Separate fullscreen capture from core.rs

- Modularity and abstraction
- Separation of concerns

src/core.rs

@@ -5,28 +5,38 @@ use x11rb::rust_connection::RustConnection;
 use png::{BitDepth, ColorType, Encoder};
 use std::fs::File;
 
-/// Holds an open X11 connection and basic screen geometry.
-/// All screenshot binaries share this context.
+/// Stores:
+/// - a connection to the X11 server (`conn`)
+/// - the root window (the whole screen)
+/// - the screen width and height in pixels
+///
+/// Every screenshot function uses this.
 pub struct XContext {
     pub conn: RustConnection,
-    pub root: u32,   // The root window of the screen (represents the whole desktop)
-    pub width: u16,  // Screen width in pixels
-    pub height: u16, // Screen height in pixels
+    pub root: u32,
+    pub width: u16,
+    pub height: u16,
 }
 
 impl XContext {
+    /// Create a new connection to the X11 server and read basic screen info.
+    ///
+    /// This does not take a screenshot. It only connects and stores
+    /// values we need later.
     pub fn new() -> anyhow::Result<Self> {
-        // Establish a connection to the running X server.
-        // `screen_num` selects which screen we are connected to (multi-head setups).
+        // Connect to the X11 server running on the system.
         let (conn, screen_num) = RustConnection::connect(None)?;
 
-        // Extract the geometry information from the X11 setup.
-        // These fields are Copy, so borrowing ends immediately.
-        let screen = &conn.setup().roots[screen_num];
+        // Read screen information from the X11 setup data.
+        // We copy only the simple numeric values we need.
+        let setup = conn.setup();
+        let screen = &setup.roots[screen_num];
+
         let root = screen.root;
         let width = screen.width_in_pixels;
         let height = screen.height_in_pixels;
 
+        // Return our context object.
         Ok(Self {
             conn,
             root,
@@ -36,66 +46,67 @@ impl XContext {
     }
 }
 
-/// Capture the *entire* root window (full screen) into an RGBA8 buffer.
+/// Capture a rectangular part of the screen.
 ///
-/// X11 provides the `GetImage` request, which returns raw pixel data from a drawable.
-/// We request the `ZPixmap` format, which returns packed pixel values.
+/// `ctx`  – connection + screen info  
+/// `x,y` – top-left corner of the rectangle  
+/// `w,h` – size of the rectangle  
 ///
-/// Many x11rb code generators include a named constant for `ZPixmap`.
-/// If not available, its protocol value is simply `2`.
-pub fn capture_root_image(ctx: &XContext) -> anyhow::Result<Vec<u8>> {
-    // ZPixmap corresponds to the X11 format that returns full pixel values.
-    // Using a literal here avoids dependency on generated constant names.
+/// Returns: a `Vec<u8>` containing RGBA pixel data.
+pub fn capture_rect(ctx: &XContext, x: u16, y: u16, w: u16, h: u16) -> anyhow::Result<Vec<u8>> {
+    // X11 uses the number 2 for "ZPixmap", which means: return full pixel data.
     let format_zpixmap: u8 = 2;
 
-    // Send the GetImage request for the entire screen.
+    // Ask X11 to send us the raw pixels in the requested rectangle.
+    // This returns X11's native pixel format (usually B,G,R,unused).
     let reply = ctx
         .conn
         .get_image(
-            format_zpixmap.into(), // ZPixmap format
-            ctx.root,              // The root window (whole desktop)
-            0,                     // X origin
-            0,                     // Y origin
-            ctx.width,             // Width to capture
-            ctx.height,            // Height to capture
-            u32::MAX,              // Plane mask: capture all pixel planes
+            format_zpixmap.into(),
+            ctx.root,
+            x as i16,
+            y as i16,
+            w,
+            h,
+            u32::MAX,
         )?
         .reply()?;
 
     let data = reply.data;
-    let depth = reply.depth;
 
