frontend/wayland: wp_viewporter support (#3445)

Accidentally all squashed together into a single mega-commit, here's
`wp_viewporter`.

You can test with the `weston-scaler` client in... weston, and also with
canonical/wlcs#340

include/core/mir/geometry/dimensions.h

@@ -200,6 +200,12 @@ template<typename T, typename Scalar>
 inline constexpr DeltaX<T> operator/(DeltaX<T> const& dx, Scalar scale) { return DeltaX<T>{dx.as_value() / scale}; }
 template<typename T, typename Scalar>
 inline constexpr DeltaY<T> operator/(DeltaY<T> const& dy, Scalar scale) { return DeltaY<T>{dy.as_value() / scale}; }
+
+// Can divide a coördinate by a distance to get a scalar
+template<typename T, typename U>
+inline constexpr auto operator/(X<T> const& x, Width<U> const& width) { return x.as_value() / width.as_value(); }
+template<typename T, typename U>
+inline constexpr auto operator/(Y<T> const& y, Height<U> const& height) { return y.as_value() / height.as_value(); }
 } // namespace
 
 // Converting between types is fine, as long as they are along the same axis

include/core/mir/geometry/size.h

@@ -20,6 +20,9 @@
 #include "forward.h"
 #include "dimensions.h"
 #include <ostream>
+#include <limits>
+#include <concepts>
+#include <type_traits>
 
 namespace mir
 {
@@ -32,6 +35,13 @@ struct Point;
 template<typename T>
 struct Displacement;
 
+template<typename T, typename U>
+concept is_exactly_representable = requires
+{
+    typename std::enable_if<std::numeric_limits<T>::digits >= std::numeric_limits<U>::digits, U>::type;
+    requires std::floating_point<T>;
+};
+
 template<typename T>
 struct Size
 {
@@ -41,6 +51,14 @@ struct Size
     constexpr Size(Size const&) noexcept = default;
     Size& operator=(Size const&) noexcept = default;
 
+    template<typename U>
+        requires is_exactly_representable<T, U>
+    constexpr Size(Size<U> const& other) noexcept
+        : width{Width<T>{other.width}},
+          height{Height<T>{other.height}}
+    {
+    }
+
     template<typename U>
     explicit constexpr Size(Size<U> const& other) noexcept
         : width{Width<T>{other.width}},

include/platform/mir/graphics/renderable.h

@@ -51,6 +51,13 @@ class Renderable
     virtual std::shared_ptr<Buffer> buffer() const = 0;
 
     virtual geometry::Rectangle screen_position() const = 0;
+    /**
+     * The region of \ref buffer that should be sampled from.
+     *
+     * This is in buffer coordinates, so {{0, 0}, {buffer->size().width, buffer->size().height} means
+     * “use the whole buffer”.
+     */
+    virtual geometry::RectangleD src_bounds() const = 0;
     virtual std::optional<geometry::Rectangle> clip_area() const = 0;
 
     // These are from the old CompositingCriteria. There is a little bit

src/gl/tessellation_helpers.cpp

@@ -20,26 +20,51 @@
 namespace mg = mir::graphics;
 namespace mgl = mir::gl;
 namespace geom = mir::geometry;
+
+namespace
+{
+struct SrcTexCoords
+{
+    GLfloat top;
+    GLfloat bottom;
+    GLfloat left;
+    GLfloat right;
+};
+
+auto tex_coords_from_rect(geom::Size buffer_size, geom::RectangleD sample_rect) -> SrcTexCoords
+{
+    /* GL Texture coordinates are normalised to the size of the buffer, so (0.0, 0.0) is the top-left
+     * and (1.0, 1.0) is the bottom-right
+     */
+    SrcTexCoords coords;
+    coords.top = sample_rect.top() / buffer_size.height;
+    coords.bottom = sample_rect.bottom() / buffer_size.height;
+    coords.left = sample_rect.left() / buffer_size.width;
+    coords.right = sample_rect.right() / buffer_size.width;
+    return coords;
+}
+}
+
 mgl::Primitive mgl::tessellate_renderable_into_rectangle(
     mg::Renderable const& renderable, geom::Displacement const& offset)
 {
     auto rect = renderable.screen_position();
     rect.top_left = rect.top_left - offset;
-    GLfloat left = rect.top_left.x.as_int();
-    GLfloat right = left + rect.size.width.as_int();
-    GLfloat top = rect.top_left.y.as_int();
-    GLfloat bottom = top + rect.size.height.as_int();
+    GLfloat const left = rect.top_left.x.as_int();
+    GLfloat const right = left + rect.size.width.as_int();
+    GLfloat const top = rect.top_left.y.as_int();
+    GLfloat const bottom = top + rect.size.height.as_int();
 
