Fix buffer pool race condition, pixman transform, and pixel conversion

Mark reused buffers as busy before returning from nextBuffer (before,
they only got marked busy on init).

Re-attach wl_buffer listener after re-initializing a buffer. This lets
the re-inited buffer still get a released event.

Combine scale and translate matrices with multiply instead of overwriting

Use appendAssumeCapacity for pixel conversion loop (since we already
initialized the list with the correct size).

README.md

@@ -11,6 +11,7 @@ SPDX-License-Identifier: GPL-3.0-or-later
 These are in rough order of my priority, though no promises I do them in this order.
 
 - [ ] Switch all structs to idiomatic Zig init/deinit pattern (init returns value, caller decides stack/heap)
+- [ ] Implement runtime log levels
 - [ ] Support per-host config using properties
 - [ ] Support a basic bar
 - [ ] Support starting programs at WM launch

src/BufferPool.zig

@@ -61,6 +61,7 @@ fn findSuitableBuffer(buffer_pool: *BufferPool, wl_shm: *wl.Shm, width: u31, hei
         const buffer: *Buffer = @fieldParentPtr("node", node);
         if (buffer.busy) continue;
         if (buffer.width == width and buffer.height == height) {
+            buffer.busy = true;
             return buffer;
         } else {
             first_unbusy_buffer = buffer;
@@ -72,6 +73,7 @@ fn findSuitableBuffer(buffer_pool: *BufferPool, wl_shm: *wl.Shm, width: u31, hei
     if (first_unbusy_buffer) |buffer| {
         buffer.deinit();
         buffer.* = try Buffer.init(wl_shm, width, height);
+        buffer.setListener();
         return buffer;
     }
 

src/Output.zig

@@ -223,7 +223,7 @@ pub fn initWallpaperLayerSurface(output: *Output) !void {
     }
 
     if (output.wl_surface) |_| {
-        log.debug("Skipping adding a second wallpaper surface to {s}", .{output.name orelse "some output"});
+        // This output already has a layer surface, we can exit early
         return;
     }
 
@@ -344,15 +344,13 @@ pub fn renderWallpaper(output: *Output) !void {
 
     const buffer: *Buffer = try context.buffer_pool.nextBuffer(context.wl_shm, width * scale, height * scale);
 
-    const pix = pixman.Image.createBitsNoClear(image_format, image_width, image_height, image_data, image_stride);
-    if (pix == null) {
-        log.err("failed to copy the background image for rendering", .{});
+    const pix = pixman.Image.createBitsNoClear(image_format, image_width, image_height, image_data, image_stride) orelse {
+        log.err("Failed to copy the wallpaper image for rendering", .{});
         return error.ImageCopyError;
-    }
-    defer _ = pix.?.unref();
+    };
+    defer _ = pix.unref();
 
-    // Get scale for our image compared to the monitor's scale
-    // XXX: This sucks in Zig but also I'm sure there's a better way to write it
+    // Calculate image scale
     var sx: f64 = @as(f64, @floatFromInt(image_width)) / @as(f64, @floatFromInt(width * scale));
     var sy: f64 = calculate_scale(image_height, height, scale);
 
@@ -360,19 +358,31 @@ pub fn renderWallpaper(output: *Output) !void {
     sx = s;
     sy = s;
 
+    // Calculate translation offsets to center the image on the output.
+    // If the scaled image is larger than the output, the offset crops equally from both sides.
     const tx: f64 = calculate_transform(image_width, width, sx);
     const ty: f64 = calculate_transform(image_height, height, sy);
 
+    // Build a combined source-to-destination transform matrix.
+    // Pixman transforms map destination pixels back to source pixels, so:
+    //   t_scale: maps a destination pixel to the corresponding source pixel (scaling)
+    //   t_trans: shifts the sampling point to center the image
+    //   t = t_trans * t_scale: first scale, then translate (in source space)
+    var t_scale: pixman.FTransform = undefined;
+    var t_trans: pixman.FTransform = undefined;
     var t: pixman.FTransform = undefined;
+    // t2 is the fixed-point version of t, which is what pixman actually uses internally
     var t2: pixman.Transform = undefined;
 
-    pixman.FTransform.initTranslate(&t, tx, ty);
-    pixman.FTransform.initScale(&t, sx, sy);
+    pixman.FTransform.initScale(&t_scale, sx, sy);
+    pixman.FTransform.initTranslate(&t_trans, tx, ty);
+    pixman.FTransform.multiply(&t, &t_trans, &t_scale);
     _ = pixman.Transform.fromFTransform(&t2, &t);
-    _ = pix.?.setTransform(&t2);
-    _ = pix.?.setFilter(.best, &[_]pixman.Fixed{}, 0);
+    _ = pix.setTransform(&t2);
+    _ = pix.setFilter(.best, &[_]pixman.Fixed{}, 0);
 
-    pixman.Image.composite32(.src, pix.?, null, buffer.pixman_image, 0, 0, 0, 0, 0, 0, width * scale, height * scale);
+    // Combine the transformed source image into the buffer.
+    pixman.Image.composite32(.src, pix, null, buffer.pixman_image, 0, 0, 0, 0, 0, 0, width * scale, height * scale);
 
     log.info("render: {}x{} (scaled from {}x{})", .{ width * scale, height * scale, image_width, image_height });
 

src/WallpaperImage.zig

@@ -35,7 +35,7 @@ pub fn create(image_path: []const u8) !*WallpaperImage {
         const g: u32 = @intCast(pixels[i].g);
         const b: u32 = @intCast(pixels[i].b);
         const new_val: u32 = (a << 24) + (r << 16) + (g << 8) + b;
-        try wallpaper_image.pixels.append(utils.allocator, new_val);
+        wallpaper_image.pixels.appendAssumeCapacity(new_val);
     }
 
     wallpaper_image.image = pixman.Image.createBits(.a8r8g8b8, @intCast(image.width), @intCast(image.height), @ptrCast(@alignCast(wallpaper_image.pixels.items.ptr)), @intCast(image.width * image.pixelFormat().pixelStride())) orelse return error.FailedToCreatePixmanImage;

src/main.zig

@@ -134,10 +134,6 @@ fn registryListener(registry: *wl.Registry, event: wl.Registry.Event, globals: *
     }
 }
 
-pub const std_options = std.Options{
-    .log_level = .debug,
-};
-
 const std = @import("std");
 const mem = std.mem;
 const fatal = std.process.fatal;