Saving YUVY image from V4L2 buffer to file

Saving YUVY image from V4L2 buffer to file

[Owen O' Hehir]

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);
        pixel++;
        cb= *(p+pixel);    //modified U
        pixel++;
        y2=*(p+pixel);
        pixel++;
        cr= *(p+pixel);    //modified V
        pixel++;

        r =y1 + (1.402*cb);
        g = y1 - (0.344*cb) - (0.714*cr);
            b = y1 + (1.772*cr);

        if (r > 255) r = 255;
        if (g > 255) g = 255;
        if (b > 255) b = 255;

        if (r < 0) r = 0;
        if (g < 0) g = 0;
        if (b < 0) b = 0;

            fprintf( fp, "%c%c%c",r,g,b);

        //Second pixel,reuse cb & cr, new y value

        r =y2 + (1.402*cb);
        g = y2 - (0.344*cb) - (0.714*cr);
        b = y2 + (1.772*cr);

        if (r > 255) r = 255;
        if (g > 255) g = 255;
        if (b > 255) b = 255;

        if (r < 0) r = 0;
        if (g < 0) g = 0;
        if (b < 0) b = 0;

        fprintf( fp, "%c%c%c",r,g,b);

            }

        fclose( fp );
        fprintf( stderr, "frame saved\n" );

    fflush (stdout);
}

However I'm only getting a green frame out. Could anybody point me in the right direction? 

Many thanks,

Owen
 		 	   		  
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Re: Saving YUVY image from V4L2 buffer to file

[Devin Heitmueller]

On Wed, Feb 3, 2010 at 12:40 PM, Owen O' Hehir <oo_hehir@hotmail.com> 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:

Independent of the conversion function, are you sure you are getting a
valid YUV frame at all?  A completely green frame is what you will get
back if you had a buffer which was memset(0).  Hence it's possible
that the data you are passing *into* your conversion function is
completely blank.

Devin

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RE: Saving YUVY image from V4L2 buffer to file

[Owen O' Hehir]

Devin,

Many thanks for the quick reply.

Yes I'm getting some sort of an image. When I was experimenting I managed to get an image but in grayscale & showing multiple copies of the same image covering the top half of the image. I imagine it was distorted because I was not converting to RGB correctly.

All the best,

 

Owen




> Date: Wed, 3 Feb 2010 12:54:02 -0500
> Subject: Re: Saving YUVY image from V4L2 buffer to file
> From: dheitmueller@kernellabs.com
> To: oo_hehir@hotmail.com
> CC: video4linux-list@redhat.com
> 
> On Wed, Feb 3, 2010 at 12:40 PM, Owen O' Hehir <oo_hehir@hotmail.com> 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:
> 
> Independent of the conversion function, are you sure you are getting a
> valid YUV frame at all?  A completely green frame is what you will get
> back if you had a buffer which was memset(0).  Hence it's possible
> that the data you are passing *into* your conversion function is
> completely blank.
> 
> Devin
> 
> -- 
> Devin J. Heitmueller - Kernel Labs
> http://www.kernellabs.com
 		 	   		  
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Re: Saving YUVY image from V4L2 buffer to file

[Devin Heitmueller]

On Wed, Feb 3, 2010 at 1:06 PM, Owen O' Hehir <oo_hehir@hotmail.com> wrote:
>
> Devin,
>
> Many thanks for the quick reply.
>
> Yes I'm getting some sort of an image. When I was experimenting I managed to get an image but in grayscale & showing multiple copies of the same image covering the top half of the image. I imagine it was distorted because I was not converting to RGB correctly.
>
> All the best,

Well, a picture is worth a thousand words, so if you perhaps would
consider throwing one up on imagebin and providing a link, someone
might be able to give you some insight as to the nature of the
problem.

Devin

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Re: Saving YUVY image from V4L2 buffer to file

[Darren Longhorn]

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.

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RE: Saving YUVY image from V4L2 buffer to file

[Charlie X. Liu]

Why don't you directly set it with: fmt.fmt.pix.pixelformat =
V4L2_PIX_FMT_BGR24, instead of converting?

