DWF and DGN underlays: attach, display, snaps, frames and references

DWFATTACH / -DWFATTACH and DGNATTACH / -DGNATTACH attach a DWF / DWFx sheet
or a DGN (V7 or V8) model with the reference's prompts; ATTACH routes the
file types to them. The sheet or model is read into vector paths, drawn as a
transparent underlay raster (contrast, fade, monochrome and background
adjustment apply) and offers its geometry to object snaps. Definitions are
kept in ACAD_DWFDEFINITIONS / ACAD_DGNDEFINITIONS, the External References
palette lists and reloads them, and image textures get line-keeping mip
levels so thin underlay lines stay visible when zoomed out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/pipeline/image_gpu.rs

@@ -220,6 +220,7 @@ impl ImageGpu {
                     "image.texture:{}:{}:{}",
                     model.file_path, x.content_start, y.content_start
                 );
+                let levels = mip_chain(&model.pixels, model.width, x.data_start, y.data_start, width, height);
                 let texture = device.create_texture(&wgpu::TextureDescriptor {
                     label: Some(&tex_label),
                     size: wgpu::Extent3d {
@@ -227,7 +228,7 @@ impl ImageGpu {
                         height,
                         depth_or_array_layers: 1,
                     },
-                    mip_level_count: 1,
+                    mip_level_count: levels.len() as u32,
                     sample_count: 1,
                     dimension: wgpu::TextureDimension::D2,
                     format: wgpu::TextureFormat::Rgba8Unorm,
@@ -235,22 +236,27 @@ impl ImageGpu {
                         | wgpu::TextureUsages::COPY_DST,
                     view_formats: &[],
                 });
-                queue.write_texture(
-                    texture.as_image_copy(),
-                    &model.pixels[((y.data_start as usize * model.width as usize
-                        + x.data_start as usize)
-                        * 4)..],
-                    wgpu::TexelCopyBufferLayout {
-                        offset: 0,
-                        bytes_per_row: Some(4 * model.width),
-                        rows_per_image: Some(model.height),
-                    },
-                    wgpu::Extent3d {
-                        width,
-                        height,
-                        depth_or_array_layers: 1,
-                    },
-                );
+                for (level, (w, h, data)) in levels.iter().enumerate() {
+                    queue.write_texture(
+                        wgpu::TexelCopyTextureInfo {
+                            texture: &texture,
+                            mip_level: level as u32,
+                            origin: wgpu::Origin3d::ZERO,
+                            aspect: wgpu::TextureAspect::All,
+                        },
+                        data,
+                        wgpu::TexelCopyBufferLayout {
+                            offset: 0,
+                            bytes_per_row: Some(4 * w),
+                            rows_per_image: Some(*h),
+                        },
+                        wgpu::Extent3d {
+                            width: *w,
+                            height: *h,
+                            depth_or_array_layers: 1,
+                        },
+                    );
+                }
                 let tex_view =
                     texture.create_view(&wgpu::TextureViewDescriptor::default());
                 let bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
@@ -302,6 +308,52 @@ struct TileRange {
     data_end: u32,
 }
 
+/// A tile's pixels and its smaller levels down to 1×1, so a far view
+/// samples a level near its own size. A level keeps the most opaque of each
+/// 2×2 block unless all four are equally opaque (then their average): thin
+/// lines on a transparent page (PDF, DWF, DGN underlays) stay visible when
+/// zoomed out instead of falling between samples, and opaque images are
+/// averaged as usual.
+fn mip_chain(pixels: &[u8], stride: u32, x0: u32, y0: u32, w: u32, h: u32) -> Vec<(u32, u32, Vec<u8>)> {
+    let (stride, x0, y0) = (stride as usize, x0 as usize, y0 as usize);
+    let mut base = Vec::with_capacity(w as usize * h as usize * 4);
+    for row in 0..h as usize {
+        let start = ((y0 + row) * stride + x0) * 4;
+        base.extend_from_slice(&pixels[start..start + w as usize * 4]);
+    }
+    let mut levels = vec![(w, h, base)];
+    loop {
+        let (pw, ph, prev) = levels.last().expect("base level");
+        if *pw == 1 && *ph == 1 {
+            break;
+        }
+        let (pw, ph) = (*pw as usize, *ph as usize);
+        let (nw, nh) = ((pw / 2).max(1), (ph / 2).max(1));
+        let mut next = Vec::with_capacity(nw * nh * 4);
+        for y in 0..nh {
+            for x in 0..nw {
+                let block = [(0, 0), (1, 0), (0, 1), (1, 1)].map(|(dx, dy)| {
+                    let at = (((2 * y + dy).min(ph - 1)) * pw + (2 * x + dx).min(pw - 1)) * 4;
+                    [prev[at], prev[at + 1], prev[at + 2], prev[at + 3]]
+                });
+                if block.iter().all(|p| p[3] == block[0][3]) {
+                    let mut sum = [0u32; 4];
+                    for p in &block {
+                        for c in 0..4 {
+                            sum[c] += p[c] as u32;
+                        }
+                    }
+                    next.extend(sum.map(|v| ((v + 2) / 4) as u8));
+                } else {
+                    next.extend(*block.iter().max_by_key(|p| p[3]).expect("four pixels"));
+                }
+            }
+        }
+        levels.push((nw as u32, nh as u32, next));
+    }
+    levels
+}
+
 fn tile_ranges(size: u32, limit: u32) -> Vec<TileRange> {
     if size <= limit {
         return vec![TileRange {