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Microsoft Screen Codec

2012-06-20T08:36:40Z

Nkd: Add MSS2 spec

Also known as Windows Media Screen Codec.

* FourCCs: MSS1, MSS2, MSA1
* Samples:
** http://samples.mplayerhq.hu/V-codecs/MSS1/
** http://samples.mplayerhq.hu/V-codecs/MSS2/

MSA1 is created by Live Meeting 2007

== Some details about format ==

Both MSS1 and MSS2 are quite close (thus are decoded with single decoder). They employ arithmetic coding - real one, with probability coding. This coding is used with several adaptive models, which look a bit like PPM.

=== MSS1 details ===

MSS1 (aka Windows Media Screen V7 codec) compresses only palettised images.

==== Extradata format ====
(for some reason, data in .wmv is stored in big-endian order)

4- 7 header length
8-11 major version (1 for MSS1, 2 for MSS2)
12-15 minor version
16-19 display width
20-23 display height
24-27 coded width
28-31 coded height
32-35 frames per second (float)
36-39 bitrate
40-43 max lead time (float)
44-47 max lag time (float)
48-51 max seek time (float)
52-55 nFreeColors
56-823 palette (256 RGB triplets)
Only for MSS2:
824-827 threadingSplit (domain: -1, 0, 1..codedH)
828-831 numSymbolsEscapeModel (domain: 0..256)

Both width and height must be in the range 1..4096.

==== Frame format ====

Codec uses arithmetic decoders for all operations and adaptive models. All code for them is suspiciously similar to the one in [http://www.stanford.edu/class/ee398a/handouts/papers/WittenACM87ArithmCoding.pdf | 1987 paper by Witten, Neal and Cleary].

Codec uses delta compression and can change top palette entries with every intra frame:

is_inter = coder->decode_bit();
if (!is_inter) {
if (nFreeColors) {
num_entries = coder->decode_number(nFreeColors + 1);
for (i = 0; i < num_entries; i++) {
pal[(256 - nFreeColors) + i].R = coder->decode_bits(8);
pal[(256 - nFreeColors) + i].G = coder->decode_bits(8);
pal[(256 - nFreeColors) + i].B = coder->decode_bits(8);
}
}
recursive_decode_intra(0, 0, width, height);
} else {
recursive_decode_inter(0, 0, width, height);
}

Frame coding is done by recursively partitioning picture horizontally or vertically and coding partitions in some way:

recursive_decode_intra(x, y, width, height) {
mode = coder->decode_model(split_mode_model);
switch (mode) {
case 0:
pivot = decode_pivot(height);
recursive_decode_intra(x, y, width, pivot);
recursive_decode_intra(x, y + pivot, width, height - pivot);
break;
case 1:
pivot = decode_pivot(width);
recursive_decode_intra(x, y, pivot, height);
recursive_decode_intra(x + pivot, y, width - pivot, height
break;
case 2:
mode = coder->decode_model(intra_decode_model);
if (!mode) {
pix = decode_pixel();
fill_rect(x, y, width, height, pixel);
} else {
decode_area(x, y, width, height);
}
break;
}
}

recursive_decode_inter(x, y, width, height) {
mode = coder->decode_model(split_mode_model);
switch (mode) {
case 0:
pivot = decode_pivot(height);
recursive_decode_inter(x, y, width, pivot);
recursive_decode_inter(x, y + pivot, width, height - pivot);
break;
case 1:
pivot = decode_pivot(width);
recursive_decode_inter(x, y, pivot, height);
recursive_decode_inter(x + pivot, y, width - pivot, height
break;
case 2:
mode = coder->decode_model(inter_decode_model);
if (!mode) {
pix = decode_pixel();
// same meaning as mask values, see below
// for MSS2, pix == 4 means a motion compensated rectangle
if (pix != 0xFF) {
copy_rect(x, y, width, height, pixel);
} else {
mode = coder->decode_model(intra_decode_model);
if (!mode) {
pix = decode_pixel();
fill_rect(x, y, width, height, pixel);
} else {
decode_area(x, y, width, height);
}
}
} else {
// this decoded change mask first and then
// checks - if mask value is 0xFF then decode pixel
// otherwise copy if from the previous frame
mask = decode_area(x, y, width, height);
decode_area_masked(x, y, width, height);
}
break;
}
}

Mask values:
{| border="1"
! Type !! Value in MSS1 !! Value in MSS2
|-
| copy from same location || 0x80 || 0x02
|-
| copy motion compensated || N/A || 0x04
|-
| decode new || 0xFF || 0x01
|-
|}

In decode_area_masked(), decode new pixels as described in "Context modeller" even if the neighboring pixels were copied.

