c - Understanding cairo_rotate -


i trying understand cairo_rotate function.

consider following code. have expected cross drawn, horizontal line (before rotation) drawn. mistake?

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

you rotating around origin, @ top-left corner of image. rotate around center of image, must translate well:

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 center cairo_rotate(cr,90.*(m_pi/180.)); cairo_translate(cr,-width/2,-height/2); // translate origin cairo_move_to(cr,0,height/2.); cairo_line_to(cr,width,height/2.); cairo_stroke(cr);

depending on application, might make sense draw relative center:

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

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