y4mscaler - Online in the Cloud

PROGRAM:

NAME

y4mscaler - Scale/crop/translate a YUV4MPEG2 stream

SYNOPSIS

y4mscaler [options] < Y4Mstream > Y4Mstream

DESCRIPTION

y4mscaler is a general-purpose video scaler which operates on YUV4MPEG2 streams, as
produced and consumed by the MJPEGtools such as lav2yuv and mpeg2enc(1).

y4mscaler is meant to be used in a pipeline. Thus, input is from stdin, and output is to
stdout.

The essential function of y4mscaler is to scale a specified "active" region of the input
stream (the source) into a specified active region of the output stream (the target).
Pixels outside of the active region of the source are ignored; pixels outside of the
active region of the target are filled with a background color. The source may
additionally have a matte applied to it; pixels outside the source matte are set to a
separately specified background color.

y4mscaler correctly handles chroma subsampling, and thus it can also perform chroma
subsampling conversions. The YUV4MPEG2 stream format supports three varieties of 4:2:0
subsampling, as well as 4:1:1, 4:2:2, 4:4:4, a 4:4:4 modes with an alpha channel, and a
monochrome luma-only mode. (See "NOTES ON CHROMA MODES AND SUBSAMPLING".)

y4mscaler can perform simple interlacing conversions: switching from top-field-first to
bottom-field-first and vice-versa (by lossily discarding the first field), and creating a
progressive stream from interlaced by discarding every other field (effectively halving
the vertical resolution).

The source and target are defined by many, many parameters, but y4mscaler has many, many
heuristics built-in to automagically set them appropriately. Most source parameters are
taken from the input stream header. Remaining source and target parameters which are not
specified by the user are guessed in a sane manner.

y4mscaler includes preset parameters for a number of common target streams: DVD, VideoCD
(VCD), SuperVCD (SVCD), associated still image formats, and DV.

EXAMPLES

To create a stream appropriate for use in an SVCD:

y4mscaler -O preset=svcd

To create a stream for a VideoCD (a non-interlaced format), from a DV source (an
interlaced format), shifting the input frame 4 pixels to the left:

y4mscaler -I ilace=bottom-only -I active=-4+0cc -O preset=vcd

To take a widescreen NTSC DV source, and convert it to a letterboxed stream, with blue
bars on the top and bottom:

y4mscaler -O sar=ntsc -O bg=RGB:0,0,255

To take a widescreen NTSC DV source, and convert it to a "fullscreen" stream (i.e. the
sides are clipped, just like on TV):

y4mscaler -O sar=ntsc -O infer=clip

To take a centered, letterboxed NTSC source, and convert it to a widescreen (16:9) format
stream for DVD, with the black bars removed:

y4mscaler -O preset=dvd -O sar=ntsc_wide -O infer=clip

To take the center 100x100 pixel chunk of an NTSC DV stream, surround it with a 20-pixel
blue border, and blow that up to a full-screen SuperVCD stream:

y4mscaler -I active=140x140+0+0cc -I matte=100x100+0+0cc -I bg=RGB:0,0,255 -O
preset=svcd

OPTIONS

The first three options, -v, -V, and -h, are simple straightforward options which take
either no arguments or one numeric argument.

-v [0,1,2]
Set verbosity level.
0 = warnings and errors only.
1 = add informative messages, too (default).
2 = add chatty debugging message, too.

-V Show version information and exit.

-h Show a help message (synopsis of options).

The -I, -O, and -S options each take one argument of the form parameter=value, which
specify parameters for the input, output, and scaling, respectively. These options can be
used repeatedly to specify multiple parameters. The parameter names and values are not
case-sensitive. Definitions of the form "parameter=[AAA|BBB|CCC]" mean that only one of
the listed keywords AAA, BBB, or CCC may be chosen. Succeeding options will override
earlier ones.

