Apple_ProRes_White_Paper|江阴雨辰互联

2023年11月29日发(作者：什么牌子手提电脑好用耐用)

Apple ProRes

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Apple ProRes

Contents

3 Introduction

4 Apple ProRes Family Overview

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Apple ProRes

Introduction

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Apple ProRes

Family Overview

All members of the Apple ProRes family provide an unparalleled combination of

multistream, real-time editing performance coupled with impressive image quality at

reduced storage rates. Additionally, all five codecs preserve standard-definition (SD),

HD, 2K, 4K, and 5K frame sizes at full resolution.

As a variable bit rate (VBR) codec technology, Apple ProRes uses fewer bits on simple

frames that would not benefit from encoding at a higher data rate. All Apple ProRes

codecs are frame-independent (or “intra-frame”) codecs, meaning that each frame is

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Apple ProRes

• Apple ProRes 422: Apple ProRes 422 offers nearly all the benefits of its big brother,

Apple ProRes 422 (HQ), but at a significantly lower data rate. It provides visually

lossless coding performance for the same full-width, 10-bit, 4:2:2 sequences as

Apple ProRes 422 (HQ) with even better multistream real-time editing performance.

• Apple ProRes 422 (LT): Like Apple ProRes 422 (HQ) and Apple ProRes 422, the

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Properties of Digital Images

The technical properties of digital images correspond to different aspects of image

quality. For example, high-resolution HD images can carry more detail than their

lower-resolution SD counterparts. 10-bit images can carry finer gradations of color,

thereby avoiding the banding artifacts that can occur in 8-bit images.

The role of a codec is to preserve image quality as much as possible at a particular

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Chroma Sampling

Color images require three channels of information. In computer graphics, a pixel’s

color is typically defined by R, G, and B values. In traditional digital video, a pixel is

represented by Y’, C

, and C values, where Y’ is the “luma” or grayscale value and

and C contain the “chroma” or color-difference information. Because the eye is

less sensitive to fine chroma detail, it is possible to average together and encode fewer

and C samples with little visible quality loss for casual viewing. This technique,

known as chroma subsampling, has been used widely to reduce the data rate of video

signals. However, excessive chroma subsampling can degrade quality for compositing,

color correction, and other image-processing operations. The Apple ProRes family

handles today’s popular chroma formats as follows:

• 4:4:4 is the highest-quality format for preserving chroma detail. In 4:4:4 image sources,

there is no subsampling, or averaging, of chroma information. There are three unique

samples, either Y’, C

, and C or R, G, and B, for every pixel location. Apple ProRes 4444

fully supports 4:4:4 image sources, from either RGB or Y’CC color spaces. The fourth

“4” means that Apple ProRes 4444 can also carry a unique alpha-channel sample for

every pixel location. Apple ProRes 4444 is intended to support 4:4:4:4 RGB+Alpha

sources exported from computer graphics applications such as Motion, as well as

4:4:4 video sources from high-end devices such as dual-link HDCAM-SR.

• 4:2:2 is considered a high-quality, professional video format in which the chroma

values of Y’C

BRBR

C images are averaged together such that there is one C and one C

sample, or one “C

/C chroma pair,” for each Y’ (luma) sample. This minimal chroma

subsampling has traditionally been considered adequate for high-quality compositing

and color correction, although better results can be achieved using 4:4:4 sources.

4:2:2 sources are generated by many popular higher-end video camcorder formats,

including DVCPRO HD, AVC-Intra/100, and XDCAM HD422/50. All Apple ProRes 422

family members fully support the chroma resolution inherent in 4:2:2 video formats.

• 4:2:0 and 4:1:1 have the least chroma resolution of the formats mentioned here, with

just one C

/C chroma pair for every four luma samples. These formats are used in a

variety of consumer and professional video camcorders. Depending on the quality of

a camera’s imaging system, 4:2:0 and 4:1:1 formats can provide excellent viewing

quality. However, in compositing workflows it can be difficult to avoid visible artifacts

around the edges of a composited element. HD 4:2:0 formats include HDV, XDCAM HD,

and AVC-Intra/50. 4:1:1 is used in DV. All Apple ProRes 422 formats can support 4:2:0 or

4:1:1 sources if the chroma is upsampled to 4:2:2 prior to encoding.

