How to make SDL(2) refresh the convas content on resize?

Question

(premise: the (pseudo) code is in Rust, however, answers in any language are accepted, since the Rust SDL library mimics closely the standard one).

I have a very minimal SDL2 template I use when I need to draw an image, display it (in a maximized window), and wait for a keypress.

The current logic works, however, it has a niggle: after the window is maximized (and the canvas updated), if I resize the window, redrawing doesn't occur.

This is my current pseudo-code:

let sdl_context = sdl2::init().unwrap();

let window = sdl_context
    .video()
    .unwrap()
    .window(""window_title"", width as u32, height as u32)
    .maximized()
    .position_centered()
    .resizable()
    .build()
    .unwrap();

let mut event_pump = sdl_context.event_pump().unwrap();

let mut canvas = window.into_canvas().present_vsync().build().unwrap();

canvas.set_logical_size(640, 480).unwrap();

// Needed to redraw, as the window has been maximized above.
//
event_pump.pump_events();

////////////////////////////////////////
// Here some pixels are drawn.
////////////////////////////////////////

canvas.present();

// Wait for a keypress or quit event.
//
for event in event_pump.wait_iter() {
    match event {
        Event::KeyDown { .. } => std::process::exit(0),
        Event::KeyUp { .. } => std::process::exit(0),
        Event::Quit { .. } => std::process::exit(0),
        _ => {}
    }
}

If I run this code, and resize the window without pressing any key (or quitting), the canvas doesn't redraw.

How should I change the code to make it redraw?

(note that I'm not using textures, because they were making the code more complex, and in the cases I use this template for, either speed is not a concern or there isn't a measurable speed improvement)

Jmb · Accepted Answer · 2020-10-29 07:42:30Z

2

There is a WindowEvent::Resized event, so the following should work (untested):

for event in event_pump.wait_iter() {
    match event {
        Event::KeyDown { .. } => std::process::exit(0),
        Event::KeyUp { .. } => std::process::exit(0),
        Event::Quit { .. } => std::process::exit(0),
        Event::WindowEvent (WindowEvent::Resized (w, h)) => unimplemented!(), // Redraw the window
        _ => {}
    }
}

answered Oct 29, 2020 at 7:42

Jmb

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3 Comments

Marcus Over a year ago

Does the redraw imply that I need to keep an internal copy of the image to be drawn? Or there is an SDL function to refresh?

Jmb Over a year ago

You need to keep a copy of the image or to redo the drawing operations taking into account the new window size.

Marcus Over a year ago

I see; that's something I wanted to avoid, but it turns out it's necessary. Thanks!

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