==== other decoding routines ====

Decoding pivot point:

decode_pivot(ref_value) {
edge = coder->decode_model(edge_model);
coord = coder->decode_model(pivot_model) + 1;
if (coord > 2)
coord = coder->decode_number((ref_value + 1) / 2 - 2) + 3;
if (edge)
return ref_value - coord;
else
return coord;
}

Decoding pixels is not that trivial.
Codec uses neighbour pixels (left, top-left, top, top-right) to form a cache which is used
along with cached move-to-front queue and several models to restore pixel.

==== Models ====

Models are reinitialised at every intraframe. Initially all symbols have weigth = 1.
With every update weight is increased by one and when they're too large they get rescaled.

Rescaling weights is performed when total cumulative probability is bigger than threshold, which can be static or adaptive.
Static threshold is calculated as <code>num_symbols * symbol_threshold</code>, adaptive one is recalculated every time as
<code>min(0x3FFF, ((2 * weights[num_symbols] - 1) / 2 + 4 * cumulative_probability[0]) / (2 * weights[num_symbols] - 1))</code>.

Scaling weights is simply <code>weight' = (weight + 1) >> 1</code>.

Main models:

{| border="1"
! Name !! Purpose !! Number of symbols !! Threshold per symbol
|-
| intra_decode_model || region decoding mode for intra (solid fill or not) || 2 || adaptive
|-
| inter_decode_model || region decoding mode for inter (full region decoder or masked) || 2 || adaptive
|-
| split_mode_model || region split mode (horizontal/vertical/none) || 3 || 50
|-
| edge_model || signals from which edge pivot point is decoded || 2 || 50
|-
| pivot_model || rough coordinates for pivot point (1, 2, escape) || 3 || 15
|-
|}

==== Context modeller ====

Context modeller is used for modelling pixel context by using its neighbours and caching last decoded values.
There are two context modellers used by decoder — one for decoding picture data (in both kinds of frames),
another one is used solely for decoding mask in interframes.

Modeller components (values in {brackets} are for MSS2):

* last decoded pixels cache (8 for picture data, 2 {3} for mask), initially filled with 0, 1, 2... and reset to that every intraframe
* primary model for decoding pixel (<code>(cache_size + 1)</code> symbols, <code>15</code> symbol threshold)
* escape model for decoding pixel value not in cache (<code>256</code> {<code>numSymbolsEscapeModel</code>} symbols, <code>50</code> symbol threshold)
* secondary models for context-modelled pixels, four layers of models for different combinations of non-equal neighbours:
** first layer - 1x4 models (<code>2</code> symbols, adaptive symbol threshold)
** second layer - 7x4 models (<code>3</code> symbols, <code>15</code> symbol threshold)
** third layer - 6x4 models (<code>4</code> symbols, <code>15</code> symbol threshold)
** fourth layer - 1x4 models (<code>5</code> symbols, <code>15</code> symbol threshold)

Decoding top left pixel (for it no neighbourhood is provided):

val = coder->decode_model(modeller->primary_model);
if (val < modeller->cache_size) {
pix = modeller->cache[pix];
if pix is found in the provided neighbourhood, insert it to the first position in the cache
(it doesn't matter if it's already in the cache)
else move it to the first position shifting other values by one
} else {
pix = coder->decode_model(modeller->escape_model);
if pix is found in cache, move it to the first position shifting other values by one
else just insert it at the first position in cache
}

Decoding other pixels:

get neighbourhood (left, top, top-right and top-left pixels)
select secondary model depending on neighbourhood
if decoded value is less than number of neighbours, pick corresponding neighbour
else decode pixel like top left one but provide neighbourhood for the reference this time

Determine neighborhood as:
If top pixel isn't available (first row): top = top-right = top-left = left
(left is available, as it was decoded above)

