[libcamera-devel] [PATCH 19/21] qcam: viewfinder: Avoid memory copy when conversion isn't needed

[Kieran Bingham]

Hi Laurent,

On 23/03/2020 14:22, Laurent Pinchart wrote:
> If the frame buffer format is identical to the display format, the
> viewfinder still invokes the converter to perform what is essentially a
> slow memcpy(). Make is possible to skip that operation by creating a

s/is/it/

> QImage referencing the buffer memory instead. A reference to the frame
> buffer is kept internally, and released when the next buffer is queued,
> pushing the current one out.
> 
> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

Some questions ... but nothing blocking I don't think

Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

> ---
>  src/qcam/viewfinder.cpp | 59 +++++++++++++++++++++++++++++------------
>  src/qcam/viewfinder.h   |  1 +
>  2 files changed, 43 insertions(+), 17 deletions(-)
> 
> diff --git a/src/qcam/viewfinder.cpp b/src/qcam/viewfinder.cpp
> index 31b358da47dc..4c35659e24aa 100644
> --- a/src/qcam/viewfinder.cpp
> +++ b/src/qcam/viewfinder.cpp
> @@ -7,6 +7,8 @@
>  
>  #include "viewfinder.h"
>  
> +#include <utility>
> +
>  #include <QImage>
>  #include <QImageWriter>
>  #include <QMutexLocker>
> @@ -16,7 +18,7 @@
>  #include "format_converter.h"
>  
>  ViewFinder::ViewFinder(QWidget *parent)
> -	: QWidget(parent)
> +	: QWidget(parent), buffer_(nullptr)
>  {
>  }
>  
> @@ -27,17 +29,23 @@ ViewFinder::~ViewFinder()
>  int ViewFinder::setFormat(const libcamera::PixelFormat &format,
>  			  const QSize &size)
>  {
> -	int ret;
> +	image_ = QImage();
> +
> +	/*
> +	 * If format conversion is needed, configure the converter and allocate
> +	 * the destination image.
> +	 */
> +	if (format != DRM_FORMAT_ARGB8888) {

Is this the only format which doesn't require converting? Can Qt support
any other formats for direct render? (I'm assuming things like a
compatible format which doesn't have alpha blending or such, but that
alpha is ignored perhaps?)

> +		int ret = converter_.configure(format, size);
> +		if (ret < 0)
> +			return ret;
>  
> -	ret = converter_.configure(format, size);
> -	if (ret < 0)
> -		return ret;
> +		image_ = QImage(size, QImage::Format_RGB32);
> +	}
>  
>  	format_ = format;
>  	size_ = size;
>  
> -	image_ = QImage(size_, QImage::Format_RGB32);
> -
>  	updateGeometry();
>  	return 0;
>  }
> @@ -49,19 +57,36 @@ void ViewFinder::render(libcamera::FrameBuffer *buffer, MappedBuffer *map)
>  		return;
>  	}
>  
> -	QMutexLocker locker(&mutex_);
> -
> -	/*
> -	 * \todo We're not supposed to block the pipeline handler thread
> -	 * for long, implement a better way to save images without
> -	 * impacting performances.
> -	 */
> +	unsigned char *memory = static_cast<unsigned char *>(map->memory);
> +	size_t size = buffer->metadata().planes[0].bytesused;
> +
> +	{
> +		QMutexLocker locker(&mutex_);
> +
> +		if (format_ == DRM_FORMAT_ARGB8888) {
> +			/*
> +			 * If the frame format is identical to the display
> +			 * format, create a QImage that references the frame
> +			 * and store a reference to the frame buffer. The
> +			 * previously stored frame buffer, if any, will be
> +			 * released.
> +			 */
> +			image_ = QImage(memory, size_.width(), size_.height(),
> +					size / size_.height(), QImage::Format_RGB32);

Is that stride 'guaranteed'?
Or should we get it from somewhere else?

If we can't get this from the buffer - do we need to add a todo note for
the future?

> +			std::swap(buffer, buffer_);


Do we need to release buffer_ on stop at all?


> +		} else {
> +			/*
> +			 * Otherwise, convert the format and release the frame
> +			 * buffer immediately.
> +			 */
> +			converter_.convert(memory, size, &image_);
> +		}
> +	}
>  
> -	converter_.convert(static_cast<unsigned char *>(map->memory),
> -			   buffer->metadata().planes[0].bytesused, &image_);
>  	update();
>  
> -	renderComplete(buffer);
> +	if (buffer)
> +		renderComplete(buffer);
>  }
>  
>  QImage ViewFinder::getCurrentImage()
> diff --git a/src/qcam/viewfinder.h b/src/qcam/viewfinder.h
> index 54c0fa9dd7a0..b5153160f70e 100644
> --- a/src/qcam/viewfinder.h
> +++ b/src/qcam/viewfinder.h
> @@ -52,6 +52,7 @@ private:
>  	libcamera::PixelFormat format_;
>  	QSize size_;
>  
> +	libcamera::FrameBuffer *buffer_;
>  	QImage image_;
>  	QMutex mutex_; /* Prevent concurrent access to image_ */
>  };
> 

-- 
Regards
--
Kieran