-I input_parameter
Specify parameters for the source/input stream. All '-I' arguments are evaluated in
order, and later arguments on the command-line will override earlier ones. All '-I'
arguments are evaluated before any '-O' arguments.

active=WxH+X+Yaa
Specify the active region of the source frame, which is scaled to fit the active
region of the target frame. The default is the full frame. (The "WxH" may be
omitted, and the region size defaults to the size of of the source frame.) W and
H are width and height. X and Y are the offset of the anchor point. "aa" is the
anchor mode (default: TL); see "NOTES ON REGION GEOMETRY" for details.
Example: active=200x180+30+24cc

matte=WxH+X+Y
Specify a matte region for the source frame. All pixels outside of this region
are set to the source background color. The default matte is the full frame. (The
"WxH" may be omitted, and the region size defaults to the size of of the source
frame.) W and H are width and height. X and Y are the offset of the anchor
point. "aa" is the anchor mode (default: TL); see "NOTES ON REGION GEOMETRY" for
details.
Example: matte=200x180+30+24cc

bg=RGB:r,g,b
bg=YCBCR:y,cb,cr
bg=RGBA:r,g,b,a
bg=YCBCRA:y,cb,cr,a
Set the source background color. Pixels outside of the source's matte region are
set to this color. One can specify the color as either a R'G'B' or Y'CbCr triplet.
For example, the default color is black, specified as "bg=YCBCR:16,128,128" or
"bg=RGB:0,0,0". The 'A' versions will set the alpha (transparency) value of the
color. The alpha range is [0,255] for RGBA and [16,235] for YCBCRA. The default
is fully-opaque (255 for RGBA, 235 for YCBCRA).

norm=[NTSC|PAL|SECAM]
Specify the "norm" of the source stream. This is normally inferred from the
stream header.

ilace=[NONE|TOP_FIRST|BOTTOM_FIRST|TOP_ONLY|BOTTOM_ONLY]
Specify the interlacing used by the source stream. NONE, TOP_FIRST, and
BOTTOM_FIRST correspond to non-interlaced, top-field-first, and bottom-field-
first. These values are normally inferred from the stream header; specifying them
will override the stream header.
TOP_ONLY and BOTTOM_ONLY specify that only the top or bottom field of each frame
should be used; the other field is discarded. These options can only be used with
an interlaced input, and cause the interlaced stream to be treated as a
progressive stream with half the height. (This is particularly useful in creating
a VCD from a full-size interlaced input stream.) These two special options can
only be used when the source is a pure progressive stream (as opposed to a
YUV4MPEG2 "mixed-mode" stream).

chromass=[420JPEG|420MPEG2|420PALDV|444|422|411|mono|444alpha]
Specify the chroma subsampling mode used in the source stream. This parameter is
inferred from the stream header, so this keyword should almost never be used in a
source specification. The only useful reason to specify this keyword is to
override one variety of 4:2:0 with another. Any other use will cause processing to
fail.

sar=N:D
sar=[NTSC|PAL|NTSC_WIDE|PAL_WIDE]
Specify the sample-aspect-ratio of the source stream. The value can either be or
numeric ratio (such as "10:11") or one of the keywords, which correspond to the
CCIR-601 values for 4:3 or 16:9 displays, respectively. This parameter is usually
inferred from the stream header.

-O output_parameter
Specify parameters for the destination/output stream. All '-O' arguments are
evaluated in order, and later arguments on the command-line will override earlier
ones. All '-O' arguments are evaluated after any '-I' arguments.

size=WxH
size=SRC
Set the output/target frame size, as width W and height H in pixels. Use the
keyword SRC to specify that the target frame size should match the source frame
size.

active=WxH+X+Yaa
Specify the active region of the target frame, into which the active region of the
source frame is scaled. The default is the full target frame. (The "WxH" may be
omitted, and the region size defaults to the size of of the target frame.) W and
H are width and height. X and Y are the offset of the anchor point. "aa" is the
anchor mode (default: TL); see "NOTES ON REGION GEOMETRY" for details.
Example: active=200x180+30+24cc

bg=RGB:r,g,b
bg=YCBCR:y,cb,cr
bg=RGBA:r,g,b,a
bg=YCBCRA:y,cb,cr,a
Set the target background color. Pixels outside of the target's active region are
set to this color. One can specify the color as either a R'G'B' or Y'CbCr triplet.
For example, the default color is black, specified as "bg=YCBCR:16,128,128" or
"bg=RGB:0,0,0". The 'A' versions will set the alpha (transparency) value of the
color. The alpha range is [0,255] for RGBA and [16,235] for YCBCRA. The default
is fully-opaque (255 for RGBA, 235 for YCBCRA).