     mgl::Primitive rectangle;
     rectangle.type = GL_TRIANGLE_STRIP;
 
-    GLfloat const tex_right = 1.0f;
-    GLfloat const tex_bottom = 1.0f;
+    auto const [tex_top, tex_bottom, tex_left, tex_right] =
+        tex_coords_from_rect(renderable.buffer()->size(), renderable.src_bounds());
 
     auto& vertices = rectangle.vertices;
-    vertices[0] = {{left,  top,    0.0f}, {0.0f,      0.0f}};
-    vertices[1] = {{left,  bottom, 0.0f}, {0.0f,      tex_bottom}};
-    vertices[2] = {{right, top,    0.0f}, {tex_right, 0.0f}};
+    vertices[0] = {{left,  top,    0.0f}, {tex_left, tex_top}};
+    vertices[1] = {{left,  bottom, 0.0f}, {tex_left, tex_bottom}};
+    vertices[2] = {{right, top,    0.0f}, {tex_right, tex_top}};
     vertices[3] = {{right, bottom, 0.0f}, {tex_right, tex_bottom}};
     return rectangle;
 }

src/server/frontend_wayland/CMakeLists.txt

@@ -63,6 +63,7 @@ set(
   ${PROJECT_SOURCE_DIR}/src/include/server/mir/frontend/pointer_input_dispatcher.h
   session_credentials.cpp
   ${PROJECT_SOURCE_DIR}/src/include/server/mir/frontend/buffer_stream.h
+  wp_viewporter.cpp             wp_viewporter.h
 )
 
 add_custom_command(

src/server/frontend_wayland/wayland_connector.cpp

@@ -28,6 +28,7 @@
 #include "wayland_executor.h"
 #include "desktop_file_manager.h"
 #include "foreign_toplevel_manager_v1.h"
+#include "wp_viewporter.h"
 
 #include "mir/main_loop.h"
 #include "mir/thread_name.h"
@@ -331,6 +332,8 @@ mf::WaylandConnector::WaylandConnector(
 
     shm_global = std::make_unique<WlShm>(display.get(), executor);
 
+    viewporter = std::make_unique<WpViewporter>(display.get());
+
     char const* wayland_display = nullptr;
 
     if (auto const display_name = getenv("WAYLAND_DISPLAY"))

src/server/frontend_wayland/wayland_connector.h

@@ -85,6 +85,7 @@ class WlSeat;
 class WlShm;
 class WlSubcompositor;
 class WlSurface;
+class WpViewporter;
 class DesktopFileManager;
 
 class WaylandExtensions
@@ -210,6 +211,7 @@ class WaylandConnector : public Connector
     std::shared_ptr<DesktopFileManager> desktop_file_manager;
     std::unique_ptr<WlDataDeviceManager> data_device_manager_global;
     std::unique_ptr<WlShm> shm_global;
+    std::unique_ptr<WpViewporter> viewporter;
     std::shared_ptr<Executor> const executor;
     std::shared_ptr<graphics::GraphicBufferAllocator> const allocator;
     std::shared_ptr<shell::Shell> const shell;

src/server/frontend_wayland/wl_surface.cpp

@@ -16,7 +16,7 @@
 
 #include "wl_surface.h"
 
