Module screenshot 

Screenshot capture handle for agg-gui apps.

The GL rendering harness (GlGfxCtx::read_screenshot on the desktop GL path + the equivalent WebGL2 read-back in the WASM harness) produces a top-down RGBA8 buffer of the current back buffer. This module supplies the small shared-state handle that a button or hotkey uses to request a capture and that a widget uses to display the result.

Threading / ownership

All fields are Rc<...> — single-threaded, cheap to clone. Never transfer a ScreenshotHandle across threads.

Wiring on native (winit + glow)

let shot = agg_gui::ScreenshotHandle::new();

// In a button's on_click:
let req = shot.request.clone();
Button::new("📷 Capture", font).on_click(move || req.set(true))

// In the event loop, AFTER render_frame but BEFORE swap_buffers:
if shot.request.get() {
    let (rgba, w, h) = gl_ctx.read_screenshot();
    *shot.image.borrow_mut() = Some((rgba, w, h));
    shot.request.set(false);
}

// Display: pass `shot.image` to `ImageView`.

Wiring on WASM

Same Rust-side flow — the browser’s WebGL2 context still provides glReadPixels, so GlGfxCtx::read_screenshot() works unchanged. The JS side needs no special code beyond driving the animation loop:

// In the WASM render export (called from JS requestAnimationFrame):
if shot.request.get() {
    let (rgba, w, h) = gl_ctx.read_screenshot();  // must be BEFORE presenting
    *shot.image.borrow_mut() = Some((rgba, w, h));
    shot.request.set(false);
}

Note for the LLM / future dev: on WASM, read_screenshot MUST be called before the browser composites the canvas (i.e. within the same rAF tick, before yielding). Because WebGL uses a preserved-drawing-buffer only when explicitly requested, calling it outside that window yields a blank image. The natural “after paint, before yield” position in the render function is correct.

If the app wants to TRIGGER a browser download instead of displaying in-canvas, export a WASM function that calls read_screenshot, encode with the png crate via agg_gui::encode_png_rgba (if available in the surrounding app), and pass the bytes to a JS helper that creates a Blob + URL.createObjectURL + synthetic <a download> click.

Structs

ScreenshotHandle

Shared capture state. Clone freely; all inner fields are Rc<...>.

Enums

ScreenshotExportError

Error returned by screenshot export helpers.

ScreenshotExportOutcome

Result of a platform screenshot export operation.

Functions

copy_rgba_to_clipboard

Copy a top-down RGBA8 screenshot to the system clipboard.

download_rgba_as_png

Download or save a top-down RGBA8 screenshot as a PNG.

encode_png_rgba

Encode a top-down RGBA8 image as a PNG.

run_frame_with_capture

Run one frame through the screenshot capture flow.