[Interest] Convertion between QImage to QPixmap taking to much time.

Gunnar Sletta gunnar at sletta.org
Wed Oct 21 11:27:05 CEST 2015


You construct your QImage from a uchar* right? This constructor does not take a deep copy of the image data you pass in, it just stores it. That means that if you for some reason delete the source data before Qt takes it into use (which happens typically on the next paint cycle or in your custom graphics item), you will have a dangling pointer in the QImage you are drawing and that will typically crash.

cheers,
Gunnar

> On 20 Oct 2015, at 21:32, Celal SAVUR <c.savur at gmail.com> wrote:
> 
> Thank you Allan and Gunnar,
> 
> I have tried the some of your method but no luck. When I used Qt::NoFormatConversion flag. If I  do not enable the openGL for my graphic view, It does not draw a image and it just crash. but whenever I enable the openGL, it draw but speed is same, in addition to that sometimes, graphicview cannot draw all image, it just draw half of it and rest of image is black.
> 
> I try also try to implement my own GraphicsItem but when I call painter.drawimage(0,0, image); it crash. I could not figure out why It crash. With same code, if I change drawimage to drawline, it works fine.
> 
> 
> Do you think is a good to convert raw image data to a known format (XPM) on fly and load to Qpixmap ? If yes , do you have any suggestion to do that.
> 
> thank you in advance.
> 
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> Celal SAVUR
> 
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> 
> On Mon, Oct 19, 2015 at 7:09 AM, Gunnar Sletta <gunnar at sletta.org> wrote:
> 
> > On 19 Oct 2015, at 13:00, Gunnar Sletta <gunnar at sletta.org> wrote:
> >
> >
> >> On 17 Oct 2015, at 17:28, Celal SAVUR <c.savur at gmail.com> wrote:
> >>
> >> Hello everyone,
> >>
> >> I have some performance issue when I am using QPixmap.
> >>
> >> I am getting image from buffer and I create QImage. After that using QPixmap::fromImage(), creating pixmap  and set it to QGraphicPixmapItem to be drawn in scene.
> >>
> >> When I put the timer to see how much time Qt spending to create QPixmap, I realize that I converting  from Qimage to QPixmap taking ~20 times more time than crating Qimage from buffer.
> >
> > If you are using the QImage(uchar *data, int width, int height, ..) overload, this is surprising as creating the image from raw pixel data only has to allocate the QImage d-pointer and otherwise use the data as is.
> >
> > Converting that to a QPixmap will entail a QImage::convertToFormat() to map the image into a system specific pixel format, usually QImage::RGB32 or QImage::ARGB32_Premultiplied, but any given QPA backend may override that with an other format if that is more compatible with its raster backingstore.
> >
> >> By the way, I am getting image approximately every 200 ms some times faster than that.
> >>
> >> Do you have any suggestion to increase the performance?
> >> Can I draw QImage into scene  without Qpixmap convertion? How ?
> >
> > There are a couple of options.
> >
> > If you can provide the pixel data in a format compatible with the backingstore, the conversion can be simpler or just a no-op.
> >
> > You can also draw the image directly using QPainter::drawImage(). If you didn't set the QGraphicsView viewport to be an OpenGL enabled viewport, then the image will be converted to the right format on the fly. The actual drawing will then be a bit slower, but as you avoid the conversion, it should be a net win.
> >
> > You can do that by implementing your own QGraphicsItem and draw the image in the virtual paint() function.
> 
> Bah.. Completely forgot about the Qt::NoImageConversion flag that Allan mentioned. That saves you going through QPainter::drawImage() and a bit of implementation work :)
> 
> >
> > If you have a GL viewport, then the image needs to be uploaded to a GL texture. QPainter will do this and put the image into some GL_RGB or GL_RGBA data and might perform additional conversion before that, but drawing the image using QPainter::drawImage might still result in less conversion.
> >
> > Of course, if you have a GL viewport and some non-standard format, you can potentially speed it up further by implementing a QGraphicsItem which does raw GL, encapsulated by QPainter::beginNativePainting()/endNativePainting() and then upload to a texture of your choosing and then draw the texture with GL commands.
> >
> > cheers,
> > Gunnar
> >
> >>
> >>
> >>
> >> Thank you.
> >>
> >> Celal
> >>
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