drawing bezier paths with cairo - c++

Question

I need to draw a collection of Bezier paths with the help of Cairo in C++.
So far I can draw a single path.
But I need to draw many paths consisting of multiple bezier curves and lines.
I load collection of paths from the external dictionaty generated by python.
Here, for this minimal example I introduce a path by the variable path0.
It is a vector, consisting of 2 subvectors. The first one represents a bezier spline, while the second one is a simple straight line.
The first subvector naturally has 4 components, while the second one has just 2.
Here is the example of my code.
#include <iostream>
#include <vector>
#include "cairo.h"
#include <string>
int main(int argc, char* argv[])
{
cairo_surface_t* surface;
cairo_t* cr;
surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 1000, 1000);
cr = cairo_create(surface);
cairo_scale(cr, 1, 1);
cairo_set_line_width(cr, 1);
cairo_set_source_rgb(cr, 0, 0, 0);
// Here goes the svg-path. It is a bezier curve and unconnected line.
std::vector<std::vector<std::vector<double>>> path0 = { {{82.801,-204.48},{66.241,-204.48},{52.561,-214.56},{52.561,-235.44}},{{277.921,-192.24},{277.921,-173.5}} };
// Here I put the initial point in the middle of the canvas.
double x0 = 300+path0[0][0][0];
double y0 = 700+ path0[0][0][1];
cairo_move_to(cr, x0, y0);
// Parsing path
for (int i = 0; i < path0.size(); i++) {
if (path0[i].size() == 4) {
cairo_rel_curve_to(cr, path0[i][1][0], path0[i][1][1], path0[i][2][0], path0[i][2][1], path0[i][3][0], path0[i][3][1]);
cairo_stroke(cr);
}
if (path0[i].size() == 2) {
cairo_rel_line_to(cr, symbol[0][i][1][0], symbol[0][i][1][1]);
cairo_stroke(cr);
}
}
cairo_surface_write_to_png(surface, "stroke.png");
cairo_destroy(cr);
cairo_surface_destroy(surface);
return 0;
}
The point is it I see only one curve on the canvas. Probably, the second one is outside.
Could you help me to understand what I'm doing wrong?
Yaroslav.

Answer 1

Could you clarify what you are expecting to see? With the following change, I get something like two lines, but I am not sure it is what you want to see.
--- t.cpp.orig 2020-10-13 17:00:12.404871474 +0200
+++ t.cpp 2020-10-13 17:04:20.573101739 +0200
## -29,13 +29,12 ## int main(int argc, char* argv[])
for (int i = 0; i < path0.size(); i++) {
if (path0[i].size() == 4) {
cairo_rel_curve_to(cr, path0[i][1][0], path0[i][1][1], path0[i][2][0], path0[i][2][1], path0[i][3][0], path0[i][3][1]);
- cairo_stroke(cr);
}
if (path0[i].size() == 2) {
- cairo_rel_line_to(cr, symbol[0][i][1][0], symbol[0][i][1][1]);
- cairo_stroke(cr);
+ cairo_rel_line_to(cr, path0[i][1][0], path0[i][1][1]);
}
}
+ cairo_stroke(cr);
cairo_surface_write_to_png(surface, "stroke.png");
cairo_destroy(cr);
cairo_surface_destroy(surface);
Result is:
What happens in the above program is:
First you call cairo_move_to(cr, 382.8, 495.5). This decides where the following curve begins.
Then there is call to cairo_rel_curve_to(). This adds a curve from the current point to something like 382.8+52.5, 495.5-235.4. The other two points describe how the curve is bent.
Next there is a call to cairo_rel_line_to(cr, 277.9, -173.5);.
In my version, this adds a line segment starting from the end of the curve.
In your version, the call to cairo_stroke() deleted the path. Since there is no current point, the line_to does not do anything (well, it sets the current point for following drawing commands).