Save QImage from BYTE buffer segfaults ?

[Christian Ehrlicher]

@R-P-H said in Save QImage from BYTE buffer segfaults ?:

I assumed that if buffer is being populated the same way each time and that if it works sometimes, then it should be fine.

Even if it does it would depend on your image size (or better: bytes per line). If it's a multiple of 4 it does not crash, if it's something else the alignment is wrong.

[R-P-H]

@SGaist said in Save QImage from BYTE buffer segfaults ?:

That's a bit short on details. An API call can be a method from a library your are using or a remote call to a REST service.

What kind of image are you getting ?

It's from a library. The API reference is very short on details. It's a 256 gray-level image.

[R-P-H]

@Christian-Ehrlicher said in Save QImage from BYTE buffer segfaults ?:

@R-P-H said in Save QImage from BYTE buffer segfaults ?:

I assumed that if buffer is being populated the same way each time and that if it works sometimes, then it should be fine.

Even if it does it would depend on your image size (or better: bytes per line). If it's a multiple of 4 it does not crash, if it's something else the alignment is wrong.

The image size is always the same (260 x 300 px).

[SGaist]

And one byte per pixel ?

[R-P-H]

@SGaist said in Save QImage from BYTE buffer segfaults ?:

And one byte per pixel ?

That is correct yes.

[SGaist]

Can you get the stack trace of the crash ?

[R-P-H]

@SGaist said in Save QImage from BYTE buffer segfaults ?:

Can you get the stack trace of the crash ?

I'm testing the application on another machine using the executable. I don't think I'd be able to get it without QT or another IDE right ?

[SGaist]

Windows machine ?

[R-P-H]

@SGaist said in Save QImage from BYTE buffer segfaults ?:

Windows machine ?

Yes it is. Windows 7 and also Windows 10.

[mranger90]

try:
BYTE *buf = new BYTE[imWidth * imHeight];

[R-P-H]

@mranger90 said in Save QImage from BYTE buffer segfaults ?:

try:
BYTE *buf = new BYTE[imWidth * imHeight];

Hi, it is not crashing anymore, however the image seems to be coming out completely black. I will confirm tomorrow if the image is supposed to be completely black or not...

[Jarek B]

From the documentation
https://doc.qt.io/qt-5/qimage.html#QImage-4
data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

So (as far as I understand) that is not enough to have memory buffer aligned to 32 bit address, also each line have to be aligned to 32 bit address. So if you have image that is 260x300 pixels, a whole line will take 288 bytes. So for whole image you should have 288*300 bytes which is more than you have provided.

[R-P-H]

@Jarek-B said in Save QImage from BYTE buffer segfaults ?:

From the documentation
https://doc.qt.io/qt-5/qimage.html#QImage-4
data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

So (as far as I understand) that is not enough to have memory buffer aligned to 32 bit address, also each line have to be aligned to 32 bit address. So if you have image that is 260x300 pixels, a whole line will take 288 bytes. So for whole image you should have 288*300 bytes which is more than you have provided.

Hi, interesting...I have not seen any examples like that. How are you arriving at 288 bytes ? Are you suggesting I make the buffer 288x300 bytes instead ?

[mranger90]

@R-P-H Of course it's black, the data never gets initialized.
Also, the reason why it doesn't crash is because:
BYTE *buf = new BYTE(x * y); // allocates a single byte, initialized to the value of (x * y)
BYTE *buf = new BYTE[x * y]; // allocates an array of bytes whose size is x * y

[R-P-H]

@mranger90 said in Save QImage from BYTE buffer segfaults ?:

@R-P-H Of course it's black, the data never gets initialized.
Also, the reason why it doesn't crash is because:
BYTE *buf = new BYTE(x * y); // allocates a single byte, initialized to the value of (x * y)
BYTE *buf = new BYTE[x * y]; // allocates an array of bytes whose size is x * y

I don't quite understand what you're saying to be honest. Where are you saying the issue is ?

[mranger90]

@R-P-H here is the amended code

// allocates a buffer of imWidth * imHeight bytes, which  depending on the compiler 
// settings will either be initialized to zero, or may contain garbage
BYTE *buf = new BYTE[imWidth * imHeight];

// creates a QImage based on the data in the buffer, 
QImage img(buf, imWidth, imHeight, QImage::Format_Grayscale8);

// saves the data
img.save("image.bmp", "BMP");
delete [] buf;

The point is that you never put actual image data into the buffer.

[R-P-H]

@mranger90 said in Save QImage from BYTE buffer segfaults ?:

@R-P-H here is the amended code

// allocates a buffer of imWidth * imHeight bytes, which  depending on the compiler 
// settings will either be initialized to zero, or may contain garbage
BYTE *buf = new BYTE[imWidth * imHeight];

// creates a QImage based on the data in the buffer, 
QImage img(buf, imWidth, imHeight, QImage::Format_Grayscale8);

// saves the data
img.save("image.bmp", "BMP");
delete [] buf;

The point is that you never put actual image data into the buffer.

This is already what I did according to your first post. The buffer IS populated, I just didn't show that API call. I will modify my original post to make this clear...

So the result is a completely black image. This may be normal but I will have to check with the API provider.

EDIT: I have edited my original post.

[SGaist]

Is that some private library you can't share ? Or would it be possible to have more information about that API ?

[R-P-H]

@SGaist said in Save QImage from BYTE buffer segfaults ?:

Is that some private library you can't share ? Or would it be possible to have more information about that API ?

Your assumption is correct. There really aren't any additional details on using this API function. It simply takes as input a reference to the buffer and a unique id of the capturing device. It then populates the buffer in place. The example in the documentation initializes the buffer the same way as shown in my original post.

If we are sure the error is on the API side, then they will need to fix that.

I made the assumption that the error was in saving the image to disk.

[SGaist]

Then are you sure their BYTE type is not something custom ? Do they also show how to access the data ?

Is it the Windows BYTE type ?