# Image Formats

The V4L2 API was primarily designed for devices exchanging image data with applications. The v4l2_pix_format structure defines the format and layout of an image in memory. Image formats are negotiated with the &VIDIOC-S-FMT; ioctl. (The explanations here focus on video capturing and output, for overlay frame buffer formats see also &VIDIOC-G-FBUF;.)

# struct v4l2_pix_format

&cs-str;
__u32 | width | Image width in pixels.
__u32 | height | Image height in pixels.
Applications set these fields to request an image size, drivers return the closest possible values. In case of planar formats the width and height applies to the largest plane. To avoid ambiguities drivers must return values rounded up to a multiple of the scale factor of any smaller planes. For example when the image format is YUV 4:2:0, width and height must be multiples of two.
__u32 | pixelformat | The pixel format or type of compression, set by the application. This is a little endian four character code. V4L2 defines standard RGB formats in , YUV formats in , and reserved codes in
&v4l2-field; | field | Video images are typically interlaced. Applications can request to capture or output only the top or bottom field, or both fields interlaced or sequentially stored in one buffer or alternating in separate buffers. Drivers return the actual field order selected. For details see .
__u32 | bytesperline | Distance in bytes between the leftmost pixels in two adjacent lines.

Both applications and drivers can set this field to request padding bytes at the end of each line. Drivers however may ignore the value requested by the application, returning width times bytes per pixel or a larger value required by the hardware. That implies applications can just set this field to zero to get a reasonable default.

Video hardware may access padding bytes, therefore they must reside in accessible memory. Consider cases where padding bytes after the last line of an image cross a system page boundary. Input devices may write padding bytes, the value is undefined. Output devices ignore the contents of padding bytes.

When the image format is planar the bytesperline value applies to the largest plane and is divided by the same factor as the width field for any smaller planes. For example the Cb and Cr planes of a YUV 4:2:0 image have half as many padding bytes following each line as the Y plane. To avoid ambiguities drivers must return a bytesperline value rounded up to a multiple of the scale factor.

__u32 | sizeimage | Size in bytes of the buffer to hold a complete image, set by the driver. Usually this is bytesperline times height. When the image consists of variable length compressed data this is the maximum number of bytes required to hold an image.
&v4l2-colorspace; | colorspace | This information supplements the pixelformat and must be set by the driver, see .
__u32 | priv | Reserved for custom (driver defined) additional information about formats. When not used drivers and applications must set this field to zero.

# Standard Image Formats

In order to exchange images between drivers and applications, it is necessary to have standard image data formats which both sides will interpret the same way. V4L2 includes several such formats, and this section is intended to be an unambiguous specification of the standard image data formats in V4L2.

V4L2 drivers are not limited to these formats, however. Driver-specific formats are possible. In that case the application may depend on a codec to convert images to one of the standard formats when needed. But the data can still be stored and retrieved in the proprietary format. For example, a device may support a proprietary compressed format. Applications can still capture and save the data in the compressed format, saving much disk space, and later use a codec to convert the images to the X Windows screen format when the video is to be displayed.

Even so, ultimately, some standard formats are needed, so the V4L2 specification would not be complete without well-defined standard formats.

The V4L2 standard formats are mainly uncompressed formats. The pixels are always arranged in memory from left to right, and from top to bottom. The first byte of data in the image buffer is always for the leftmost pixel of the topmost row. Following that is the pixel immediately to its right, and so on until the end of the top row of pixels. Following the rightmost pixel of the row there may be zero or more bytes of padding to guarantee that each row of pixel data has a certain alignment. Following the pad bytes, if any, is data for the leftmost pixel of the second row from the top, and so on. The last row has just as many pad bytes after it as the other rows.

In V4L2 each format has an identifier which looks like `PIX_FMT_XXX`, defined in the videodev.h header file. These identifiers represent four character codes which are also listed below, however they are not the same as those used in the Windows world.

