(cr);
    cairo_set_source_rgba(cr, 0.0, 0.0, 1.0, 1.0);
    cairo_translate(cr, width/4.0, 3.0*height/4.0);
    cairo_rotate(cr, M_PI/2.0);
    cairo_move_to(cr, 100.0, 0.0);
    cairo_curve_to(cr, 50.0, -50.0, -50.0, -50.0, -100.0, 0.0);
    cairo_curve_to(cr, -50.0, 50.0, 50.0, 50.0, 100.0, 0.0);
    cairo_path_extents(cr, &x1, &y1, &x2, &y2);
    cairo_fill(cr);
    cairo_set_source_rgba(cr, 0.0, 0.0, 0.0, 1.0);
    cairo_rectangle(cr, x1, y1, x2-x1, y2-y1);
    cairo_stroke(cr);
    cairo_restore(cr);
    
    printf("Output to bounding_box.png\n");
    cairo_surface_write_to_png(surface, "bounding_box.png");

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

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