Understanding cairo_scale()

Question

I am trying to understand the cairo_scale() function. There is a documentation but I still don't get it.

Consider the following code

cairo_scale(cr,1./WIDTH,1./HEIGHT);
cairo_move_to(cr,0,0);
cairo_line_to(cr,1,1);
cairo_stroke(cr); //draw a line across the image

and (the working version) without scale

cairo_move_to(cr,0,0);
cairo_line_to(cr,WIDTH,HEIGHT);
cairo_stroke(cr);

As far as I understand cairo_scale(1/width,1/height) should scale all following verbs, so the x and y ranges are from 0 to 1 (instead of 0 to WIDTH and 0 to HEIGHT).

Unfortunately it does not work and I cannot see anything. What is my mistake?

Answer 1

You can think of cairo_scale() as multiplying all the following coordinates with its argument.

In your case this means you are drawing a line from (0, 0) to (1/width, 1/height) which is less than a pixel.

You want to call cairo_scale(cr, WIDTH, HEIGHT) instead.