If right pixel isn't available (last column): top-right = top

If left pixel isn't available (first column): left = top-left = top

If neither right nor left are available (single column): top-right = top-left = left = top

note: pixels outside the current area aren't considered available

Determine secondary model as:

layer = number of different neighborhoods (1 if all equal, 4 if all different, 2 if ABBB or AABB
or ABBA or any other such combination, 3 if ABCC or ABBC or ABCA or any other such combination)

sublayer = identify which neighborhoods are equal to each other. For example:

if layer == 1: # all equal
sublayer = 0

if layer == 2: # 2-2 or 3-1
if top == topLeft:
if topRight == topLeft:
sublayer = 3
elsif left == topLeft:
sublayer = 2
else:
sublayer = 4
elsif topRight == topLeft:
if left == topLeft:
sublayer = 1
else:
sublayer = 5
else
if left == topLeft:
sublayer = 6
else:
sublayer = 0

if layer == 3: # 2-1-1
if top == topLeft:
sublayer = 0
elsif topRight == topLeft:
sublayer = 1
elsif left == topLeft:
sublayer = 2
elsif topRight == top:
sublayer = 3
elsif left == top:
sublayer = 4
else
sublayer = 5

if layer == 4: # all different
sublayer = 0

subsublayer = 0
if left-left pixel is available (column >= 2) and its value is equal to the left pixel:
subsublayer += 1
if top-top pixel is available (row >= 2) and its value is equal to the top pixel:
subsublayer += 2

Last decoded pixels cache use:

This cache internally has 4 more entries (12 total for picture data, 6 {7} for mask). The extra entries are to skip neighboring colors which we already know aren't the ones we're looking for.

Example:

Get neiborhood pixels, in this order: topLeft = 140, top = 134, topRight = 140, left = 136
Remove duplicates: [140, 134, 136]
We have 3 unique colors, therefore we use the third layer in the secondary model.
Since topRight == topLeft, we use sublayer 1. The subsublayer doesn't mater for the sake of this example.

Now we fetch a value x using the corresponding secondary model:

if x == 0, output 140

if x == 1, output 134

if x == 2, output 136

if x == 3, the secondary model can't code the color. Fall back to the primary model to try and decode it from the cache.

Assume the cache contents are [25, 140, 136, 134, 50, 23, ...

If the primary model returned 0, output 25

If it returned 1, since we know the color isn't 134, 136, or 140, output 50

If it returned 2, output 23, and so on, until 8 which means the color isn't in the cache either and we have to fall back to the escape model. In this example, the last cache entry was unreachable. For the top-left pixel, there are zero neighbors and the last 4 entries are unreachable.

=== MSS2 (Windows Media Video 9 Screen codec) details ===

In MSS2, the frame header, RLE modes, palette updates, motion vector coding
and WMV9 data are not arithmetic coded, whereas the rectangle info data and
paletted recursive subdivision modes are. Each block is byte-aligned and
consumes a integral number of bytes. The coders have to be re-initialized
between blocks, even if they are of the same type.

alignByteStream() {
< Align to byte boundary discarding any partially read bytes.
When using VLC decoding, use
get_bits_count() + 7 >> 3;
to determine the number of consumed bytes. For the AC
portions, use
ac2_get_consumed_byes(); >
}

RLE555Decode(x, y, w, h) {
//outputs RGB555

if (!isIntra) {
x = get_bits(12);
w = x - get_bits(12) + 1;
y = y + get_bits(12);
h = y - get_bits(12) + 1;
}

for each pixel in the (x, y, w, h) rectangle:
read a byte, and switch:
0..127, 134..255:
read another byte b and put the color (byte<<8 && b)
#note that if the byte was >133, this will generate colors >32767
128: copy from prev line
129: copy from prev frame (leave unchanged)
130..133:
r = 0
repeat (value-130) times:
r = (r << 8) + (read another byte)
r += 1
repeat the previous decoded symbol r times
#the previous symbol can be a color or a copy instruction
}

RLEDecode(x, y, w, h) {
if (!isIntra) {
x = get_bits(12);
y = get_bits(12);
w = get_bits(12) + 1;
h = get_bits(12) + 1;
}

// This mode uses a single tree of VLC codes.
// It is built using the code lengths, which are read as follows:
usedCodes = 0
currentCodeLength = 1
loop:
remainingCodes = (1 << currentCodeLength) - usedCodes
codesOfThisLength = get_bits(ceil_log2(remainingCodes + 1))
if codesOfThisLength == remainingCodes:
we're done, all of the remainign codes have the current length
otherwise, for each codesOfThisLength:
x = get 8 bits
if x < 190:
addcode(get_bits1() + (x << 1) - 190, currentCodeLength)
if x < 204 - isIntra:
addcode(x, currentCodeLength)
#if x >= 204:
addcode(x + 14 - isIntra, currentCodeLength)
usedCodes = (usedCodes + codesOfThisLength) << 1
currentCodeLength++

