3 files changed, 195 insertions, 0 deletions
diff --git a/include/wlr/types/wlr_damage_ring.h b/include/wlr/types/wlr_damage_ring.h
new file mode 100644
index 00000000..9415a556
--- /dev/null
+++ b/include/wlr/types/wlr_damage_ring.h
@@ -0,0 +1,81 @@
+/*
+ * This an unstable interface of wlroots. No guarantees are made regarding the
+ * future consistency of this API.
+ */
+#ifndef WLR_USE_UNSTABLE
+#error "Add -DWLR_USE_UNSTABLE to enable unstable wlroots features"
+#endif
+
+#ifndef WLR_TYPES_WLR_DAMAGE_RING_H
+#define WLR_TYPES_WLR_DAMAGE_RING_H
+
+#include <stdbool.h>
+
+/* For triple buffering, a history of two frames is required. */
+#define WLR_DAMAGE_RING_PREVIOUS_LEN 2
+
+struct wlr_box;
+
+struct wlr_damage_ring {
+	int32_t width, height;
+
+	// Difference between the current buffer and the previous one
+	pixman_region32_t current;
+
+	// private state
+
+	pixman_region32_t previous[WLR_DAMAGE_RING_PREVIOUS_LEN];
+	size_t previous_idx;
+};
+
+void wlr_damage_ring_init(struct wlr_damage_ring *ring);
+
+void wlr_damage_ring_finish(struct wlr_damage_ring *ring);
+
+/**
+ * Set ring bounds and damage the ring fully.
+ *
+ * Next time damage will be added, it will be cropped to the ring bounds.
+ * If at least one of the dimensions is 0, bounds are removed.
+ *
+ * By default, a damage ring doesn't have bounds.
+ */
+void wlr_damage_ring_set_bounds(struct wlr_damage_ring *ring,
+	int32_t width, int32_t height);
+
+/**
+ * Add a region to the current damage.
+ *
+ * Returns true if the region intersects the ring bounds, false otherwise.
+ */
+bool wlr_damage_ring_add(struct wlr_damage_ring *ring,
+	pixman_region32_t *damage);
+
+/**
+ * Add a box to the current damage.
+ *
+ * Returns true if the box intersects the ring bounds, false otherwise.
+ */
+bool wlr_damage_ring_add_box(struct wlr_damage_ring *ring,
+	const struct wlr_box *box);
+
+/**
+ * Damage the ring fully.
+ */
+void wlr_damage_ring_add_whole(struct wlr_damage_ring *ring);
+
+/**
+ * Rotate the damage ring. This needs to be called after using the accumulated
+ * damage, e.g. after rendering to an output's back buffer.
+ */
+void wlr_damage_ring_rotate(struct wlr_damage_ring *ring);
+
+/**
+ * Get accumulated damage, which is the difference between the current buffer
+ * and the buffer with age of buffer_age; in context of rendering, this is
+ * the region that needs to be redrawn.
+ */
+void wlr_damage_ring_get_buffer_damage(struct wlr_damage_ring *ring,
+	int buffer_age, pixman_region32_t *damage);
+
+#endif
diff --git a/types/meson.build b/types/meson.build
index 16870b83..53bf852b 100644
--- a/types/meson.build
+++ b/types/meson.build
@@ -30,6 +30,7 @@ wlr_files += files(
 	'wlr_buffer.c',
 	'wlr_compositor.c',
 	'wlr_cursor.c',
+	'wlr_damage_ring.c',
 	'wlr_data_control_v1.c',
 	'wlr_drm.c',
 	'wlr_export_dmabuf_v1.c',
diff --git a/types/wlr_damage_ring.c b/types/wlr_damage_ring.c
new file mode 100644
index 00000000..378051c7
--- /dev/null
+++ b/types/wlr_damage_ring.c
@@ -0,0 +1,113 @@
+#include <limits.h>
+#include <stdlib.h>
+#include <string.h>
+#include <pixman.h>
+#include <wlr/types/wlr_damage_ring.h>
+#include <wlr/util/box.h>
+#include "util/signal.h"