# Colorspaces

[intro]

Gamma Correction

  [to do]

  E'R = f(R)

  E'G = f(G)

  E'B = f(B)

Construction of luminance and color-difference signals

  [to do]

  E'Y = CoeffR E'R + CoeffG E'G + CoeffB E'B

  (E'R - E'Y) = E'R - CoeffR E'R - CoeffG E'G - CoeffB E'B

  (E'B - E'Y) = E'B - CoeffR E'R - CoeffG E'G - CoeffB E'B

Re-normalized color-difference signals

  The color-difference signals are scaled back to unity range [-0.5;+0.5]:

  KB = 0.5 / (1 - CoeffB)

  KR = 0.5 / (1 - CoeffR)

  PB = KB (E'B - E'Y) = 0.5 (CoeffR / CoeffB) E'R + 0.5 (CoeffG / CoeffB) E'G + 0.5 E'B

  PR = KR (E'R - E'Y) = 0.5 E'R + 0.5 (CoeffG / CoeffR) E'G + 0.5 (CoeffB / CoeffR) E'B

Quantization

  [to do]

  Y' = (Lum. Levels - 1) · E'Y + Lum. Offset

  CB = (Chrom. Levels - 1) · PB + Chrom. Offset

  CR = (Chrom. Levels - 1) · PR + Chrom. Offset

  Rounding to the nearest integer and clamping to the range [0;255] finally yields the digital color components Y'CbCr stored in YUV images.

## ITU-R Rec. BT.601 color conversion

Forward Transformation

    int ER, EG, EB;         /* gamma corrected RGB input [0;255] */
    int Y1, Cb, Cr;         /* output [0;255] */

    double r, g, b;         /* temporaries */
    double y1, pb, pr;

    int
    clamp (double x)
    {
    	int r = x;      /* round to nearest */

    	if (r < 0)         return 0;
    	else if (r > 255)  return 255;
    	else               return r;
    }

    r = ER / 255.0;
    g = EG / 255.0;
    b = EB / 255.0;

    y1  =  0.299  * r + 0.587 * g + 0.114  * b;
    pb  = -0.169  * r - 0.331 * g + 0.5    * b;
    pr  =  0.5    * r - 0.419 * g - 0.081  * b;

    Y1 = clamp (219 * y1 + 16);
    Cb = clamp (224 * pb + 128);
    Cr = clamp (224 * pr + 128);

    /* or shorter */

    y1 = 0.299 * ER + 0.587 * EG + 0.114 * EB;

    Y1 = clamp ( (219 / 255.0)                    *       y1  + 16);
    Cb = clamp (((224 / 255.0) / (2 - 2 * 0.114)) * (EB - y1) + 128);
    Cr = clamp (((224 / 255.0) / (2 - 2 * 0.299)) * (ER - y1) + 128);

Inverse Transformation

    int Y1, Cb, Cr;         /* gamma pre-corrected input [0;255] */
    int ER, EG, EB;         /* output [0;255] */

    double r, g, b;         /* temporaries */
    double y1, pb, pr;

    int
    clamp (double x)
    {
    	int r = x;      /* round to nearest */

    	if (r < 0)         return 0;
    	else if (r > 255)  return 255;
    	else               return r;
    }

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

    r = 1.0 * y1 + 0     * pb + 1.402 * pr;
    g = 1.0 * y1 - 0.344 * pb - 0.714 * pr;
    b = 1.0 * y1 + 1.772 * pb + 0     * pr;

    ER = clamp (r * 255); /* [ok? one should prob. limit y1,pb,pr] */
    EG = clamp (g * 255);
    EB = clamp (b * 255);

## enum v4l2_colorspace

Identifier | Value | Description | Chromaticities The coordinates of the color primaries are given in the CIE system (1931) | White Point | Gamma Correction | Luminance E'Y | Quantization
--- | --- | --- | --- | --- | --- | --- | ---
Red | Green | Blue | Y' | Cb, Cr
`V4L2_COLORSPACE_SMPTE170M` | 1 | NTSC/PAL according to , | x = 0.630, y = 0.340 | x = 0.310, y = 0.595 | x = 0.155, y = 0.070 | x = 0.3127, y = 0.3290, Illuminant D65 | E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < I | 0.299 E'R + 0.587 E'G + 0.114 E'B | 219 E'Y + 16 | 224 PB,R + 128
`V4L2_COLORSPACE_SMPTE240M` | 2 | 1125-Line (US) HDTV, see | x = 0.630, y = 0.340 | x = 0.310, y = 0.595 | x = 0.155, y = 0.070 | x = 0.3127, y = 0.3290, Illuminant D65 | E' = 4 I for I ≤0.0228, 1.1115 I0.45 - 0.1115 for 0.0228 < I | 0.212 E'R + 0.701 E'G + 0.087 E'B | 219 E'Y + 16 | 224 PB,R + 128
`V4L2_COLORSPACE_REC709` | 3 | HDTV and modern devices, see | x = 0.640, y = 0.330 | x = 0.300, y = 0.600 | x = 0.150, y = 0.060 | x = 0.3127, y = 0.3290, Illuminant D65 | E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < I | 0.2125 E'R + 0.7154 E'G + 0.0721 E'B | 219 E'Y + 16 | 224 PB,R + 128

