centered backgrounds are now an option

Author: https://github.com/alexispurslane
Commit: https://github.com/SerenityOS/serenity/commit/da9c70598f9
Pull-request: https://github.com/SerenityOS/serenity/pull/118
Reviewed-by: https://github.com/awesomekling

Base/home/anon/WindowManager.ini

@@ -1,6 +1,6 @@
 [Screen]
-Width=2560
-Height=1440
+Width=1920
+Height=1080
 
 [Cursor]
 Arrow=/res/cursors/arrow.png
@@ -36,4 +36,4 @@ MenuSelectionColor=132,53,26
 DoubleClickSpeed=250
 
 [Background]
-Mode=tile
+Mode=center

Servers/WindowServer/WSCompositor.cpp

@@ -95,8 +95,10 @@ void WSCompositor::compose()
                 m_wallpaper_mode == WallpaperMode::Unchecked) {
                 m_back_painter->blit(dirty_rect.location(), *m_wallpaper, dirty_rect);
             } else if (m_wallpaper_mode == WallpaperMode::Center) {
-                // TODO: Implement centered wallpaper
-                m_back_painter->blit(dirty_rect.location(), *m_wallpaper, dirty_rect);
+                Point offset{ ws.size().width() / 2 - m_wallpaper->size().width() / 2,
+                              ws.size().height() / 2 - m_wallpaper->size().height() / 2 };
+                m_back_painter->blit_offset(dirty_rect.location(), *m_wallpaper,
+                                            dirty_rect, offset);
             } else if (m_wallpaper_mode == WallpaperMode::Tile) {
                 m_back_painter->blit_tiled(dirty_rect.location(), *m_wallpaper, dirty_rect);
             }

SharedGraphics/Painter.cpp

@@ -301,6 +301,43 @@ void Painter::blit_tiled(const Point& position, const GraphicsBitmap& source, co
     ASSERT_NOT_REACHED();
 }
 
+void Painter::blit_offset(const Point& position,
+                          const GraphicsBitmap& source,
+                          const Rect& src_rect,
+                          const Point& offset)
+{
+    auto dst_rect = Rect(position, src_rect.size()).translated(translation());
+    auto clipped_rect = dst_rect.intersected(clip_rect());
+    if (clipped_rect.is_empty())
+        return;
+    const int first_row = (clipped_rect.top() - dst_rect.top());
+    const int last_row = (clipped_rect.bottom() - dst_rect.top());
+    const int first_column = (clipped_rect.left() - dst_rect.left());
+    RGBA32* dst = m_target->scanline(clipped_rect.y()) + clipped_rect.x();
+    const size_t dst_skip = m_target->pitch() / sizeof(RGBA32);
+
+    if (source.format() == GraphicsBitmap::Format::RGB32 || source.format() == GraphicsBitmap::Format::RGBA32) {
+        int x_start = first_column + src_rect.left();
+        for (int row = first_row; row <= last_row; ++row) {
+            int sr = row - offset.y() + src_rect.top();
+            if (sr >= source.size().height() || sr < 0) {
+                dst += dst_skip;
+                continue;
+            }
+            const RGBA32* sl = source.scanline(sr);
+            for (int x = x_start; x < clipped_rect.width() + x_start; ++x) {
+                int sx = x - offset.x();
+                if (sx < source.size().width() && sx >= 0)
+                    dst[x - x_start] = sl[sx];
+            }
+            dst += dst_skip;
+        }
+        return;
+    }
+
+    ASSERT_NOT_REACHED();
+}
+
 void Painter::blit_with_alpha(const Point& position, const GraphicsBitmap& source, const Rect& src_rect)
 {
     ASSERT(source.has_alpha_channel());

SharedGraphics/Painter.h

@@ -28,6 +28,7 @@ public:
     void blit(const Point&, const GraphicsBitmap&, const Rect& src_rect, float opacity = 1.0f);
     void blit_dimmed(const Point&, const GraphicsBitmap&, const Rect& src_rect);
     void blit_tiled(const Point&, const GraphicsBitmap&, const Rect& src_rect);
+    void blit_offset(const Point&, const GraphicsBitmap&, const Rect& src_rect, const Point&);
     void draw_text(const Rect&, const char* text, int length, const Font&, TextAlignment = TextAlignment::TopLeft, Color = Color::Black, TextElision = TextElision::None);
     void draw_text(const Rect&, const char* text, int length, TextAlignment = TextAlignment::TopLeft, Color = Color::Black, TextElision = TextElision::None);
     void draw_text(const Rect&, const String&, const Font&, TextAlignment = TextAlignment::TopLeft, Color = Color::Black, TextElision = TextElision::None);