Opening a png of type gio::File in cairo

[youpie123]

Hello,

I am trying to create a libadwaita application in Rust. I want to open two images and overlay them on top of eachother. I am however struggling to understand the documentation. I used an AI assistant which recommended me to use cairo and use create_from_png(). To open the files.

I have already built the logic to allow the user to select a file and have a gio::File which I want to insert. When I do ImageSurface::create_from_png(&mut file); and run the program, I get the error:

error[E0277]: the trait bound `libadwaita::gio::File: std::io::Read` is not satisfied
   --> src/window.rs:138:51
    |
138 |         let image = ImageSurface::create_from_png(&mut file);
    |                     ----------------------------- ^^^^^^^^^ the trait `std::io::Read` is not implemented for `libadwaita::gio::File`
    |                     |
    |                     required by a bound introduced by this call

So I assume the type I need to use must implement std::io::Read. I can do file.read(None) which gives me a FileInputStream. But I think this is a different implementation of read.
What do I need to do differently to make this work, Can I even make this work with a gio::File or do I need to use something else?

Any tips are much appreciated!

[gwillems]

Hi,

For opening images from gio::File, just use Picture.for_file().

For overlaying 2 images, an Overlay should do the trick.

@youpie123

I used an AI assistant which recommended me to use cairo and use create_from_png(). To open the files.

Could work, but that way sounds incredibly complicated…
By the way, cairo is the old gtk3 method for custom drawing. In gtk4, prefer snapshots.

[youpie123]

thank you for the reply, that could indeed be an easy sollution. However I want to be able to save to overlayed images. I am trying to create an application to easilly add an icon to a folder like this:

[gwillems]

Ah, OK, thanks for the details!

Looks like create_from_png() takes a file path (i.e. a string) as parameter.
If you have a gio::File you can get its path with the get_path() method.

Alternatively, you could use a GDK Texture and load it to a cairo surface (but I’ve never tried…).

[sdroege (Sebastian Dröge)]

gio::File is just like a path or URL, it does not involve an actually opened file. You first need to get an input stream from it via e.g. read() or read_async() / read_future(). Once you have that you can convert that input stream into something that implements the std::io::Read trait via into_read(), for example.

[youpie123]

Hi, thank you very much for that tip. Doing file.read(cancellable).unwrap().into_read() gave me the result I needed. Many thanks