`V4L2_COLORSPACE_BT878`

4

Broken Bt878 extents The ubiquitous Bt878 video capture chip quantizes E'Y to 238 levels, yielding a range of Y' = 16 … 253, unlike Rec. 601 Y' = 16 … 235. This is not a typo in the Bt878 documentation, it has been implemented in silicon. The chroma extents are unclear. ,

?

?

?

?

?

0.299 E'R + 0.587 E'G + 0.114 E'B

237 E'Y + 16

224 PB,R + 128 (probably)

`V4L2_COLORSPACE_470_SYSTEM_M` | 5 | M/NTSC No identifier exists for M/PAL which uses the chromaticities of M/NTSC, the remaining parameters are equal to B and G/PAL. according to , | x = 0.67, y = 0.33 | x = 0.21, y = 0.71 | x = 0.14, y = 0.08 | x = 0.310, y = 0.316, Illuminant C | ? | 0.299 E'R + 0.587 E'G + 0.114 E'B | 219 E'Y + 16 | 224 PB,R + 128
`V4L2_COLORSPACE_470_SYSTEM_BG` | 6 | 625-line PAL and SECAM systems according to , | x = 0.64, y = 0.33 | x = 0.29, y = 0.60 | x = 0.15, y = 0.06 | x = 0.313, y = 0.329, Illuminant D65 | ? | 0.299 E'R + 0.587 E'G + 0.114 E'B | 219 E'Y + 16 | 224 PB,R + 128
`V4L2_COLORSPACE_JPEG` | 7 | JPEG Y'CbCr, see , | ? | ? | ? | ? | ? | 0.299 E'R + 0.587 E'G + 0.114 E'B | 256 E'Y + 16 Note JFIF quantizes Y'PBPR in range [0;+1] and [-0.5;+0.5] to 257 levels, however Y'CbCr signals are still clamped to [0;255]. | 256 PB,R + 128
`V4L2_COLORSPACE_SRGB` | 8 | [?] | x = 0.640, y = 0.330 | x = 0.300, y = 0.600 | x = 0.150, y = 0.060 | x = 0.3127, y = 0.3290, Illuminant D65 | E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < I | n/a

# Indexed Format

In this format each pixel is represented by an 8 bit index into a 256 entry ARGB palette. It is intended for Video Output Overlays only. There are no ioctls to access the palette, this must be done with ioctls of the Linux framebuffer API.

## Indexed Image Format

Identifier | Code |  | Byte 0
--- | --- | --- | ---
|  | Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0
`V4L2_PIX_FMT_PAL8` | 'PAL8' |  | i7 | i6 | i5 | i4 | i3 | i2 | i1 | i0

# RGB Formats

&sub-packed-rgb; &sub-sbggr8; &sub-sgbrg8; &sub-sgrbg8; &sub-sbggr16;

# YUV Formats

YUV is the format native to TV broadcast and composite video signals. It separates the brightness information (Y) from the color information (U and V or Cb and Cr). The color information consists of red and blue color difference signals, this way the green component can be reconstructed by subtracting from the brightness component. See for conversion examples. YUV was chosen because early television would only transmit brightness information. To add color in a way compatible with existing receivers a new signal carrier was added to transmit the color difference signals. Secondary in the YUV format the U and V components usually have lower resolution than the Y component. This is an analog video compression technique taking advantage of a property of the human visual system, being more sensitive to brightness information.

