feat(image): use cairo conversion for png images, reduce pixbuf overhead

src/state.rs

@@ -1,9 +1,13 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 // Copyright (C) 2026, Nathan Gill
 
-use std::sync::{
-    Arc,
-    atomic::{AtomicBool, Ordering},
+use std::{
+    fs::File,
+    io::Seek,
+    sync::{
+        Arc,
+        atomic::{AtomicBool, Ordering},
+    },
 };
 
 use anyhow::{Result, anyhow, bail};
@@ -27,7 +31,6 @@ use wayland_protocols::ext::session_lock::v1::client::{
 };
 use zeroize::Zeroizing;
 
-use crate::auth::{AtomicAuthState, AuthState};
 use crate::cairo_ext::{ImageSurfaceExt, SubpixelOrderExt};
 use crate::config::NLockConfig;
 use crate::util::BackgroundType;
@@ -36,6 +39,10 @@ use crate::{
     seat::{NLockSeat, NLockXkb},
     surface::NLockSurface,
 };
+use crate::{
+    auth::{AtomicAuthState, AuthState},
+    util::detect_png,
+};
 
 pub struct NLockState {
     pub config: NLockConfig,
@@ -191,11 +198,24 @@ impl NLockState {
             return Ok(());
         }
 
-        let pixbuf = Pixbuf::from_file(self.config.image.path.clone())?;
-        let pixbuf = pixbuf
-            .apply_embedded_orientation()
-            .ok_or(anyhow!("Failed to apply embedded image orientation"))?;
-        self.background_image = Some(ImageSurface::create_from_pixbuf(&pixbuf)?);
+        let mut image_file = File::open(&self.config.image.path)?;
+        let is_png = detect_png(&mut image_file)?;
+        image_file.rewind()?;
+
+        // if it's a PNG, Cairo can handle it directly, pixbuf conversion is expensive
+        if is_png {
+            let image_surface = ImageSurface::create_from_png(&mut image_file)?;
+            self.background_image = Some(image_surface);
+        } else {
+            let pixbuf = Pixbuf::from_read(image_file)?;
+            let pixbuf = pixbuf
+                .apply_embedded_orientation()
+                .ok_or(anyhow!("Failed to apply embedded image orientation"))?;
+
+            let image_surface = ImageSurface::create_from_pixbuf(&pixbuf)?;
+            self.background_image = Some(image_surface);
+        }
+
         self.config.general.bg_type = BackgroundType::Image;
 
         Ok(())

src/util.rs

@@ -1,7 +1,11 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 // Copyright (C) 2026, Nathan Gill
 
-use std::{os::fd::OwnedFd, str::FromStr};
+use std::{
+    io::{self, Read},
+    os::fd::OwnedFd,
+    str::FromStr,
+};
 
 use clap::ValueEnum;
 use nix::{
@@ -168,3 +172,21 @@ pub fn is_eintr(err: &std::io::Error) -> bool {
         None => false,
     }
 }
+
+const PNG_SIG: [u8; 8] = [0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A];
+
+// Detect if a source stream starts with a PNG signature.
+// Intended use case is for detecting whether a file is PNG or not.
+pub fn detect_png<R>(r: &mut R) -> io::Result<bool>
+where
+    R: Read,
+{
+    let mut buf = [0; 8];
+    let n = r.read(&mut buf)?;
+
+    if n < 8 || !buf.eq(&PNG_SIG) {
+        return Ok(false);
+    }
+
+    Ok(true)
+}