[PATCH v1 3/3] libcamera: virtual: Speed up test pattern animation

[Barnabás Pőcze]

After the initial generation, when a frame is requested,
the test pattern generator rotates the image left by 1 column.

The current approach has two shortcomings:
  (1) it allocates a temporary buffer to hold one column;
  (2) it swaps two columns at a time.

The test patterns are simple ARGB images, in row-major order,
so doing (2) works against memory prefetching. This can be
addressed by doing the rotation one row at a time as that way
the image is addressed in a purely linear fashion. Doing so also
eliminates the need for a dynamically allocated temporary buffer,
as the required buffer now only needs to hold one sample,
which is 4 bytes in this case.

In an optimized build, this results in about a 2x increase in the
number of frames per second as reported by `cam`. In an unoptimized,
ASAN and UBSAN intrumented build, the difference is even bigger,
which is useful for running lc-compliance in CI in a reasonable time.

Signed-off-by: Barnabás Pőcze <pobrn at protonmail.com>
---
 .../virtual/test_pattern_generator.cpp        | 68 +++++++++----------
 .../pipeline/virtual/test_pattern_generator.h |  4 --
 2 files changed, 34 insertions(+), 38 deletions(-)

diff --git a/src/libcamera/pipeline/virtual/test_pattern_generator.cpp b/src/libcamera/pipeline/virtual/test_pattern_generator.cpp
index 47d341919..eeadc1646 100644
--- a/src/libcamera/pipeline/virtual/test_pattern_generator.cpp
+++ b/src/libcamera/pipeline/virtual/test_pattern_generator.cpp
@@ -5,6 +5,8 @@
  * Derived class of FrameGenerator for generating test patterns
  */
 
+#include <string.h>
+
 #include "test_pattern_generator.h"
 
 #include <libcamera/base/log.h>
@@ -13,6 +15,37 @@
 
 #include <libyuv/convert_from_argb.h>
 
+namespace {
+
+template<size_t SampleSize>
+void rotateLeft1Sample(uint8_t *samples, size_t width)
+{
+	if (width <= 0)
+		return;
+
+	const size_t stride = width * SampleSize;
+	uint8_t first[SampleSize];
+
+	memcpy(first, &samples[0], SampleSize);
+	for (size_t i = 0; i < stride - SampleSize; i += SampleSize)
+		memcpy(&samples[i], &samples[i + SampleSize], SampleSize);
+	memcpy(&samples[stride - SampleSize], first, SampleSize);
+}
+
+template<size_t SampleSize>
+void rotateLeft1Column(const libcamera::Size &size, uint8_t *image)
+{
+	if (size.width < 2)
+		return;
+
+	const size_t stride = size.width * SampleSize;
+
+	for (size_t i = 0; i < size.height; i++, image += stride)
+		rotateLeft1Sample<SampleSize>(image, size.width);
+}
+
+}
+
 namespace libcamera {
 
 LOG_DECLARE_CATEGORY(Virtual)
@@ -27,7 +60,7 @@ int TestPatternGenerator::generateFrame(const Size &size,
 
 	const auto &planes = mappedFrameBuffer.planes();
 
-	shiftLeft(size);
+	rotateLeft1Column<kARGBSize>(size, template_.get());
 
 	/* Convert the template_ to the frame buffer */
 	int ret = libyuv::ARGBToNV12(template_.get(), size.width * kARGBSize,
@@ -40,39 +73,6 @@ int TestPatternGenerator::generateFrame(const Size &size,
 	return ret;
 }
 
-void TestPatternGenerator::shiftLeft(const Size &size)
-{
-	/* Store the first column temporarily */
-	auto firstColumn = std::make_unique<uint8_t[]>(size.height * kARGBSize);
-	for (size_t h = 0; h < size.height; h++) {
-		unsigned int index = h * size.width * kARGBSize;
-		unsigned int index1 = h * kARGBSize;
-		firstColumn[index1] = template_[index];
-		firstColumn[index1 + 1] = template_[index + 1];
-		firstColumn[index1 + 2] = template_[index + 2];
-		firstColumn[index1 + 3] = 0x00;
-	}
-
-	/* Overwrite template_ */
-	uint8_t *buf = template_.get();
-	for (size_t h = 0; h < size.height; h++) {
-		for (size_t w = 0; w < size.width - 1; w++) {
-			/* Overwrite with the pixel on the right */
-			unsigned int index = (h * size.width + w + 1) * kARGBSize;
-			*buf++ = template_[index]; /* B */
-			*buf++ = template_[index + 1]; /* G */
-			*buf++ = template_[index + 2]; /* R */
-			*buf++ = 0x00; /* A */
-		}
-		/* Overwrite the new last column with the original first column */
-		unsigned int index1 = h * kARGBSize;
-		*buf++ = firstColumn[index1]; /* B */
-		*buf++ = firstColumn[index1 + 1]; /* G */
-		*buf++ = firstColumn[index1 + 2]; /* R */
-		*buf++ = 0x00; /* A */
-	}
-}
-
 void ColorBarsGenerator::configure(const Size &size)
 {
 	constexpr uint8_t kColorBar[8][3] = {
diff --git a/src/libcamera/pipeline/virtual/test_pattern_generator.h b/src/libcamera/pipeline/virtual/test_pattern_generator.h
index 05f4ab7a7..2a51bd31a 100644
--- a/src/libcamera/pipeline/virtual/test_pattern_generator.h
+++ b/src/libcamera/pipeline/virtual/test_pattern_generator.h
@@ -29,10 +29,6 @@ public:
 protected:
 	/* Buffer of test pattern template */
 	std::unique_ptr<uint8_t[]> template_;
-
-private:
-	/* Shift the buffer by 1 pixel left each frame */
-	void shiftLeft(const Size &size);
 };
 
 class ColorBarsGenerator : public TestPatternGenerator
-- 
2.47.1