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

[Laurent Pinchart]

Hi Kieran,

On Mon, Mar 23, 2020 at 05:23:06PM +0000, Kieran Bingham wrote:
> 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?)

See the last patch in the series :-)

> > +		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?

We should get it from the buffer, but we don't support strides yet. I'll
add a todo.

> > +			std::swap(buffer, buffer_);
> 
> Do we need to release buffer_ on stop at all?

Yes. It's in the next patch, I'll move it here.

> > +		} 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,

Laurent Pinchart