c - 了解 cairo_rotate

Question

我正在尝试理解 cairo_rotate 函数。

考虑以下代码。我本来希望画一个十字，但只画了水平线(旋转之前)。我的错误是什么？

cairo_move_to(cr,0,HEIGHT/2.); 
cairo_line_to(cr,WIDTH,HEIGHT/2.);
cairo_stroke(cr); //horizontal line
cairo_rotate(cr,90.*(M_PI/180.));
cairo_move_to(cr,0,HEIGHT/2.);
cairo_line_to(cr,WIDTH,HEIGHT/2.);
cairo_stroke(cr); //this line isn't painted

Answer 1

您正在围绕位于图像左上角的原点旋转。要围绕图像的中心旋转，您还必须平移:

cairo_move_to(cr,0,HEIGHT/2.); 
cairo_line_to(cr,WIDTH,HEIGHT/2.);
cairo_stroke(cr);
cairo_translate(cr,WIDTH/2,HEIGHT/2); // translate origin to the center
cairo_rotate(cr,90.*(M_PI/180.));
cairo_translate(cr,-WIDTH/2,-HEIGHT/2); // translate origin back
cairo_move_to(cr,0,HEIGHT/2.);
cairo_line_to(cr,WIDTH,HEIGHT/2.);
cairo_stroke(cr);

根据您的应用程序，实际绘制相对于中心的所有内容也可能有意义:

int half_w = WIDTH/2;
int half_h = HEIGHT/2;
cairo_translate(cr, half_w, half_h);
cairo_move_to(cr, -half_w, 0); 
cairo_line_to(cr, half_w, 0);
cairo_stroke(cr); // horizontal line
cairo_rotate(cr, 90.*(M_PI/180.));
cairo_move_to(cr, -half_w, 0);
cairo_line_to(cr, half_w, 0);
cairo_stroke(cr); // vertical line