-    // Modern X11 desktops typically use depth 24 (RGB) or 32 (RGB with padding).
-    if depth != 24 && depth != 32 {
-        anyhow::bail!("Unsupported X11 depth: {depth}");
-    }
-
-    // X11 typically stores pixels in little-endian B,G,R,(unused) format.
-    // We convert it to standard RGBA because PNG expects that.
-    let mut out = Vec::with_capacity((ctx.width as usize) * (ctx.height as usize) * 4);
+    // Prepare a buffer for RGBA output.
+    // Each pixel = 4 bytes (R, G, B, A).
+    let mut out = Vec::with_capacity((w as usize) * (h as usize) * 4);
 
+    // Convert each incoming pixel from X11's format (B, G, R, X)
+    // into the common RGBA format that PNG expects.
     for chunk in data.chunks_exact(4) {
         let b = chunk[0];
         let g = chunk[1];
         let r = chunk[2];
-        out.extend_from_slice(&[r, g, b, 0xFF]); // Always opaque
+
+        // A (alpha) is set to 255 = fully opaque.
+        out.extend_from_slice(&[r, g, b, 0xFF]);
     }
 
     Ok(out)
 }
 
-/// Save a raw RGBA8 buffer as a PNG image at `path`.
+/// Save an RGBA pixel buffer as a PNG file.
 ///
-/// The png crate handles all compression and file formatting.
-/// We simply specify:
-/// - RGBA layout
-/// - 8 bits per channel
+/// `path`   – where to write the file  
+/// `width`  – image width in pixels  
+/// `height` – image height in pixels  
+/// `rgba`   – pixel data in RGBA format  
 pub fn save_png(path: &str, width: u16, height: u16, rgba: &[u8]) -> anyhow::Result<()> {
     let file = File::create(path)?;
+
+    // Create a PNG encoder for an RGBA 8-bit image.
     let mut encoder = Encoder::new(file, width.into(), height.into());
     encoder.set_color(ColorType::Rgba);
     encoder.set_depth(BitDepth::Eight);
 
+    // Write PNG header + pixel data.
     let mut writer = encoder.write_header()?;
     writer.write_image_data(rgba)?;
     writer.finish()?;

src/fullscreen.rs

@@ -1,38 +1,42 @@
 mod core;
 
 use chrono::Local;
-use core::{XContext, capture_root_image, save_png};
+use core::{XContext, capture_rect, save_png};
 use std::fs;
 
 fn main() -> anyhow::Result<()> {
-    // Establish connection to X11 and gather screen info.
+    // Create a connection to the X11 server and read screen information.
     let ctx = XContext::new()?;
 
-    // Capture the entire screen as an RGBA8 buffer.
-    let img = capture_root_image(&ctx)?;
+    // Capture the entire screen.
+    // We pass (0,0) as the top-left corner and use the screen width/height.
+    let img = capture_rect(&ctx, 0, 0, ctx.width, ctx.height)?;
 
-    // Build an output path:
-    //   ~/Pictures/Screenshots/screenshot_YYYYMMDDHHMMSS.png
+    // Build the output file path.
+    // Example:
+    //   ~/Pictures/Screenshots/screenshot_20250116094530.png
     //
-    // Using timestamps avoids overwriting previous screenshots,
-    // and matches the behavior of many modern screenshot tools.
+    // Using a timestamp prevents overwriting old screenshots.
     let timestamp = Local::now().format("%Y%m%d%H%M%S").to_string();
 
+    // Start from the user's home directory (fallback: current dir).
     let mut path = dirs::home_dir().unwrap_or_else(|| ".".into());
     path.push("Pictures");
     path.push("Screenshots");
 
-    // Ensure the directory exists
+    // Make sure the directory exists.
     fs::create_dir_all(&path)?;
 
+    // Add the actual filename.
     path.push(format!("screenshot_{timestamp}.png"));
 
-    // Convert PathBuf → &str (safe because it’s UTF-8 on all Linux systems).
+    // Convert path to string for save_png().
     let path_str = path.to_string_lossy();
 
-    // Write the PNG file to disk.
+    // Write the captured RGBA image to a PNG file.
     save_png(&path_str, ctx.width, ctx.height, &img)?;
 
     println!("Saved {path_str}");
+
     Ok(())
 }