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

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 `std::vector<char>`:

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 `std::vector<char>`:

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 `std::vector<char>`:

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.