// main decoding loop
for each pixel in the (x, y, w, h) rectangle:
read an VLC code using the tree generated above, and switch:
0-255: put that color
256-267:
q = value - 256
if q == 11:
q = get_bits(4) + 10
if !q: r=1
else: r = get_bits(q) + 1
while q--:
r += 1 << q
repeat the previous symbol r times
268: copy from prev line
269: copy from prev frame (leave unchanged)

alignByteStream();
}

SubDivDecode(contextSet, x, y, w, h) {
< load the corresponding contextSet, with includes
all the 5 main models plus both color contexts >
if (isIntra) {
reset_contextSet();
recursive_decode_intra(x, y, w, h); // same as MSS1, with the differences outlined above
} else
recursive_decode_inter(x, y, w, h); // same as MSS1, with the differences outlined above
alignByteStream();
}

The WMV9 rectangle coordinates are read like a tree, but
we're only interested on the list that will be placed on root_rect.children.
There shouldn't be any grandchildren. levelIsPal determines whether
the root rect is a paletted subdivision node and its children are WMV9 nodes
or viceversa. In practice it should always be 1. There's a limit of 20 WMV9
rectangles per frame.

WMV9RectRecursive(rect, offset, depth, levelIsPal, flagsRead) {
int n = 0;
while (ac2_get_bit()) {
if (!n)
new_rect.x = ac2_get_number(rect.w);
else
new_rect.x = ac2_get_number(rect.h - rect.children[n-1].x) +
rect.children[n-1].x;
new_rect.y = ac2_get_number(rect.h);
new_rect.w = ac2_get_number(rect.w - new_rect.x);
new_rect.h = ac2_get_number(rect.h - new_rect.y);
new_rect.children = [];
rect.children += new_rect; // append to list
n++;
}
if (!levelIsPAL && !flagsRead) {
if (offset == 0) {
if (maskWMV9 = ac2_get_bit())
maskWMV9color = ac2_get_number(256);
}
WMV9RectIsCoded[offset] = ac2_get_number(2);
flagsRead = 1;
}
n = 0;
foreach crect in rect.children {
WMV9RectRecursive(crect, n, depth + 1, !levelIsPal, flagsRead);
n++;
}
}

WMV9RectInfoDecode() {

topLevelIsPal = ac2_get_bit();

root_rect.x = 0;
root_rect.y = 0;
root_rect.w = codedWidth;
root_rect.h = Height;
root_rect.children = [];

WMV9RectRecursive(root_rect, 0, 0, topLevelIsPal, 0);

alignByteStream();
}

decodeWMV9Rect(rect) {
< Initialize a WMV9 decoder. The sequence header, which is usually
stored in the extradata for WM9 files, is implicit here, as follows:

codec tag = MKTAG('W', 'M', 'V', '9')
codedWidth/Height = rect.w/h (might be odd, round up if necessary)

profile = PROFILE_MAIN
res_y411 = 0
res_sprite = 0
frmrtq_postproc = 7
bitrtq_postproc = 31
s.loop_filter = 1
res_x8 = 0
multires = 0
res_fasttx = 1
fastuvmc = 0
extended_mv = 0
dquant = 1
vstransform = 1
res_transtab = 0
overlap = 0
s.resync_marker = 0
rangered = 0
s.max_b_frames = 0
quantizer_mode = 0
finterpflag = 0
res_rtm_flag = 1

Then read the frame header (ff_vc1_parse_frame_header()) and
frame data (vc1_decode_i_blocks()).

If maskWMV9 is set, blit the resulting picture only over the pixels
that had maskWMV9color. >
}

==== Main frame decoding ====

===== Header =====

isIntra = get_bits1();
if (isIntra)
skip_bits(7);
hasWMV9 = getBits1();
hasMotionVector = isIntra ? 0 : getBits1();
isRLE = getBits1();
isRLE555 = isRLE && getBits1();
if (threadingSplit > 0)
splitPosition = threadingSplit
else if (threadingSplit == -1) {
if (getBits1()) {
if (getBits1()) {
if (getBits1())
splitPosition = get_bits(16);
else
splitPosition = get_bits(12);
} else splitPosition = get_bits(8) << 4;
splitPosition = readSplit
} else {
splitPosition = isIntra ? codedHeight / 2 : oldSplitPosition;
}
oldSplitPosition = splitPosition;
}

if (threadingSplit) {
rectangle1 = {0, 0, codedWidth, splitPosition};
rectangle2 = {0, splitPosition, codedWidth, codedHeight - splitPosition};
} else {
rectangle1 = {0, 0, codedWidth, codedHeight};
}

alignByteStream();