ilace=[NONE|TOP_FIRST|BOTTOM_FIRST]
Specify the interlacing used by the target stream. NONE, TOP_FIRST, and
BOTTOM_FIRST correspond to non-interlaced, top-field-first, and bottom-field-
first. The default if to match the source stream.
If the source and target are both interlaced, but with different modes (i.e. one
is bottom-first, and the other is top-first), then y4mscaler will convert one mode
to the other by dropping the first source field.

chromass=[420JPEG|420MPEG2|420PALDV|444|422|411|mono|444alpha]
Specify the chroma subsampling mode to be used in the target stream. The default
is to match the source mode. See "NOTES ON CHROMA MODES AND SUBSAMPLING" for more
information.

sar=N:D
sar=[SRC|NTSC|PAL|NTSC_WIDE|PAL_WIDE]
Specify the sample-aspect-ratio of the source stream. The value can either be or
numeric ratio (such as "10:11") or one of the keywords, which correspond to the
CCIR-601 values for 4:3 or 16:9 displays, respectively. The keyword SRC specifies
that the target SAR should match the source.

scale=N/D
Xscale=N/D
Yscale=N/D
Set the scaling ratios, as a fraction; for example, scale=1/2. "scale=" sets both
X and Y factors simultaneously. "Xscale=" and "Yscale=" can be used to set them
independently.

infer=[PAD|CLIP|PRESERVE_X|PRESERVE_Y]
Set the mode used to infer scaling ratios from active regions and SAR's. The
keywords are mutually exclusive. The default is PAD.

infer=[SIMPLIFY|EXACT]
Set whether the above heuristic uses exact ratios, or whether it is allowed to
slightly adjust active regions to simplify the scaling ratios. The keywords are
mutually exclusive. The default is SIMPLIFY.

align=[TL|TC|TR|CL|CC|CR|BL|BC|BR]
Set the alignment point between the source and target active regions. The
keywords specify "top-left", "top-center", "top-right", etc. The specified corner
or point from the source region will be mapped to the same spot in the target
region; and cropping or padding which is applied to the active regions will
preserve this mapping. The default is CC, for "center-center", i.e. the source
and target regions are mutually centered. The keywords are mutually exclusive.
The default is CC. See "NOTES ON SOURCE AND TARGET ALIGNMENT" for details.

preset=[VCD|CVD|SVCD|DVD|DVD_WIDE|DV|DV_WIDE|
SVCD_STILL_HI|SVCD_STILL_LO|VCD_STILL_HI|VCD_STILL_LO|
ATSC_720P|ATSC_1080I|ATSC_1080P]
Use preset target parameters for several common output formats. Individual
parameters can be overridden by following with more "-O" settings. These keywords
are mutually exclusive. For the details of what settings these preset keywords
imply, see "NOTES ON TARGET PRESETS".

VCD - 352-wide VideoCD, progressive

CVD - 352-wide (full-height) ChinaVideoDisc

SVCD - 480-wide SuperVCD

DVD - 720-wide DVD

DVD_WIDE - 720-wide DVD, anamorphic pixels

DV - 720-wide DV (bottom-field-first, 4:1:1)

DV_WIDE - 720-wide DV, anamorphic pixels

SVCD_STILL_HI - high-resolution SVCD still image

SVCD_STILL_LO - low-resolution SVCD still image

VCD_STILL_HI - high-resolution VCD still image

VCD_STILL_LO - low-resolution SVCD still image

ATSC_720P - ATSC 720p (progressive HDTV)

ATSC_1080I - ATSC 1080i (interlaced HDTV)

ATSC_1080P - ATSC 1080p (HDTV)

-S scaling_parameter
Specify parameters for the scaling engine. All '-S' arguments are evaluated in
order, and later arguments on the command-line will override earlier ones.

mode=MONO
Request monochrome scaling. The source is treated as monochrome and its chroma
channels are ignored. The chroma channels of the output stream will be zeroed to
yield a grayscale output.

mode=LINESWITCH
Request line swapping. Effectively, the top and bottom fields within each frame
will be swapped. This may help with malformed streams that have a messed up
spatial order. This option is only effective on interlaced streams.

scaler=scaler-name
Use a particular scaling engine. The available engines are:
'default' - Matto's Generic Scaler (the default)

option=scaler-option
Specify an option for the chosen scaling engine. To see all the available
options, use "option=help".

