[HELP] how to get image data from a cairo::Surface?

[algebnaly]

I want to get drawing result in cairo::Surface.
for example, I have tried the following

fn main() {
    let surface = cairo::ImageSurface::create(cairo::Format::ARgb32, 1920, 1080).unwrap();
    let ctx = cairo::Context::new(surface).unwrap();
    ctx.set_source_rgba(1.0, 1.0, 0.0, 1.0);
    ctx.move_to(0.0, 0.0);
    ctx.line_to(100.0, 100.0);
    ctx.stroke().unwrap();
    let surface: cairo::Surface = ctx.target();
    let maped: cairo::MappedImageSurface = surface.map_to_image(None).unwrap();
    let mut img_surface: cairo::ImageSurface = maped.clone();
    let img_surface_data: cairo::ImageSurfaceData = img_surface.data().expect("why panic here?");
    let data: Vec<u8> = img_surface_data.to_vec();
    println!("{}",data[0]);
}

but rust compiler report panic message:

thread 'main' panicked at src/main.rs:11:72:
why panic here?: NonExclusive
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

it panic at let img_surface_data: cairo::ImageSurfaceData = img_surface.data().expect("why panic here?");

is the above approach to get data from a surface is wrong? and how to get image data from a surface(I am ok to copy data in to a Vec)?

[sdroege]

You need to make sure that there's only a single reference to the surface. Yours will have one more from the context, and another one from maped.clone().

[algebnaly]

You need to make sure that there's only a single reference to the surface. Yours will have one more from the context, and another one from maped.clone().

the problem is, if I just call data() on maped,

fn main() {
    let surface = cairo::ImageSurface::create(cairo::Format::ARgb32, 1920, 1080).unwrap();
    let ctx = cairo::Context::new(surface).unwrap();
    ctx.set_source_rgba(1.0, 1.0, 0.0, 1.0);
    ctx.move_to(0.0, 0.0);
    ctx.line_to(100.0, 100.0);
    ctx.stroke().unwrap();
    let surface: cairo::Surface = ctx.target();
    let mut maped: cairo::MappedImageSurface = surface.map_to_image(None).unwrap();
    let img_surface_data: cairo::ImageSurfaceData = maped.data().unwrap();
    let data: Vec<u8> = img_surface_data.to_vec();
    println!("{}",data[0]);
}

rust compiler tell me that

trait `DerefMut` is required to modify through a dereference, but it is not implemented for `MappedImageSurface`

and it seems that no other way to get image data from a cairo::Surface

[sdroege]

We should probably add an unsafe getter for the data, yes.

For what you're doing here, the following code would work:

fn main() {
    let mut surface = cairo::ImageSurface::create(cairo::Format::ARgb32, 1920, 1080).unwrap();

    let ctx = cairo::Context::new(&surface).unwrap();  // <---- passing a reference here allows you to access it again later
    ctx.set_source_rgba(1.0, 1.0, 0.0, 1.0);
    ctx.move_to(0.0, 0.0);
    ctx.line_to(100.0, 100.0);
    ctx.stroke().unwrap();
    drop(ctx); // <---- makes sure that the surface is not referenced by the context anymore and `surface` is the only owner

    let img_surface_data: cairo::ImageSurfaceData = surface.data().unwrap();
    let data: Vec<u8> = img_surface_data.to_vec();
    println!("{}", data[0]);
}