CODEWITHSUNDEEP
c++ccairogtkmm
BobMorane
BobMorane

Reputation: 4296

DrawingArea: fill area outside a region

I have followed this gtkmm tutorial on how to draw shapes and fill them with colors (e.g. A red disc on a transparent background). I was also able, from this example, to derive another example with a red disc on a blue background.

However, what I would really need is a transparent disc with a blue background that fills everything minus the disc area, which should stay transparent.

So with cairo, the usual workflow is:

  1. Create a surface
  2. Draw a shape (e.g. draw a circle)
  3. Fill the circle, so that it becomes a disc.

I would need some workflow that achieves something like this instead:

  1. Create a surface
  2. Draw a shape (e.g. draw a circle)
  3. Fill the area outside the circle, so that I have a colored background with a transparent "hole" in the middle.

I have done some research on this on the web but all examples seem to assume that we want to fill the inner region of a shape (which I must admit is more typical).

How could I do this?

P.S. I have added the C tag because I don't mind if you prefer to use C (or even Python).

Upvotes: 1

Views: 543

Answers (2)

Uli Schlachter
Uli Schlachter

Reputation: 9867

Draw your circle and draw a rectangle containing all the visible area. Set the cairo fill rule to even/odd. Fill. Done.

cairo_save(cr); // Save the state
cairo_arc(cr, 42, 42, 21, 0, 2*M_PI); // Draw circle
cairo_rectangle(cr, 0, 0, width, height); // Rectangle containing everything
cairo_set_fill_rule(cr, CAIRO_FILL_RULE_EVEN_ODD);
cairo_fill(cr);
cairo_restore(cr); // Restore default fill rule (optional; pairs with save above)

Upvotes: 4

Kyno
Kyno

Reputation: 29

IMHO, The function of 'Draw outside the circle' is complex to the graphic framework. It may also be ambiguous if you draw more than one circle filled outside.

As graphic shapes drawn later are placed 'on' the ones drawn former. What is needed is that draw a rectangle to fill the entire graphic context before drawing other shapes. This is defined as clear with the background color in some frameworks.

the workflow would seem like:
1. Create the surface.
2. Draw the background colored with what outside the circle.
3. Draw the circle filled with a specific color, e.g. white.
As a result, the circle would cover the background.

If insist on draw the circle first, please search Flood Fill Algorithm, which is used to draw on images. However, it is needless and costly to achieve the screen pixels and play such algorithms when drawing on screen.

I find Example Application: Creating a Clock with Cairo in the later section of the book you provide. That seems help.

Upvotes: 1

Related Questions