For the default engine, the available scaler-options select the filter kernel:

box - box filter

linear - linear interpolation

quadratic - quadratic interpolation

cubic - cubic interpolation, Mitchell-Netravali spline

cubicCR - cubic interpolation, Catmull-Rom spline

cubicB - cubic interpolation, B-spline

cubicK4 - Keys 4th-order cubic

sinc:N - sinc with Lanczos window, N cycles

To select kernels for the x and y scaling directions independently, use two kernel
names separated by a comma, e.g. option=box,quadratic.

sinc:N will give the best quality results (least aliasing), but is the slowest.
The quality improves with larger values of N, as does processing time. cubic is
generally regarded in the graphics world as the 3rd-order cubic spline with the
best trade-off between smoothing and aliasing. box yields the worst quality
results (most aliasing), but is the fastest. The default kernel is cubicK4, which
has a flatter passband and sharper cutoff than cubic. (It requires the same
computational power as sinc:4, but produces less ringing artifacts.)

NOTES ON TARGET PRESETS

The following table details the settings provided by the various target "preset="
keywords. When two values are given the primary is for NTSC streams; the value in
{braces} is for PAL streams. If interlace value is unspecified, it is inherited from the
source, otherwise the indicated target interlacing is required.

Preset Frame Size Interlace SAR Subsampling
-----------------------------------------------------------------------
VCD 352x240{288} none 10:11{59:54} 4:2:0-JPEG
CVD 352x480{576} --- 20:11{59:27} 4:2:0-MPEG2
SVCD 480x480{576} --- 15:11{59:36} 4:2:0-MPEG2
DVD 720x480{576} --- 10:11{59:54} 4:2:0-MPEG2
DVD_WIDE 720x480{576} --- 40:33{118:81} 4:2:0-MPEG2
DV 720x480{576} bottom-first 10:11{59:54} 4:1:1
DV_WIDE 720x480{576} bottom-first 40:33{118:81} 4:1:1
SVCD_STILL_HI 704x480{576} none 10:11{59:54} 4:2:0-MPEG2
SVCD_STILL_LO 480x480{576} none 15:11{59:36} 4:2:0-MPEG2
VCD_STILL_HI 704x480{576} none 10:11{59:54} 4:2:0-JPEG
VCD_STILL_LO 352x240{288} none 10:11{59:54} 4:2:0-JPEG
ATSC_720p 1280x720 none 1:1 4:2:0-MPEG2
ATSC_1080i 1920x1080 (required) 1:1 4:2:0-MPEG2
ATSC_1080p 1920x1080 none 1:1 4:2:0-MPEG2

NOTES ON REGION GEOMETRY

Active and matte regions are specified using a geometry string of the form "WxH+X+Yaa".
The "WxH" part specifies the size of the region, as a Width and Height in pixels. (In
some cases, the "WxH" may be omitted, and the region size defaults to the full frame
size.) The "+X+Y" specifies the position of the region, as an offset relative to the
anchor point specified by "aa".

The "aa" code can be one of TL, TC, TR, CL, CC, CR, BL, BC, or BR. These stand for "top-
left", "top-center", ..., "bottom-center", "bottom-right". These codes are not case-
sensitive.

The "+X+Y" specifies the offset of the region's anchor point from the frame's anchor
point. For example, "+20+30TL" means that the top-left corner of the region will be
offset 20 pixels to the right and 30 pixels down from the top-left corner of the frame.

The offset values can also be negative. For example, "-4+0CC" means that the center
(vertical and horizontal) of the region is offset 4 pixels to the left of the center of
the frame.

The default anchoring point for geometry strings is TL, i.e. the top-left corner.

NOTES ON SOURCE AND TARGET ALIGNMENT

Often, the source and target active regions do not match exactly. This happens when,
using the given or calculated scaling ratios, the source region scales to a different size
or shape than the target region. In this case, the source and target regions are mutually
clipped, so that only the portion of the source which fits will be scaled into the target.

Before any clipping or padding, the source and target regions are aligned so that the
points specified via the "align=aa" parameter coincide. The "aa" code specifies an anchor
point as described above.

