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Re: Saving YUVY image from V4L2 buffer to file




Andy Walls wrote:
> On Wed, 2010-02-03 at 18:38 +0000, Darren Longhorn wrote:
>> Owen O' Hehir wrote:
>>> Hello All,
>>>
>>> I'm trying to save a captured image from a USB camera to a file. The capture is based on V4L2 video capture example from the V4L2 API spec. http://v4l2spec.bytesex.org/spec/a16706.htm
>>>
>>> The V4L2 set pointers (via mmap) to to the USB image (in YUV 4:2:2 (YUYV)) and as far as I can see the simplest way to save the image in a recognised format is in RGB format, specifically in PPM (Netpbm color image format).
>>>
>>> As such I've expanded the process_image function:
>>>
>>>
>>> static void
>>> process_image                   (const void *           p)
>>> {
>>>     static int count = 0;
>>>
>>>     static int r,g,b;
>>>     static int y1,y2,cb,cr;
>>>
>>>     int pixel=0;
>>>
>>>         FILE* fp = fopen("datadump", "w" );
>>>         // Write PNM header
>>>         fprintf( fp, "P6\n" );
>>>         fprintf( fp, "# YUV422 frame -> RGB \n" );
>>>         fprintf( fp, "%d %d\n", userfmt.fmt.pix.width, userfmt.fmt.pix.height );
>>>
>>>         fprintf( fp, "255\n" );
>>>
>>>         while(pixel < (userfmt.fmt.pix.width * userfmt.fmt.pix.height)){
>>>
>>>         y1 = *(p+pixel);
>> Are you sure that's your real code? I don't think you should dereference
>> a void pointer like that.
> 
> Old-ish C-compilers treated that as a char * in that case.  The behavior
> is unreliable of course.  This certainly could be a cause of problems.

Ah, yes. Well remembered!

Cheers

Darren

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