LibWeb: Paint `backdrop-filter` effects!

This implements all the filters other than `saturate()`,
`hue-rotate()`, and `drop-shadow()`.

There are still a lot of FIXMEs to handle in the actual implementation
though, particularly around supporting transforms, but this handles
the most common use cases :^)

5 files changed, 159 insertions, 0 deletions

diff --git a/Userland/Libraries/LibWeb/CMakeLists.txt b/Userland/Libraries/LibWeb/CMakeLists.txt
index f297d213b8..2037d55dfa 100644
--- a/Userland/Libraries/LibWeb/CMakeLists.txt
+++ b/Userland/Libraries/LibWeb/CMakeLists.txt
@@ -335,6 +335,7 @@ set(SOURCES
     Painting/CanvasPaintable.cpp
     Painting/CheckBoxPaintable.cpp
     Painting/GradientPainting.cpp
+    Painting/FilterPainting.cpp
     Painting/ImagePaintable.cpp
     Painting/InlinePaintable.cpp
     Painting/LabelablePaintable.cpp
diff --git a/Userland/Libraries/LibWeb/Painting/FilterPainting.cpp b/Userland/Libraries/LibWeb/Painting/FilterPainting.cpp
new file mode 100644
index 0000000000..639349d433
--- /dev/null
+++ b/Userland/Libraries/LibWeb/Painting/FilterPainting.cpp
@@ -0,0 +1,129 @@
+/*
+ * Copyright (c) 2022, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGfx/Filters/BrightnessFilter.h>
+#include <LibGfx/Filters/ContrastFilter.h>
+#include <LibGfx/Filters/GrayscaleFilter.h>
+#include <LibGfx/Filters/InvertFilter.h>
+#include <LibGfx/Filters/OpacityFilter.h>
+#include <LibGfx/Filters/SepiaFilter.h>
+#include <LibGfx/Filters/StackBlurFilter.h>
+#include <LibWeb/Layout/Node.h>
+#include <LibWeb/Painting/BorderRadiusCornerClipper.h>
+#include <LibWeb/Painting/FilterPainting.h>
+
+namespace Web::Painting {
+
+void apply_filter_list(Gfx::Bitmap& target_bitmap, Layout::Node const& node, Span<CSS::FilterFunction const> filter_list)
+{
+    for (auto& filter_function : filter_list) {
+        // See: https://drafts.fxtf.org/filter-effects-1/#supported-filter-functions
+        filter_function.visit(
+            [&](CSS::Filter::Blur const& blur) {
+                // Applies a Gaussian blur to the input image.
+                // The passed parameter defines the value of the standard deviation to the Gaussian function.
+                Gfx::StackBlurFilter filter { target_bitmap };
+                filter.process_rgba(blur.resolved_radius(node), Color::Transparent);
+            },
+            [&](CSS::Filter::Color const& color) {
+                auto amount = color.resolved_amount();
+                auto amount_clamped = clamp(amount, 0.0f, 1.0f);
+                auto apply_color_filter = [&](Gfx::ColorFilter const& filter) {
+                    const_cast<Gfx::ColorFilter&>(filter).apply(target_bitmap, target_bitmap.rect(), target_bitmap, target_bitmap.rect());
+                };
+                switch (color.operation) {
+                case CSS::Filter::Color::Operation::Grayscale: {
+                    // Converts the input image to grayscale. The passed parameter defines the proportion of the conversion.
+                    // A value of 100% is completely grayscale. A value of 0% leaves the input unchanged.
+                    apply_color_filter(Gfx::GrayscaleFilter { amount_clamped });
+                    break;
+                }
+                case CSS::Filter::Color::Operation::Brightness: {
+                    // Applies a linear multiplier to input image, making it appear more or less bright.
+                    // A value of 0% will create an image that is completely black. A value of 100% leaves the input unchanged.
+                    // Values of amount over 100% are allowed, providing brighter results.
+                    apply_color_filter(Gfx::BrightnessFilter { amount });
+                    break;
+                }