For example, "align=BC" specifies that the bottom-center of the source region should get
mapped to the bottom-center of the target region. In other words, the source region will
be horizontally centered and vertically aligned to the bottom of the target region before
clipping:

---------------- source
|abcdefghijklmn|
---|opqrstuvwxyz01|--- target ----------------
| |234567890ABCDE| | |234567890ABCDE|
| |FGHIJKLMNOPQRS| | |FGHIJKLMNOPQRS|
| |TUVWXYZabcdefg| | |TUVWXYZabcdefg|
---------------------- ----------------
Before Mutually Clipped

If instead "align=TR" were centered, the source would be clipped in a different place, and
scaled into a different region of the target frame:

---------------------- ----------------
| |abcdefghijklmn| |abcdefghijklmn|
| |opqrstuvwxyz01| |opqrstuvwxyz01|
| |234567890ABCDE| |234567890ABCDE|
------|FGHIJKLMNOPQRS| ----------------
target |TUVWXYZabcdefg| source
----------------
Before Mutually Clipped

The default alignment mode is "CC", that is, the source and target are mutually centered.

NOTES ON SCALE FACTOR INFERENCE

If the X and Y scaling factors are not explicitly provided, y4mscaler will infer the
factors from the source and target active regions and sample aspect ratios (SAR's).

If the active regions are not compatible shape-wise (given the SAR's), the source and
target regions will be clipped or padded according to one of four policies. The policy is
selected using the "infer=" parameter and one of the keywords PAD, CLIP, PRESERVE_X, or
PRESERVE_Y. (The default is PAD.)

PAD
Pick scaling factors which will pad the source, but ensure that all of the source
image content ends up in the target.

CLIP
Pick scaling factors which will clip the source, but which will fill the target
region as much as possible.

PRESERVE_X
Pick scaling factors which preserve as much of the horizontal source content as
possible.

PRESERVE_Y
Pick scaling factors which preserve as much of the vertical source content as
possible.

The policy is further affected by a choice of two other keywords, SIMPLIFY, or EXACT.
(The default is SIMPLIFY.)

EXACT
Calculate exact scaling factors.

SIMPLIFY
Adjust the active regions and scaling factors (within 10% or so), to simplify the
ratios as much as possible. (For example, crop or pad slightly to achieve a ratio
of 2/1 rather than 45/22.)

NOTES ON CHROMA MODES AND SUBSAMPLING

y4mscaler can convert streams from one chroma subsampling mode to another. Such
conversions are always lossy operations, even if the overall frame is undergoing 1/1
scaling.

y4mscaler will infer the source's subsampling mode from tags in the input stream header.
The target presets ("preset=XXX") will attempt to set the target subsampling mode
appropriately. Otherwise, by default the target subsampling mode will match the source.
One can explicitly set the subsampling mode for the source and/or the target by using the
"chromass=" parameter.

y4mscaler is capable of reading and writing streams in the 4:4:4, 4:2:2, 4:1:1, and 4:2:0
(all three varieties) subsampling modes. The first three, however, are a relatively new
addition to the YUV4MPEG2 standard, and many MJPEGtools will fail to process them
correctly, if at all. smil2yuv and raw2yuv can produce native 4:1:1 streams from NTSC DV
video, which can then be converted to 4:2:0 by y4mscaler before further processing by
other tools.

If the source has an alpha-channel (i.e. 444ALPHA mode) and the target does not, the alpha
channel will simply be discarded. On the other hand, if the target has an alpha-channel
but the source does not, a constant alpha-channel will be created using the alpha-value of
the target's background color (as set by "-O bg="). The default is fully-opaque.

Similarly, if the target has chroma channels but the source does not (i.e. a luma-only
MONO stream), then the chroma channels in the output will be set according to the
background color.

NOTES ON ANOMALOUS INTERLACE MIXTURES

The YUV4MPEG2 format allows for "mixed-mode interlacing" streams, which may contain a
mixture of progressive and interlaced frames. Each frame is tagged as temporally
interlaced or progressive, and vertically-subsampled frames (4:2:0 formats) are further
tagged as spatially interlaced or not. Unfortunately, this allows for the possibility of
anomalous frames, which happen to be temporally interlaced (fields sampled at different
times) but spatially progressive (subsampling performed across entire frame), or vice-
versa. The only reasonable thing to do with such anomalous frames is to vertically-
upsample the chroma, essentially making to problem go away as quickly as possible.

y4mscaler will only process such frames if the target output format is non-vertically-
subsampled (e.g. 4:4:4, 4:2:2, etc.) and no other vertical processing is required.
Otherwise y4mscaler will bail on processing in midstream when it encounters an anomalous
frame. If there is any possibility of encountering such an error, y4mscaler will print a
warning when processing begins.

EXIT STATUS

0 Successful program execution.

1 Usage, syntax, or operational error.

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