Then, save like:

        sprintf( ppmheader, "P6\n#ppm image\n%d %d\n255\n", info->width,
info->height);
        fwrite( ppmheader, 1, strlen(ppmheader), fptr);
        // write out the rows
        for ( i=0; i<info->height; i++)
        {
            //fwrite( &image[info->stride*i], 1, info->stride, fptr);
            for ( j=0; j<info->width; j++ ) {
            fwrite( &image[info->stride*i+j*3+2], 1, 1, fptr);
            fwrite( &image[info->stride*i+j*3+1], 1, 1, fptr);
            fwrite( &image[info->stride*i+j*3+0], 1, 1, fptr);
            }
        }


Charlie X. Liu @ Sensoray Co.


-----Original Message-----
From: video4linux-list-bounces@redhat.com
[mailto:video4linux-list-bounces@redhat.com] On Behalf Of Owen O' Hehir
Sent: Wednesday, February 03, 2010 10:06 AM
To: video4linux-list@redhat.com
Subject: RE: Saving YUVY image from V4L2 buffer to file


Devin,

Many thanks for the quick reply.

Yes I'm getting some sort of an image. When I was experimenting I managed to
get an image but in grayscale & showing multiple copies of the same image
covering the top half of the image. I imagine it was distorted because I was
not converting to RGB correctly.

All the best,

 

Owen




> Date: Wed, 3 Feb 2010 12:54:02 -0500
> Subject: Re: Saving YUVY image from V4L2 buffer to file
> From: dheitmueller@kernellabs.com
> To: oo_hehir@hotmail.com
> CC: video4linux-list@redhat.com
> 
> On Wed, Feb 3, 2010 at 12:40 PM, Owen O' Hehir <oo_hehir@hotmail.com>
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:
> 
> Independent of the conversion function, are you sure you are getting a
> valid YUV frame at all?  A completely green frame is what you will get
> back if you had a buffer which was memset(0).  Hence it's possible
> that the data you are passing *into* your conversion function is
> completely blank.
> 
> Devin
> 
> -- 
> Devin J. Heitmueller - Kernel Labs
> http://www.kernellabs.com
 		 	   		  
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Re: Saving YUVY image from V4L2 buffer to file

[Devin Heitmueller]

On Wed, Feb 3, 2010 at 1:57 PM, Charlie X. Liu <charlie@sensoray.com> wrote:
> Why don't you directly set it with: fmt.fmt.pix.pixelformat =
> V4L2_PIX_FMT_BGR24, instead of converting?

That only really works if the hardware supports providing RGB data as
opposed to YUYV (which many do not).  Or, it would work if you link
against libv4l to do the conversion in userland.

Devin

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RE: Saving YUVY image from V4L2 buffer to file

[Charlie X. Liu]

Do most of Webcams have no RGB support? I'm not experienced on that. Does
anybody know the percentage of RGB support by Webcams?


-----Original Message-----
From: Devin Heitmueller [mailto:dheitmueller@kernellabs.com] 
Sent: Wednesday, February 03, 2010 11:19 AM
To: Charlie X. Liu
Cc: Owen O' Hehir; video4linux-list@redhat.com
Subject: Re: Saving YUVY image from V4L2 buffer to file

On Wed, Feb 3, 2010 at 1:57 PM, Charlie X. Liu <charlie@sensoray.com> wrote:
> Why don't you directly set it with: fmt.fmt.pix.pixelformat =
> V4L2_PIX_FMT_BGR24, instead of converting?

That only really works if the hardware supports providing RGB data as
opposed to YUYV (which many do not).  Or, it would work if you link
against libv4l to do the conversion in userland.

Devin

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Re: Saving YUVY image from V4L2 buffer to file

[Devin Heitmueller]

On Wed, Feb 3, 2010 at 3:01 PM, Charlie X. Liu <charlie@sensoray.com> wrote:
> Do most of Webcams have no RGB support? I'm not experienced on that. Does
> anybody know the percentage of RGB support by Webcams?

