[igt-dev] [PATCH i-g-t v4 18/21] lib/igt_frame: Add a checkerboard frame comparison method

[Paul Kocialkowski]

This introduces a new frame comparison method that was designed for
patterns that follow a checkerboard style. These patterns are made of
consecutive rectangular shapes with alternating solid colors. They are
currently used for some Chamelium-based tests.

The method is particularly adapted for cases where the edges of the
shapes might be blurred (e.g. due to scaling), which makes it impossible
to use pixel-perfect or CRC-based comparisons to decide whether the
captured frame matches the reference.

Overall, this test will first detect the edges of the pattern and later
exclude them from comparison. Colors are compared between the reference
and capture with a low threshold for error. A percentage of the faulty
pixels is calculated and the captured frame is considered invalid if
more than one percent of the pixels are erroneous.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski at bootlin.com>
Reviewed-by: Lyude Paul <lyude at redhat.com>
---
 lib/igt_frame.c | 135 ++++++++++++++++++++++++++++++++++++++++++++++++
 lib/igt_frame.h |   2 +
 2 files changed, 137 insertions(+)

diff --git a/lib/igt_frame.c b/lib/igt_frame.c
index 6984c02e9912..45523a79f601 100644
--- a/lib/igt_frame.c
+++ b/lib/igt_frame.c
@@ -267,3 +267,138 @@ complete:
 
 	return match;
 }
+
+#define XR24_COLOR_VALUE(data, stride, x, y, c) \
+	*((uint8_t *)(data) + (y) * (stride) + 4 * (x) + (c))
+
+/**
+ * igt_check_checkerboard_frame_match:
+ * @reference: The reference cairo surface
+ * @capture: The captured cairo surface
+ *
+ * Checks that the reference frame matches the captured frame using a
+ * method designed for checkerboard patterns. These patterns are made of
+ * consecutive rectangular shapes with alternating solid colors.
+ *
+ * The intent of this method is to cover cases where the captured result is
+ * not pixel-perfect due to features such as scaling or YUV conversion and
+ * subsampling. Such effects are mostly noticeable on the edges of the
+ * patterns, so they are detected and excluded from the comparison.
+ *
+ * The algorithm works with two major steps. First, the edges of the reference
+ * pattern are detected on the x and y axis independently. The detection is done
+ * by calculating an absolute difference with a span of a few pixels before and
+ * after the current position on the given axis, accumulated for each color
+ * component. If the sum is above a given threshold on one of the axis, the
+ * position is marked as an edge. In the second step, the pixel values are
+ * compared per-component, excluding positions that were marked as edges or
+ * that are at a transition between an edge and a non-edge. An error threshold
+ * (for each individual color component) is used to mark the position as
+ * erroneous or not. The ratio of erroneous pixels over compared pixels (that
+ * does not count excluded pixels) is then calculated and compared to the error
+ * rate threshold to determine whether the frames match or not.
+ *
+ * Returns: a boolean indicating whether the frames match
+ */
+bool igt_check_checkerboard_frame_match(cairo_surface_t *reference,
+					cairo_surface_t *capture)
+{
+	unsigned int width, height, ref_stride, cap_stride;
+	void *ref_data, *cap_data;
+	unsigned char *edges_map;
+	unsigned int x, y, c;
+	unsigned int errors = 0, pixels = 0;
+	unsigned int edge_threshold = 100;
+	unsigned int color_error_threshold = 24;
+	double error_rate_threshold = 0.01;
+	double error_rate;
+	unsigned int span = 2;
+	bool match = false;
+
+	width = cairo_image_surface_get_width(reference);
+	height = cairo_image_surface_get_height(reference);
+
+	ref_stride = cairo_image_surface_get_stride(reference);
+	ref_data = cairo_image_surface_get_data(reference);
+	igt_assert(ref_data);
+
+	cap_stride = cairo_image_surface_get_stride(capture);
+	cap_data = cairo_image_surface_get_data(capture);
+	igt_assert(cap_data);
+
+	edges_map = calloc(1, width * height);
+	igt_assert(edges_map);
+
+	/* First pass to detect the pattern edges. */
+	for (y = 0; y < height; y++) {
+		if (y < span || y > (height - span - 1))
+			continue;
+
+		for (x = 0; x < width; x++) {
+			unsigned int xdiff = 0, ydiff = 0;
+
+			if (x < span || x > (width - span - 1))
+				continue;
+
+			for (c = 0; c < 3; c++) {
+				xdiff += abs(XR24_COLOR_VALUE(ref_data, ref_stride, x + span, y, c) -
+					     XR24_COLOR_VALUE(ref_data, ref_stride, x - span, y, c));
+				ydiff += abs(XR24_COLOR_VALUE(ref_data, ref_stride, x, y + span, c) -
+					     XR24_COLOR_VALUE(ref_data, ref_stride, x, y - span, c));
+			}
+
+			edges_map[y * width + x] = (xdiff > edge_threshold ||
+						    ydiff > edge_threshold);
+		}
+	}
+
+	/* Second pass to detect errors. */
+	for (y = 0; y < height; y++) {
+		for (x = 0; x < width; x++) {
+			bool error = false;
+
+			if (edges_map[y * width + x])
+				continue;
+
+			for (c = 0; c < 3; c++) {
+				unsigned int diff;
+
+				/* Compare the reference and capture values. */
+				diff = abs(XR24_COLOR_VALUE(ref_data, ref_stride, x, y, c) -
+					   XR24_COLOR_VALUE(cap_data, cap_stride, x, y, c));
+
+				if (diff > color_error_threshold)
+					error = true;
+			}
+
+			/* Allow error if coming on or off an edge (on x). */
+			if (error && x >= span && x <= (width - span - 1) &&
+			    edges_map[y * width + (x - span)] !=
+			    edges_map[y * width + (x + span)])
+				continue;
+
+			/* Allow error if coming on or off an edge (on y). */
+			if (error && y >= span && y <= (height - span - 1) &&
+			    edges_map[(y - span) * width + x] !=
+			    edges_map[(y + span) * width + x] && error)
+				continue;
+
+			if (error)
+				errors++;
+
+			pixels++;
+		}
+	}
+
+	free(edges_map);
+
+	error_rate = (double) errors / pixels;
+
+	if (error_rate < error_rate_threshold)
+		match = true;
+
+	igt_debug("Checkerboard pattern %s with error rate %f %%\n",
+		  match ? "matched" : "not matched", error_rate * 100);
+
+	return match;
+}
diff --git a/lib/igt_frame.h b/lib/igt_frame.h
index 11f96cbea203..f44f57d7ce73 100644
--- a/lib/igt_frame.h
+++ b/lib/igt_frame.h
@@ -38,5 +38,7 @@ void igt_write_compared_frames_to_png(cairo_surface_t *reference,
 				      const char *capture_suffix);
 bool igt_check_analog_frame_match(cairo_surface_t *reference,
 				  cairo_surface_t *capture);
+bool igt_check_checkerboard_frame_match(cairo_surface_t *reference,
+					cairo_surface_t *capture);
 
 #endif
-- 
2.20.1