QImage to Qpixmap conversion is too slow

lansing

I want to display video and show its frames to the gui. I have a processor that pumps out packed RGB data, and to display them, I have a QImage that took in the data and convert them to QPixmap for display. However the QImage to QPixmap conversion is too slow for real time play speed. It played fine when the videos are around 1080p, but with 4K video, the fps just plummeted.

QImage frame_image(data, width, height, QImage::Format_ARGB32);
QPixmap frame_pixmap = QPixmap::fromImage(frame_image);

What is a faster way to do this?

Christian Ehrlicher

@lansing said in QImage to Qpixmap conversion is too slow:

What is a faster way to do this?

Use a proper backend for your task. CPU raster is definitely not made for such kind of work. Use OenGL or similar instead.

lansing

@Christian-Ehrlicher said in QImage to Qpixmap conversion is too slow:

@lansing said in QImage to Qpixmap conversion is too slow:

What is a faster way to do this?

Use a proper backend for your task. CPU raster is definitely not made for such kind of work. Use OenGL or similar instead.

Is there any example with OpenGL? Like where do I start?

SGaist

Hi,

Can you give more information about how are these images created ? Passed around ?
Are they from a camera ?
Are you loading them from a file ?
Etc.

lansing

@SGaist

They were loaded from a video file, then go into a processor where it will spit out packed RGB data frame by frame.

I found a stackoverflow answer about passing the QImage into a QOpenGLWidget and then using its paintEvent to draw out the image, is this the way?

https://stackoverflow.com/questions/20245865/render-qimage-with-opengl

SGaist

What are you using to read your video file ?
Depending on that and the image format you could load the data directly into a texture and do the color space conversion in OpenGL rather than on the CPU side.

lansing

@SGaist said in QImage to Qpixmap conversion is too slow:

What are you using to read your video file ?
Depending on that and the image format you could load the data directly into a texture and do the color space conversion in OpenGL rather than on the CPU side.

That can be a future plan, but I want to do it one step at a time. For right now I need to know where to even start. The Qimage to QPixmap conversion is cutting my playback speed in half.

mrjj

@lansing
Hi
well try the OpenGLWidget::paintEvent code and see if its fast enough.
At least you don't have the conversion to pixmap

lansing

@mrjj

Hi This is what I have done so far:

class MyGLPainter : public QOpenGLWidget
{
    Q_OBJECT
public:
    MyGLPainter (QWidget *parent = nullptr);
    void paintEvent(QPaintEvent *) override;
    void drawImage(const QImage &image); 

private:
    QImage m_frameImage;

MyGLPainter::MyGLPainter (QWidget *parent): QOpenGLWidget(parent)  
{
}

void MyGLPainter::paintEvent(QPaintEvent *)
{
    QPainter painter(this);   
    painter.drawImage(m_frameImage.width(),m_frameImage.height(), m_frameImage);
    painter.end();
}

void MyGLPainter::drawImage(const QImage &image)
{
    m_frameImage = image;
    update();
}

But when I pass the image to drawImage(), the program will just crash. Am I doing this correctly?

mrjj

@lansing
Hi
It looks ok.
In what lines does it crash ?

lansing

@mrjj

At the line update(). It points to function QImage::operator=

Christian Ehrlicher

Do you update your image from a different thread? If so you should learn about threading: Threading Basics

lansing

@Christian-Ehrlicher

HI, is this related to the crash?

mrjj

@lansing

Hi
yes if you do it from another thread then very much yes.

lansing

@mrjj

I don't have threading in the program.

mrjj

@lansing
Hi
then something is wrong with the image then.
Else why crash on copy.

can you try with Image that you are 100% sure is valid ?

lansing

@mrjj

Hi, I have set the image to one in my resource folder for testing, now it has crashed at line m_frameImage = image. I have changed the image argument from const QImage & to QImage*, I don't know if that have a effect on it. But in debug, it did show my image dimension.

QImage *img= new QImage(":/cross.png");
myGLPainter->drawImage(img);

mrjj

@lansing
and it does still crash ?

lansing

Yes, it said

error: Debugger encountered an exception: Exception at 0x7ff71b7e9b88, code: 0xc0000005: read access violation at: 0xffffffffffffffff, flags=0x0

Christian Ehrlicher

@lansing said in QImage to Qpixmap conversion is too slow:

myGLPainter

Where do you initialize this variable?