Well, the V4L interface doesn't make the distinction between webcams
and other analog capture devices.  That said, webcams do tend to
provide some variant of RGB (and in a number of cases it uses
proprietary formats that include compression that must be handled in
userland).  So there are different formats that would all be
considered some form of RGB.

Other capture devices, such as tuners and home movie converters tend to use YUV.

So, if you're looking to write a simple app for internal use to work
with a particular webcam, you can disregard the above.  But if you are
trying to write a generic capture application that is expected to work
with many products, then you need to take into consideration all the
different formats that can be used.

Devin

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Re: Saving YUVY image from V4L2 buffer to file

[Andy Walls]

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.

Regards,
Andy

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Re: Saving YUVY image from V4L2 buffer to file

[Darren Longhorn]

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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RE: Saving YUVY image from V4L2 buffer to file - SOLVED

[Owen O' Hehir]

Hello All, 

Solved this problem.

For info for others here is a function to unpacked YUYV (Also known as YUY422 & YUY2) -> RGB and save it to a file. I've included other formats that I came across that others might find useful but are untested.

int frame_save(const void *p, const char* filename )
{

    static int packed_value, i;
    FILE* fp = fopen(filename, "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( stderr, "&userframe address %d\n", &userframe );
    // fprintf( stderr, "&p address %d\n", &p );
    // fprintf( stderr, "sizeof(userframe) %d\n", sizeof(userframe) );
    
//     switch (userfmt.fmt.pix.pixelformat){
//     case V4L2_PIX_FMT_RGB24:
//         
//         // Max val
//         // NOTE UNTESTED!!!!!!!!!
//         fprintf( fp, "255\n" );
//         fprintf( stderr, "frame_save(): RGB24 Unsupported type!\n" );
// 
//         //Write image data
//         for ( i = 0; i < ( userfmt.fmt.pix.width * userfmt.fmt.pix.height ); i++ )
//         {
//         // 3 bytes per pixel
// //         rgb = ((FRAME_RGB*)(p))[i];
// // 
// //         fprintf( fp, "%c%c%c",
// //              rgb.blue,
// //              rgb.green,
// //              rgb.red );
//         }
//         break;
//         
//     case V4L2_PIX_FMT_RGB32:
//         // NOTE UNTESTED!!!!!!!!!
//         // Max val
//         fprintf( fp, "255\n" );
//         
//         // Write image data
//         for ( i = 0; i < ( userfmt.fmt.pix.width * userfmt.fmt.pix.height ); i++ )
//         {
//         // Retrieve lower 24 bits of ARGB
//         packed_value = ((int*)(p))[i] & 0x00ffffff;
//         
//         fprintf( fp, "%c%c%c",
//              ( packed_value & 0x00ff0000 ) >> 16, // Blue
//              ( packed_value & 0x0000ff00 ) >>  8, // Green
//              ( packed_value & 0x000000ff )        // Red
//              );
//         }
//         break;
//         
//     case V4L2_PIX_FMT_RGB565:
//     case V4L2_PIX_FMT_RGB555:
//         // NOTE UNTESTED!!!!!!!!!
//         // Max val
//         fprintf( fp, "65535\n" );
//         
//         // Write image data
//         for ( i = 0; i < ( userfmt.fmt.pix.width *userfmt.fmt.pix.height ); i++ ){
//         // Retrieve 16-bit words
//         packed_value = ((short*)(p))[i];
//         
//         fprintf( fp, "%c%c",frame
//              ( packed_value & 0xff00 ) >> 8, // High
//              ( packed_value & 0x00ff )       // Low
//              );
//             }
//         break;
// 
//     case V4L2_PIX_FMT_YUYV:
        int Y0, Y1, Cb, Cr;            /* gamma pre-corrected input [0;255] */
        int ER0,ER1,EG0,EG1,EB0,EB1;    /* output [0;255] */
        double r0,r1,g0,g1,b0,b1;             /* temporaries */
        double y0,y1, pb, pr;