+                case CSS::Filter::Color::Operation::Contrast: {
+                    // Adjusts the contrast of the input. A value of 0% will create an image that is completely gray.
+                    // A value of 100% leaves the input unchanged. Values of amount over 100% are allowed, providing results with more contrast.
+                    apply_color_filter(Gfx::ContrastFilter { amount });
+                    break;
+                }
+                case CSS::Filter::Color::Operation::Invert: {
+                    // Inverts the samples in the input image. The passed parameter defines the proportion of the conversion.
+                    // A value of 100% is completely inverted. A value of 0% leaves the input unchanged.
+                    apply_color_filter(Gfx::InvertFilter { amount_clamped });
+                    break;
+                }
+                case CSS::Filter::Color::Operation::Opacity: {
+                    // Applies transparency to the samples in the input image. The passed parameter defines the proportion of the conversion.
+                    // A value of 0% is completely transparent. A value of 100% leaves the input unchanged.
+                    apply_color_filter(Gfx::OpacityFilter { amount_clamped });
+                    break;
+                }
+                case CSS::Filter::Color::Operation::Sepia: {
+                    // Converts the input image to sepia. The passed parameter defines the proportion of the conversion.
+                    // A value of 100% is completely sepia. A value of 0% leaves the input unchanged.
+                    apply_color_filter(Gfx::SepiaFilter { amount_clamped });
+                    break;
+                }
+                case CSS::Filter::Color::Operation::Saturate: {
+                    dbgln("TODO: Implement saturate() filter function!");
+                    break;
+                }
+                default:
+                    break;
+                }
+            },
+            [&](CSS::Filter::HueRotate const&) {
+                dbgln("TODO: Implement hue-rotate() filter function!");
+            },
+            [&](CSS::Filter::DropShadow const&) {
+                dbgln("TODO: Implement drop-shadow() filter function!");
+            });
+    }
+}
+
+void apply_backdrop_filter(PaintContext& context, Layout::Node const& node, Gfx::FloatRect const& backdrop_rect, BorderRadiiData const& border_radii_data, CSS::BackdropFilter const& backdrop_filter)
+{
+    // This performs the backdrop filter operation: https://drafts.fxtf.org/filter-effects-2/#backdrop-filter-operation
+
+    auto backdrop_region = backdrop_rect.to_rounded<int>();
+
+    // Note: The region bitmap can be smaller than the backdrop_region if it's at the edge of canvas.
+    Gfx::IntRect actual_region {};
+
+    // FIXME: Go through the steps to find the "Backdrop Root Image"
+    // https://drafts.fxtf.org/filter-effects-2/#BackdropRoot
+
+    // 1. Copy the Backdrop Root Image into a temporary buffer, such as a raster image. Call this buffer T’.
+    auto maybe_backdrop_bitmap = context.painter().get_region_bitmap(backdrop_region, Gfx::BitmapFormat::BGRA8888, actual_region);
+    if (actual_region.is_empty())
+        return;
+    if (maybe_backdrop_bitmap.is_error()) {
+        dbgln("Failed get region bitmap for backdrop-filter");
+        return;
+    }
+    auto backdrop_bitmap = maybe_backdrop_bitmap.release_value();
+    // 2. Apply the backdrop-filter’s filter operations to the entire contents of T'.
+    apply_filter_list(*backdrop_bitmap, node, backdrop_filter.filters());
+
+    // FIXME: 3. If element B has any transforms (between B and the Backdrop Root), apply the inverse of those transforms to the contents of T’.
+