-#include "mir/geometry/forward.h"
+#include "viewporter_wrapper.h"
 #include "wayland_utils.h"
 #include "wl_surface_role.h"
 #include "wl_subcompositor.h"
@@ -39,9 +39,12 @@
 #include "mir/scene/surface.h"
 #include "mir/shell/surface_specification.h"
 #include "mir/log.h"
+#include "wp_viewporter.h"
 
 #include <chrono>
 #include <boost/throw_exception.hpp>
+#include <cmath>
+#include <limits>
 #include <wayland-server-protocol.h>
 
 namespace mf = mir::frontend;
@@ -72,6 +75,9 @@ void mf::WlSurfaceState::update_from(WlSurfaceState const& source)
                            begin(source.frame_callbacks),
                            end(source.frame_callbacks));
 
+    if (source.viewport)
+        viewport = source.viewport;
+
     if (source.surface_data_invalidated)
         surface_data_invalidated = true;
 }
@@ -331,7 +337,19 @@ void mf::WlSurface::commit(WlSurfaceState const& state)
         input_shape = state.input_shape.value();
 
     if (state.scale)
-        inv_scale = 1.0f / state.scale.value();
+        scale = state.scale.value();
+
+    if (state.viewport)
+    {
+        viewport = std::move(state.viewport);
+    }
+
+    bool needs_buffer_submission =
+        state.scale ||                                               // If the scale has changed, or...
+        state.viewport ||                                            // ...we've added a viewport, or...
+        (viewport && viewport.value().changed_since_last_resolve()); // ...the viewport has changed...
+                                                                     // ...then we'll need to submit a new frame, even if the client hasn't
+                                                                     // attached a new buffer.
 
     auto const executor_send_frame_callbacks = [executor = wayland_executor, weak_self = mw::make_weak(this)]()
         {
@@ -366,49 +384,65 @@ void mf::WlSurface::commit(WlSurfaceState const& state)
                             }
                         });
                 };
-            std::shared_ptr<graphics::Buffer> mir_buffer;
 
             if (auto const shm_buffer = ShmBuffer::from(weak_buffer.value()))
             {
-                mir_buffer = allocator->buffer_from_shm(
+                current_buffer = allocator->buffer_from_shm(
                     shm_buffer->data(),
                     std::move(executor_send_frame_callbacks),
                     std::move(release_buffer));
                 tracepoint(
                     mir_server_wayland,
                     sw_buffer_committed,
                     wl_resource_get_client(resource),
-                    mir_buffer->id().as_value());
+                    current_buffer->id().as_value());
             }
             else
             {
-                mir_buffer = allocator->buffer_from_resource(
+                current_buffer = allocator->buffer_from_resource(
                     weak_buffer.value(),
                     std::move(executor_send_frame_callbacks),
                     std::move(release_buffer));
                 tracepoint(
                     mir_server_wayland,
                     hw_buffer_committed,
                     wl_resource_get_client(resource),
-                    mir_buffer->id().as_value());
-            }
-
-            stream->submit_buffer(mir_buffer, mir_buffer->size() * inv_scale, {{0, 0}, geom::SizeD{mir_buffer->size()}});
-            auto const new_buffer_size = mir_buffer->size() * inv_scale;
-
-            if (std::make_optional(new_buffer_size) != buffer_size_)
-            {
-                state.invalidate_surface_data(); // input shape needs to be recalculated for the new size
+                    current_buffer->id().as_value());
             }
 
-            buffer_size_ = new_buffer_size;
+            needs_buffer_submission = true;
         }
     }
     else
     {
         frame_callback_executor->spawn(std::move(executor_send_frame_callbacks));
     }
 
