Dump QImage raw pixel data into `std::vector<char>`

[Christian Ehrlicher]

@abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

std::vector<char> &fb

Is this used in another thread?

Otherwise I don't see anything obvious.

[abmyii]

@Christian-Ehrlicher I think the while loop is in another thread and VNC is accessing fb.data() throughout execution.

[Christian Ehrlicher]

@abmyii So you have your answer. Concurrent access to a memory region from two different threads.

[abmyii]

@Christian-Ehrlicher Ah I see, that makes perfect sense now! Is there a simple solution to this? I tried the following which didn't work:

std::vector<char> temp_fb(frame_size_bytes, 0);
std::memcpy(temp_fb.data(), (const char *)image.constBits(), image.byteCount());
fb = temp_fb

[Christian Ehrlicher]

Use mutexes or copy the data - I would suggest reading a little bit on how threading works. E.g. https://doc.qt.io/qt-5/threads-synchronizing.html - c++ uses the same mechanisms

[abmyii]

Whoops, I've been saying malloc (which is apparently thread-safe) all along when in fact the actual function I'm using is memcpy... Does that change things and/or can I use malloc instead?

[JonB]

@abmyii
Hence what @Christian-Ehrlicher has been saying above. malloc and memcpy do totally different things, it makes no sense to ask whether one can be used in place of the other....

[abmyii]

@JonB I understand - I got confused due to my unfamiliarity with the functions but I never asked if I could use malloc instead of memcpy - I asked if I could use push_back instead of malloc (sic).

[JonB]

@abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

I never asked if I could use malloc instead of memcpy [sic.]

in fact the actual function I'm using is memcpy... Does that change things and/or can I use malloc instead? [*sic.]

If you say so! I'll leave others to untangle, who clearly understand what you are saying better than I :) Don't worry about explaining to me.

[abmyii]

@JonB OK, you got me! Too much to keep track of...

I tried this and it worked, but is quite a lot slower than memcpy:

std::vector<unsigned char> vectorBuffer(charBuffer, charBuffer + length);
fb = vectorBuffer;

[JonB]

@abmyii
I am lost in the complexities of your questions :) But I would suggest heeding @Christian-Ehrlicher's advice, he is usually right! He last offered:

Use mutexes or copy the data

So why not pursue that?

[Christian Ehrlicher]

@abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

std::vector<unsigned char> vectorBuffer(charBuffer, charBuffer + length);
fb = vectorBuffer;

This does two memcpy for no good reason. Since I don't know what libvncserver function you use you're on your own. Read the documentation of them and also read what's needed for threadsafe access.

[abmyii]

@JonB Haha, you think you are the only one?!

I am planning to try the mutex idea - I haven't had a chance to look it up and figure out how to do it yet. A basic QMutex lock/unlock didn't fix it (though I probably applied it wrong)...

@Christian-Ehrlicher Ah, I see. What "libvncserver function" do you mean?

[Christian Ehrlicher]

@abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

What "libvncserver function" do you mean?

You are using libvncserver, not me.

Giving up here now.

[abmyii]

Going to mark this thread as closed since it seems the issue lies with VNC, not Qt. Thank you all for your suggestions and helping me to isolate the problem.

@Christian-Ehrlicher Sorry for stressing you out - I thought you were referring to a specific function for some reason. I really need to slow down when reading and comprehend rather than just having a quick look and then replying which is causing confusion on all sides. Thank you again for your patience and assistance throughout.

[heatblazer]

Hi I've done something recently I've needed so my approach was:

total_len = qimg.width() * qimg.height();
 std::vector<unsigned int> data;
 data.reserve(total_len);
 for(int i=0; i < qimg.height(); i++)
     for(int j=0; j < qimg.width(); j++){
         data.push_back(qimg.pixel(j, i));
     }

I needed it as an rgb pixels in order to do some image manipulations, but I believe you can also take the uchar data.