imgcmp: Add a --write-diff-image option

This writes a bitmap that has pixels that are identical between
the two input images blended with 0.5 alpha against white, and
differing pixels are highlighted in red.

Use like so:

    imgcmp --write-diff-image diff.webp image1.png image2.webp

Userland/Utilities/imgcmp.cpp

@@ -9,6 +9,8 @@
 #include <LibCore/MappedFile.h>
 #include <LibCore/MimeData.h>
 #include <LibGfx/ImageFormats/ImageDecoder.h>
+#include <LibGfx/ImageFormats/PNGWriter.h>
+#include <LibGfx/ImageFormats/WebPWriter.h>
 
 static ErrorOr<RefPtr<Gfx::Bitmap>> load_image(StringView path)
 {
@@ -20,10 +22,49 @@ static ErrorOr<RefPtr<Gfx::Bitmap>> load_image(StringView path)
     return TRY(decoder->frame(0)).image;
 }
 
+static ErrorOr<void> save_image(NonnullRefPtr<Gfx::Bitmap> bitmap, StringView out_path)
+{
+    if (!out_path.ends_with(".png"sv, CaseSensitivity::CaseInsensitive) && !out_path.ends_with(".webp"sv, CaseSensitivity::CaseInsensitive))
+        return Error::from_string_view("can only save to .png and .webp files"sv);
+
+    auto output_stream = TRY(Core::File::open(out_path, Core::File::OpenMode::Write));
+    auto buffered_stream = TRY(Core::OutputBufferedFile::create(move(output_stream)));
+
+    if (out_path.ends_with(".png"sv, CaseSensitivity::CaseInsensitive))
+        return Gfx::PNGWriter::encode(*buffered_stream, *bitmap);
+
+    VERIFY(out_path.ends_with(".webp"sv, CaseSensitivity::CaseInsensitive));
+    return Gfx::WebPWriter::encode(*buffered_stream, *bitmap);
+}
+
+static ErrorOr<NonnullRefPtr<Gfx::Bitmap>> make_diff_image(NonnullRefPtr<Gfx::Bitmap> first_image, NonnullRefPtr<Gfx::Bitmap> second_image)
+{
+    VERIFY(first_image->size() == second_image->size());
+
+    auto diff_image = TRY(Gfx::Bitmap::create(Gfx::BitmapFormat::BGRA8888, first_image->size()));
+
+    for (int y = 0; y < first_image->height(); ++y) {
+        for (int x = 0; x < first_image->width(); ++x) {
+            auto first_pixel = first_image->get_pixel<Gfx::StorageFormat::BGRA8888>(x, y);
+            auto second_pixel = second_image->get_pixel<Gfx::StorageFormat::BGRA8888>(x, y);
+            if (first_pixel == second_pixel) {
+                diff_image->set_pixel(x, y, first_pixel.interpolate(Gfx::Color::White, 0.5f));
+            } else {
+                diff_image->set_pixel(x, y, Gfx::Color::Red);
+            }
+        }
+    }
+
+    return diff_image;
+}
+
 ErrorOr<int> serenity_main(Main::Arguments arguments)
 {
     Core::ArgsParser args_parser;
 
+    StringView write_diff_image_path;
+    args_parser.add_option(write_diff_image_path, "Write image that highlights differing pixels", "write-diff-image", {}, "FILE");
+
     StringView first_image_path;
     args_parser.add_positional_argument(first_image_path, "Path to first input image", "FILE1");
 
@@ -40,6 +81,11 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         return 1;
     }
 
+    if (!write_diff_image_path.is_empty()) {
+        auto diff_image = TRY(make_diff_image(*first_image, *second_image));
+        TRY(save_image(diff_image, write_diff_image_path));
+    }
+
     for (int y = 0; y < first_image->physical_height(); ++y) {
         for (int x = 0; x < first_image->physical_width(); ++x) {
             auto first_pixel = first_image->get_pixel(x, y);