1 files changed, 48 insertions, 0 deletions
diff --git a/src/pdf.zig b/src/pdf.zig
index 052d34db..712750dc 100644
--- a/src/pdf.zig
+++ b/src/pdf.zig
@@ -976,6 +976,20 @@ pub const Document = struct {
         highlights: []const Highlight,
         rgba: []u8,
     ) !void {
+        return document.renderIntoAtWithPaper(page, request, raster, band, highlights, rgba, false);
+    }
+
+    /// Preserve the PDF's content coverage when the host paints paper separately.
+    pub fn renderIntoAtWithPaper(
+        document: *Document,
+        page: usize,
+        request: RenderRequest,
+        raster: Raster,
+        band: Raster.Band,
+        highlights: []const Highlight,
+        rgba: []u8,
+        transparent_paper: bool,
+    ) !void {
         if (rgba.len != band.len) return error.BadPixmap;
         if (band.height == 0 or band.y + band.height > raster.height)
             return error.BadPixmap;
@@ -999,6 +1013,7 @@ pub const Document = struct {
             @intCast(raster.stride),
             @intCast(band.y),
             @intCast(band.height),
+            @intFromBool(transparent_paper),
         ) != c.PARDES_PDF_OK) return error.RenderFailed;
     }
 
@@ -1832,6 +1847,7 @@ test "RGBA allocation and buffer-validation failures leave the document renderab
             raw_layout.stride,
             0,
             raw_layout.height,
+            0,
         ));
         for (scratch) |byte|
             try std.testing.expectEqual(@as(u8, 0xa5), byte);
@@ -2074,3 +2090,35 @@ test "offset crop and rotation keep normalized highlights pixel-aligned" {
         marked.rgba[left + 1] < 10 and marked.rgba[left + 2] > 240);
     try std.testing.expectEqualSlices(u8, plain.rgba[right .. right + 4], marked.rgba[right .. right + 4]);
 }
+
+test "transparent PDF paper preserves ink and band coverage" {
+    var document = try Document.open("docs/9p.pdf");
+    defer document.deinit();
+    const request: RenderRequest = .{ .dpi = 96, .max_dimension = 1200 };
+    const raster = try document.measureRenderAt(0, request);
+    const flattened = try std.testing.allocator.alloc(u8, raster.len);
+    defer std.testing.allocator.free(flattened);
+    const content = try std.testing.allocator.alloc(u8, raster.len);
+    defer std.testing.allocator.free(content);
+    try document.renderIntoAt(0, request, raster, raster.wholePage(), &.{}, flattened);
+    try document.renderIntoAtWithPaper(0, request, raster, raster.wholePage(), &.{}, content, true);
+    var paper: usize = 0;
+    var ink: usize = 0;
+    var edges: usize = 0;
+    for (std.mem.bytesAsSlice([4]u8, content), std.mem.bytesAsSlice([4]u8, flattened)) |pixel, reference| {
+        const a: u32 = pixel[3];
+        paper += @intFromBool(a == 0);
+        ink += @intFromBool(a == 255);
+        edges += @intFromBool(a > 0 and a < 255);
+        for (0..3) |channel| {
+            const over_white = (a * pixel[channel] + (255 - a) * 255 + 127) / 255;
+            try std.testing.expect(@abs(@as(i32, @intCast(over_white)) - @as(i32, reference[channel])) <= 2);
+        }
+    }
+    try std.testing.expect(paper > 100 and ink > 100 and edges > 100);
+    const band = raster.band(raster.height / 3, raster.height / 4);
+    const rows = try std.testing.allocator.alloc(u8, band.len);
+    defer std.testing.allocator.free(rows);
+    try document.renderIntoAtWithPaper(0, request, raster, band, &.{}, rows, true);
+    try std.testing.expectEqualSlices(u8, content[band.y * raster.stride ..][0..band.len], rows);
+}