+    // 4. Apply a clip to the contents of T’, using the border box of element B, including border-radius if specified. Note that the children of B are not considered for the sizing or location of this clip.
+    ScopedCornerRadiusClip corner_clipper { context.painter(), backdrop_region, border_radii_data };
+
+    // FIXME: 5. Draw all of element B, including its background, border, and any children elements, into T’.
+
+    // FXIME: 6. If element B has any transforms, effects, or clips, apply those to T’.
+
+    // 7. Composite the contents of T’ into element B’s parent, using source-over compositing.
+    context.painter().blit(actual_region.location(), *backdrop_bitmap, backdrop_bitmap->rect());
+}
+
+}
diff --git a/Userland/Libraries/LibWeb/Painting/FilterPainting.h b/Userland/Libraries/LibWeb/Painting/FilterPainting.h
new file mode 100644
index 0000000000..d4b3e693b6
--- /dev/null
+++ b/Userland/Libraries/LibWeb/Painting/FilterPainting.h
@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2022, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGfx/Forward.h>
+#include <LibWeb/CSS/BackdropFilter.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::Painting {
+
+void apply_filter_list(Gfx::Bitmap& target_bitmap, Layout::Node const& node, Span<CSS::FilterFunction const> filter_list);
+
+void apply_backdrop_filter(PaintContext&, Layout::Node const&, Gfx::FloatRect const&, BorderRadiiData const&, CSS::BackdropFilter const&);
+
+}
diff --git a/Userland/Libraries/LibWeb/Painting/PaintableBox.cpp b/Userland/Libraries/LibWeb/Painting/PaintableBox.cpp
index 97add1cf2f..58d6e397f7 100644
--- a/Userland/Libraries/LibWeb/Painting/PaintableBox.cpp
+++ b/Userland/Libraries/LibWeb/Painting/PaintableBox.cpp
@@ -11,6 +11,7 @@
 #include <LibWeb/Layout/BlockContainer.h>
 #include <LibWeb/Layout/InitialContainingBlock.h>
 #include <LibWeb/Painting/BackgroundPainting.h>
+#include <LibWeb/Painting/FilterPainting.h>
 #include <LibWeb/Painting/PaintableBox.h>
 #include <LibWeb/Painting/ShadowPainting.h>
 #include <LibWeb/Painting/StackingContext.h>
@@ -122,6 +123,7 @@ void PaintableBox::paint(PaintContext& context, PaintPhase phase) const
             auto border_box = absolute_border_box_rect();
             context.painter().add_clip_rect(clip_rect.to_rect().resolved(Paintable::layout_node(), border_box).to_rounded<int>());
         }
+        paint_backdrop_filter(context);
         paint_background(context);
         paint_box_shadow(context);
     }
@@ -193,6 +195,13 @@ void PaintableBox::paint_border(PaintContext& context) const
     paint_all_borders(context, absolute_border_box_rect(), normalized_border_radii_data(), borders_data);
 }
 
+void PaintableBox::paint_backdrop_filter(PaintContext& context) const
+{
+    auto& backdrop_filter = computed_values().backdrop_filter();
+    if (!backdrop_filter.is_none())
+        Painting::apply_backdrop_filter(context, layout_node(), absolute_border_box_rect(), normalized_border_radii_data(), backdrop_filter);
+}
+
 void PaintableBox::paint_background(PaintContext& context) const
 {
     // If the body's background properties were propagated to the root element, do no re-paint the body's background.
diff --git a/Userland/Libraries/LibWeb/Painting/PaintableBox.h b/Userland/Libraries/LibWeb/Painting/PaintableBox.h
index ccd917c4b1..e19c476173 100644
--- a/Userland/Libraries/LibWeb/Painting/PaintableBox.h
+++ b/Userland/Libraries/LibWeb/Painting/PaintableBox.h
@@ -118,6 +118,7 @@ protected:
     explicit PaintableBox(Layout::Box const&);
 
     virtual void paint_border(PaintContext&) const;
+    virtual void paint_backdrop_filter(PaintContext&) const;
     virtual void paint_background(PaintContext&) const;
     virtual void paint_box_shadow(PaintContext&) const;