===== Frame =====

if (isRLE555) {
RLE555Decode(rectangle1);
if (threadingSplit)
RLE555Decode(rectangle2);
return;
}

if (hasWMV9) WMV9RectInfoDecode();
if (isIntra)
getPalette(); /* similar to MSS1, but here both the number of
changed colors and the colors themselves are
directly read as bytes */
else {
if (hasMotionVector) {
mvX = get_bits(16) - codedWidth;
mvY = get_bits(16) - codedHeight;
} else {
mvX = mvY = 0;
}
}

if (isRLE) {
RLEDecode(rectangle1);
if (threadingSplit)
RLEDecode(rectangle2);
} else {
SubDivDecode(contextSet1, rectangle1);
if (threadingSplit)
SubDivDecode(contextSet2, rectangle2);
}

if (hasWMV9) {
int i = 0;
foreach rect in root_rect.children {
if (WMV9RectIsCoded[i]) {
WMV9codedFrameSize = get_le24();
decodeWMV9Rect(rect);
} else {
fillGrey(rect); // fill with 128,128,128
}
i++;
}
}

==== V2 Arithmetic Coder ====

void ac2_init(AC2 *c, GetByteContext *gb)
{
c->low = 0;
c->high = 0xFFFFFF;
c->value = bytestream2_get_be24(gb);
c->gb = gb;
}

void ac2_renorm(AC2 *c)
{
while ((c->high >> 15) - (c->low >> 15) < 2) {
if ((c->low ^ c->high) & 0x10000) {
c->high ^= 0x8000;
c->value ^= 0x8000;
c->low ^= 0x8000;
}
c->high = c->high << 8 & 0xFFFFFF | 0xFF;
c->value = c->value << 8 & 0xFFFFFF | bytestream2_get_byte(gb);
c->low = c->low << 8 & 0xFFFFFF;
}
}

int ac2_get_bit(AC2 *c);
// Identical to its MSS1 counterpart, except it renormalizes using ac2_renorm()

/* decodes a number dividing the range into two linear pieces: one
whose values have probability 1 and another whose values have
probability 2, so that it maps to n values ( range/2 < n <= range ) */
int ac2_get_scaled_value(int value, int n, int range) {
split = (n << 1) - range;
if (value > split)
return split + (value - split >> 1);
else
return value;
}

/* rescales the interval considering the piecewise linear division */
void ac2_rescale_interval(AC2 *c, int range,
int low, int high, int n) {
split = (n << 1) - range;

if (high > split)
c->high = split + (high - split << 1);
else
c->high = high;

c->high += c->low;

if (low > split)
c->low += split + (low - split << 1);
else
c->low += low;
}

int ac2_get_number(AC2 *c, int n)
{
int range = c->high - c->low + 1;
int scale = av_log2(range) - av_log2(n);
int val;

if ( n << scale > range )
scale--;

n <<= scale;

val = ac2_get_scaled_value(c->value - c->low, n, range) >> scale;

ac2_rescale_interval(c, range, val << scale, (val + 1) << scale, n);

ac2_renorm(c);

return val;
}

int ac2_get_prob(AC2 *c, int *probs)
{
int range = c->high - c->low + 1, n = *probs;
int scale = av_log2(range) - av_log2(n);
int i = 0, val;

if ( n << scale > range )
scale--;

n <<= scale;

val = ac2_get_scaled_value(c->value - c->low, n, range) >> scale;
while (probs[++i] > val) ;

ac2_rescale_interval(c, range, probs[i] << scale, probs[i-1] << scale, n);

return i;
}

int ac2_get_model_sym(AC2 *c, Model *m);
// Identical to its MSS1 counterpart, except it gets the symbol index
// using ac2_get_prob() and renormalizes using ac2_renorm()

int ac2_get_consumed_byes(AC2 *c)
{
int diff = (c->high >> 16) - (c->low >> 16);
int bp = bytestream2_tell(c->gb) - 3 << 3;
int bits = 1;

while (!(diff & 0x80)) {
bits++;
diff <<= 1;
}

return (bits + bp + 7 >> 3) + ((c->low >> 16) + 1 == c->high >> 16);
}

[[Category:Video Codecs]]
[[Category:Screen Capture Video Codecs]]
[[Category:Undiscovered Video Codecs]]

Microsoft Screen Codec

2012-06-12T09:01:24Z

Nkd: Add details about context and cache color decoding

Microsoft Screen Codec

2012-06-11T17:56:13Z

Nkd: Added MSS2 differences for the recursive decoder and model selection details