quant.go

Documentation: golang.org/x/image/vp8

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package vp8
     6  
     7  // This file implements parsing the quantization factors.
     8  
     9  // quant are DC/AC quantization factors.
    10  type quant struct {
    11  	y1 [2]uint16
    12  	y2 [2]uint16
    13  	uv [2]uint16
    14  }
    15  
    16  // clip clips x to the range [min, max] inclusive.
    17  func clip(x, min, max int32) int32 {
    18  	if x < min {
    19  		return min
    20  	}
    21  	if x > max {
    22  		return max
    23  	}
    24  	return x
    25  }
    26  
    27  // parseQuant parses the quantization factors, as specified in section 9.6.
    28  func (d *Decoder) parseQuant() {
    29  	baseQ0 := d.fp.readUint(uniformProb, 7)
    30  	dqy1DC := d.fp.readOptionalInt(uniformProb, 4)
    31  	const dqy1AC = 0
    32  	dqy2DC := d.fp.readOptionalInt(uniformProb, 4)
    33  	dqy2AC := d.fp.readOptionalInt(uniformProb, 4)
    34  	dquvDC := d.fp.readOptionalInt(uniformProb, 4)
    35  	dquvAC := d.fp.readOptionalInt(uniformProb, 4)
    36  	for i := 0; i < nSegment; i++ {
    37  		q := int32(baseQ0)
    38  		if d.segmentHeader.useSegment {
    39  			if d.segmentHeader.relativeDelta {
    40  				q += int32(d.segmentHeader.quantizer[i])
    41  			} else {
    42  				q = int32(d.segmentHeader.quantizer[i])
    43  			}
    44  		}
    45  		d.quant[i].y1[0] = dequantTableDC[clip(q+dqy1DC, 0, 127)]
    46  		d.quant[i].y1[1] = dequantTableAC[clip(q+dqy1AC, 0, 127)]
    47  		d.quant[i].y2[0] = dequantTableDC[clip(q+dqy2DC, 0, 127)] * 2
    48  		d.quant[i].y2[1] = dequantTableAC[clip(q+dqy2AC, 0, 127)] * 155 / 100
    49  		if d.quant[i].y2[1] < 8 {
    50  			d.quant[i].y2[1] = 8
    51  		}
    52  		// The 117 is not a typo. The dequant_init function in the spec's Reference
    53  		// Decoder Source Code (http://tools.ietf.org/html/rfc6386#section-9.6 Page 145)
    54  		// says to clamp the LHS value at 132, which is equal to dequantTableDC[117].
    55  		d.quant[i].uv[0] = dequantTableDC[clip(q+dquvDC, 0, 117)]
    56  		d.quant[i].uv[1] = dequantTableAC[clip(q+dquvAC, 0, 127)]
    57  	}
    58  }
    59  
    60  // The dequantization tables are specified in section 14.1.
    61  var (
    62  	dequantTableDC = [128]uint16{
    63  		4, 5, 6, 7, 8, 9, 10, 10,
    64  		11, 12, 13, 14, 15, 16, 17, 17,
    65  		18, 19, 20, 20, 21, 21, 22, 22,
    66  		23, 23, 24, 25, 25, 26, 27, 28,
    67  		29, 30, 31, 32, 33, 34, 35, 36,
    68  		37, 37, 38, 39, 40, 41, 42, 43,
    69  		44, 45, 46, 46, 47, 48, 49, 50,
    70  		51, 52, 53, 54, 55, 56, 57, 58,
    71  		59, 60, 61, 62, 63, 64, 65, 66,
    72  		67, 68, 69, 70, 71, 72, 73, 74,
    73  		75, 76, 76, 77, 78, 79, 80, 81,
    74  		82, 83, 84, 85, 86, 87, 88, 89,
    75  		91, 93, 95, 96, 98, 100, 101, 102,
    76  		104, 106, 108, 110, 112, 114, 116, 118,
    77  		122, 124, 126, 128, 130, 132, 134, 136,
    78  		138, 140, 143, 145, 148, 151, 154, 157,
    79  	}
    80  	dequantTableAC = [128]uint16{
    81  		4, 5, 6, 7, 8, 9, 10, 11,
    82  		12, 13, 14, 15, 16, 17, 18, 19,
    83  		20, 21, 22, 23, 24, 25, 26, 27,
    84  		28, 29, 30, 31, 32, 33, 34, 35,
    85  		36, 37, 38, 39, 40, 41, 42, 43,
    86  		44, 45, 46, 47, 48, 49, 50, 51,
    87  		52, 53, 54, 55, 56, 57, 58, 60,
    88  		62, 64, 66, 68, 70, 72, 74, 76,
    89  		78, 80, 82, 84, 86, 88, 90, 92,
    90  		94, 96, 98, 100, 102, 104, 106, 108,
    91  		110, 112, 114, 116, 119, 122, 125, 128,
    92  		131, 134, 137, 140, 143, 146, 149, 152,
    93  		155, 158, 161, 164, 167, 170, 173, 177,
    94  		181, 185, 189, 193, 197, 201, 205, 209,
    95  		213, 217, 221, 225, 229, 234, 239, 245,
    96  		249, 254, 259, 264, 269, 274, 279, 284,
    97  	}
    98  )
    99
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