+
+#define WLR_DAMAGE_RING_MAX_RECTS 20
+
+void wlr_damage_ring_init(struct wlr_damage_ring *ring) {
+	memset(ring, 0, sizeof(*ring));
+
+	ring->width = INT_MAX;
+	ring->height = INT_MAX;
+
+	pixman_region32_init(&ring->current);
+	for (size_t i = 0; i < WLR_DAMAGE_RING_PREVIOUS_LEN; ++i) {
+		pixman_region32_init(&ring->previous[i]);
+	}
+}
+
+void wlr_damage_ring_finish(struct wlr_damage_ring *ring) {
+	pixman_region32_fini(&ring->current);
+	for (size_t i = 0; i < WLR_DAMAGE_RING_PREVIOUS_LEN; ++i) {
+		pixman_region32_fini(&ring->previous[i]);
+	}
+}
+
+void wlr_damage_ring_set_bounds(struct wlr_damage_ring *ring,
+		int32_t width, int32_t height) {
+	if (width == 0 || height == 0) {
+		ring->width = INT_MAX;
+		ring->height = INT_MAX;
+	} else {
+		ring->width = width;
+		ring->height = height;
+	}
+	wlr_damage_ring_add_whole(ring);
+}
+
+bool wlr_damage_ring_add(struct wlr_damage_ring *ring,
+		pixman_region32_t *damage) {
+	pixman_region32_t clipped;
+	pixman_region32_init(&clipped);
+	pixman_region32_intersect_rect(&clipped, damage,
+		0, 0, ring->width, ring->height);
+	bool intersects = pixman_region32_not_empty(&clipped);
+	if (intersects) {
+		pixman_region32_union(&ring->current, &ring->current, &clipped);
+	}
+	pixman_region32_fini(&clipped);
+	return intersects;
+}
+
+bool wlr_damage_ring_add_box(struct wlr_damage_ring *ring,
+		const struct wlr_box *box) {
+	struct wlr_box clipped = {
+		.x = 0,
+		.y = 0,
+		.width = ring->width,
+		.height = ring->height,
+	};
+	if (wlr_box_intersection(&clipped, &clipped, box)) {
+		pixman_region32_union_rect(&ring->current,
+			&ring->current, clipped.x, clipped.y,
+			clipped.width, clipped.height);
+		return true;
+	}
+	return false;
+}
+
+void wlr_damage_ring_add_whole(struct wlr_damage_ring *ring) {
+	pixman_region32_union_rect(&ring->current,
+		&ring->current, 0, 0, ring->width, ring->height);
+}
+
+void wlr_damage_ring_rotate(struct wlr_damage_ring *ring) {
+	// modular decrement
+	ring->previous_idx = ring->previous_idx +
+		WLR_DAMAGE_RING_PREVIOUS_LEN - 1;
+	ring->previous_idx %= WLR_DAMAGE_RING_PREVIOUS_LEN;
+
+	pixman_region32_copy(&ring->previous[ring->previous_idx], &ring->current);
+	pixman_region32_clear(&ring->current);
+}
+
+void wlr_damage_ring_get_buffer_damage(struct wlr_damage_ring *ring,
+		int buffer_age, pixman_region32_t *damage) {
+	if (buffer_age <= 0 || buffer_age - 1 > WLR_DAMAGE_RING_PREVIOUS_LEN) {
+		pixman_region32_clear(damage);
+		pixman_region32_union_rect(damage, damage,
+			0, 0, ring->width, ring->height);
+	} else {
+		pixman_region32_copy(damage, &ring->current);
+
+		// Accumulate damage from old buffers
+		for (int i = 0; i < buffer_age - 1; ++i) {
+			int j = (ring->previous_idx + i) % WLR_DAMAGE_RING_PREVIOUS_LEN;
+			pixman_region32_union(damage, damage, &ring->previous[j]);
+		}
+
+		// Check the number of rectangles
+		int n_rects = pixman_region32_n_rects(damage);
+		if (n_rects > WLR_DAMAGE_RING_MAX_RECTS) {
+			pixman_box32_t *extents = pixman_region32_extents(damage);
+			pixman_region32_union_rect(damage, damage,
+				extents->x1, extents->y1,
+				extents->x2 - extents->x1,
+				extents->y2 - extents->y1);
+		}
+	}
+}