        // Max val
        fprintf( fp, "255\n" );
        //fprintf( stderr, "frame_save(): YUYV file type!\n" );

        while(i < (userfmt.fmt.pix.width * userfmt.fmt.pix.height/2)){

        packed_value = *((int*)p+i);

        Y0 = (char)(packed_value & 0xFF);
        Cb = (char)((packed_value >> 8) & 0xFF);
        Y1 = (char)((packed_value >> 16) & 0xFF);
        Cr = (char)((packed_value >> 24) & 0xFF);

        // Strip sign values after shift (i.e. unsigned shift)
        Y0 = Y0 & 0xFF;
        Cb = Cb & 0xFF;
        Y1 = Y1 & 0xFF;
        Cr = Cr & 0xFF;

        //fprintf( fp, "Value:%x Y0:%x Cb:%x Y1:%x Cr:%x ",packed_value,Y0,Cb,Y1,Cr);

        y0 = (255 / 219.0) * (Y0 - 16);
        y1 = (255 / 219.0) * (Y1 - 16);
        pb = (255 / 224.0) * (Cb - 128);
        pr = (255 / 224.0) * (Cr - 128);

        // Generate first pixel
        r0 = 1.0 * y0 + 0     * pb + 1.402 * pr;
        g0 = 1.0 * y0 - 0.344 * pb - 0.714 * pr;
        b0 = 1.0 * y0 + 1.772 * pb + 0     * pr;

        // Generate next pixel - must reuse pb & pr as 4:2:2
        r1 = 1.0 * y1 + 0     * pb + 1.402 * pr;
        g1 = 1.0 * y1 - 0.344 * pb - 0.714 * pr;
        b1 = 1.0 * y1 + 1.772 * pb + 0     * pr;

        ER0 = clamp (r0);
        ER1 = clamp (r1);
        EG0 = clamp (g0);
        EG1 = clamp (g1);
        EB0 = clamp (b0);
        EB1 = clamp (b1);

        fprintf( fp, "%c%c%c%c%c%c",ER0,EG0,EB0,ER1,EG1,EB1); // Output two pixels
        //fprintf( fp, "Memory:%p Pixel:%d R:%d G:%d B:%d     Pixel:%d R:%d G:%d B:%d \n",location,val,ER0,EG0,EB0,(val+1),ER1,EG1,EB1);

        i++;
        }

        //fprintf( stderr, "Size of packed_value:%d Y0:%d Cb:%d Cr:%d Y1:%d\n", sizeof(packed_value), sizeof(y0), sizeof(cb0), sizeof(y1), sizeof(cr0));

//        break;
// 
//     default:
//         // Unsupported!
//         fprintf( stderr, "frame_save(): Unsupported type!\n" );
//         return -1;
//         }

    fprintf( stderr, "frame saved\n" );
    fclose( fp );
    return 0;
}

All the best,

 

Owen


> Date: Wed, 3 Feb 2010 13:18:44 -0500
> Subject: Re: Saving YUVY image from V4L2 buffer to file
> From: dheitmueller@kernellabs.com
> To: oo_hehir@hotmail.com
> CC: video4linux-list@redhat.com
> 
> On Wed, Feb 3, 2010 at 1:06 PM, Owen O' Hehir <oo_hehir@hotmail.com> wrote:
> >
> > Devin,
> >
> > Many thanks for the quick reply.
> >
> > Yes I'm getting some sort of an image. When I was experimenting I managed to get an image but in grayscale & showing multiple copies of the same image covering the top half of the image. I imagine it was distorted because I was not converting to RGB correctly.
> >
> > All the best,
> 
> Well, a picture is worth a thousand words, so if you perhaps would
> consider throwing one up on imagebin and providing a link, someone
> might be able to give you some insight as to the nature of the
> problem.
> 
> Devin
> 
> -- 
> Devin J. Heitmueller - Kernel Labs
> http://www.kernellabs.com
 		 	   		  
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