+    if (needs_buffer_submission)
+    {
+        geom::Size logical_size;
+        geom::RectangleD src_sample;
+
+        if (viewport)
+        {
+            std::tie(src_sample, logical_size) = viewport.value().resolve_viewport(scale, current_buffer->size());
+        }
+        else
+        {
+            src_sample = geom::RectangleD{{0, 0}, current_buffer->size()};
+            logical_size = current_buffer->size() / scale;
+        }
+
+        stream->submit_buffer(current_buffer, logical_size, src_sample);
+
+        if (std::make_optional(logical_size) != buffer_size_)
+        {
+            state.invalidate_surface_data(); // input shape needs to be recalculated for the new size
+        }
+
+        buffer_size_ = logical_size;
+    }
+
     for (WlSubsurface* child: children)
     {
         child->parent_has_committed();
@@ -468,6 +502,15 @@ auto mf::WlSurface::confine_pointer_state() const -> MirPointerConfinementState
     return mir_pointer_unconfined;
 }
 
+void mf::WlSurface::associate_viewport(wayland::Weak<Viewport> viewport)
+{
+    if (this->viewport || pending.viewport)
+    {
+        BOOST_THROW_EXCEPTION((std::logic_error{"Cannot associate a viewport to a window with an existing viewport"}));
+    }
+    pending.viewport = viewport;
+}
+
 void mir::frontend::WlSurface::update_surface_spec(shell::SurfaceSpecification const& spec)
 {
     pending.surface_spec.update_from(spec);

src/server/frontend_wayland/wl_surface.h

@@ -39,6 +39,7 @@ class Executor;
 namespace graphics
 {
 class GraphicBufferAllocator;
+class Buffer;
 }
 namespace scene
 {
@@ -57,6 +58,7 @@ namespace frontend
 class WlSurface;
 class WlSubsurface;
 class ResourceLifetimeTracker;
+class Viewport;
 
 struct WlSurfaceState
 {
@@ -83,6 +85,7 @@ struct WlSurfaceState
     std::optional<geometry::Displacement> offset;
     std::optional<std::optional<std::vector<geometry::Rectangle>>> input_shape;
     std::vector<wayland::Weak<Callback>> frame_callbacks;
+    wayland::Weak<Viewport> viewport;
 
 private:
     // only set to true if invalidate_surface_data() is called
@@ -142,6 +145,13 @@ class WlSurface : public wayland::Surface
     void commit(WlSurfaceState const& state);
     auto confine_pointer_state() const -> MirPointerConfinementState;
 
+    /**
+     * Associate a viewport (buffer scale & crop metadata) with this surface
+     *
+     * \throws A std::logic_error if the surface already has a viewport associated
+     */
+    void associate_viewport(wayland::Weak<Viewport> viewport);
+
     std::shared_ptr<scene::Session> const session;
     std::shared_ptr<compositor::BufferStream> const stream;
 
@@ -155,14 +165,24 @@ class WlSurface : public wayland::Surface
     NullWlSurfaceRole null_role;
     WlSurfaceRole* role;
     std::vector<WlSubsurface*> children; // ordering is from bottom to top
+    /* We might need to resubmit the current buffer, but with different metadata
+     * For example: if a client commits a wl_surface.buffer_scale() without attaching
+     * a new buffer, we need to generate a new frame with the same content, but at the
+     * new scale.
+     *
+     * To simplify other interfaces, keep a reference to the current content so we can
+     * hide this, here, in the frontend.
+     */
+    std::shared_ptr<graphics::Buffer> current_buffer;
 
     WlSurfaceState pending;
     geometry::Displacement offset_;
-    float inv_scale{1.0f};
+    int32_t scale{1};
     std::optional<geometry::Size> buffer_size_;
     std::vector<wayland::Weak<WlSurfaceState::Callback>> frame_callbacks;
     std::optional<std::vector<mir::geometry::Rectangle>> input_shape;
     std::vector<SceneSurfaceCreatedCallback> scene_surface_created_callbacks;
+    wayland::Weak<Viewport> viewport;
 
     void send_frame_callbacks();