&sub-packed-yuv; &sub-grey; &sub-y16; &sub-yuyv; &sub-uyvy; &sub-yvyu; &sub-vyuy; &sub-y41p; &sub-yuv420; &sub-yuv410; &sub-yuv422p; &sub-yuv411p; &sub-nv12; &sub-nv16;

# Compressed Formats

## Compressed Image Formats

&cs-def;
Identifier | Code | Details
--- | --- | ---
`V4L2_PIX_FMT_JPEG` | 'JPEG' | TBD. See also &VIDIOC-G-JPEGCOMP;, &VIDIOC-S-JPEGCOMP;.
`V4L2_PIX_FMT_MPEG` | 'MPEG' | MPEG stream. The actual format is determined by extended control `V4L2_CID_MPEG_STREAM_TYPE`, see .

# Reserved Format Identifiers

These formats are not defined by this specification, they are just listed for reference and to avoid naming conflicts. If you want to register your own format, send an e-mail to the linux-media mailing list &v4l-ml; for inclusion in the `videodev2.h` file. If you want to share your format with other developers add a link to your documentation and send a copy to the linux-media mailing list for inclusion in this section. If you think your format should be listed in a standard format section please make a proposal on the linux-media mailing list.

## Reserved Image Formats

&cs-def;
Identifier | Code | Details
--- | --- | ---
`V4L2_PIX_FMT_DV` | 'dvsd' | unknown
`V4L2_PIX_FMT_ET61X251` | 'E625' | Compressed format of the ET61X251 driver.
`V4L2_PIX_FMT_HI240` | 'HI24' | 8 bit RGB format used by the BTTV driver.
`V4L2_PIX_FMT_HM12` | 'HM12' | YUV 4:2:0 format used by the IVTV driver, http://www.ivtvdriver.org/ The format is documented in the kernel sources in the file `Documentation/video4linux/cx2341x/README.hm12`
`V4L2_PIX_FMT_SPCA501` | 'S501' | YUYV per line used by the gspca driver.
`V4L2_PIX_FMT_SPCA505` | 'S505' | YYUV per line used by the gspca driver.
`V4L2_PIX_FMT_SPCA508` | 'S508' | YUVY per line used by the gspca driver.
`V4L2_PIX_FMT_SPCA561` | 'S561' | Compressed GBRG Bayer format used by the gspca driver.
`V4L2_PIX_FMT_SGRBG10` | 'DA10' | 10 bit raw Bayer, expanded to 16 bits.
`V4L2_PIX_FMT_SGRBG10DPCM8` | 'DB10' | 10 bit raw Bayer DPCM compressed to 8 bits.
`V4L2_PIX_FMT_PAC207` | 'P207' | Compressed BGGR Bayer format used by the gspca driver.
`V4L2_PIX_FMT_MR97310A` | 'M310' | Compressed BGGR Bayer format used by the gspca driver.
`V4L2_PIX_FMT_OV511` | 'O511' | OV511 JPEG format used by the gspca driver.
`V4L2_PIX_FMT_OV518` | 'O518' | OV518 JPEG format used by the gspca driver.
`V4L2_PIX_FMT_PJPG` | 'PJPG' | Pixart 73xx JPEG format used by the gspca driver.
`V4L2_PIX_FMT_SQ905C` | '905C' | Compressed RGGB bayer format used by the gspca driver.
`V4L2_PIX_FMT_MJPEG` | 'MJPG' | Compressed format used by the Zoran driver
`V4L2_PIX_FMT_PWC1` | 'PWC1' | Compressed format of the PWC driver.
`V4L2_PIX_FMT_PWC2` | 'PWC2' | Compressed format of the PWC driver.
`V4L2_PIX_FMT_SN9C10X` | 'S910' | Compressed format of the SN9C102 driver.
`V4L2_PIX_FMT_SN9C20X_I420` | 'S920' | YUV 4:2:0 format of the gspca sn9c20x driver.
`V4L2_PIX_FMT_WNVA` | 'WNVA' | Used by the Winnov Videum driver, http://www.thedirks.org/winnov/
`V4L2_PIX_FMT_TM6000` | 'TM60' | Used by Trident tm6000
`V4L2_PIX_FMT_YYUV` | 'YYUV' | unknown