••••••••••••••

••••

4:4:44:2:24:2:0 (interstitial siting)

Image pixel Chroma sample

4:1:1

•

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Sample Bit Depth

The number of bits used to represent each Y’, C, or C (or R, G, or B) image sample

determines the number of possible colors that can be represented at each pixel

location. Sample bit depth also determines the smoothness of subtle color shading

that can be represented across an image gradient, such as a sunset sky, without visible

quantization or “banding” artifacts.

Traditionally, digital images have been limited to 8-bit samples. In recent years the

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Properties of

Apple ProRes Codecs

Every image or video codec can be characterized by how well it behaves in three

critical dimensions: compression, quality, and complexity. Compression means data

reduction, or how many bits are required compared to the original image. For image

sequences or video streams, compression means data rate, expressed in bits/sec for

transmission or bytes/hour for storage. Quality describes how closely a compressed

image resembles the original. “Fidelity” would therefore be a more accurate term,

but “quality” is the term widely used. Complexity relates to how many arithmetic

operations must be computed to compress or decompress an image frame or

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Apple ProRes

The bar chart below shows how the data rate of each Apple ProRes format compares

to uncompressed, full-width (1920 x 1080), 4:4:4 12-bit and 4:2:2 10-bit image

sequences at 29.97 frames/sec. The chart shows that even the two highest-quality

Apple ProRes formats—Apple ProRes 4444 and Apple ProRes 422 (HQ)—offer

significantly lower data rates than their uncompressed counterparts.

Data Rates - Uncompressed and Apple ProRes at 1920 x 1080, 29.97 fps

3,000

4:4:4 formats

2,237

4:2:2 formats

2,250

1,500

1,326

750

330

UncompressedProResUncompressedProResProResProResProRes

12-bit 4:4:4444410-bit 4:2:2422 (HQ)422422 (LT)422 (Proxy)

[no alpha]

220

147

102

The data rates shown in the bar chart above are for “full-width” (1920 x 1080) HD frames

at 29.97 frames/sec. The Apple ProRes family also supports the 720p HD format at its full

width (1280 x 720). In addition to full-width HD formats, Apple ProRes codecs support

three different “partial-width” HD video formats used as the recording resolutions in a

number of popular HD camcorders: 1280 x 1080, 1440 x 1080, and 960 x 720.

The data rate of an Apple ProRes format is determined primarily by three key factors:

Apple ProRes codec type, encoded frame size, and frame rate. The chart below shows

some examples of how varying any one of these factors changes an Apple ProRes

format’s data rate. A table of data rates for a number of Apple ProRes formats

supported for real-time editing in Final Cut Pro can be found in the appendix.

Data Rates - Apple ProRes 422 (LT) versus Apple ProRes 422

ProRes 422 (LT)

ProRes 422

1440 x 1080

1920 x 1080

23.976 fps

1440 x 1080

1920 x 1080

101

117

1440 x 1080

1920 x 1080

29.97 fps

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Apple ProRes

Apple ProRes is a variable bit rate (VBR) video codec. This means that the number

of bits used to encode each frame within a stream is not constant, but varies from

one frame to the next. For a given video frame size and a given Apple ProRes

codec type, the Apple ProRes encoder aims to achieve a “target” number of bits per

frame. Multiplying this number by the frames per second of the video format being

encoded results in the target data rate for a specific Apple ProRes format.

Although Apple ProRes is a VBR codec, the variability is usually small. The actual data

rate is usually close to the target data rate. For a given Apple ProRes format, there is

also a maximum number of bits per frame that is never exceeded. This maximum is

approximately 10 percent more than the target number of bits per frame. The graph

below plots the actual number of bits used per frame in an example Apple ProRes

video sequence.

Compressed Frame Sizes - Apple ProRes 422

800000

Max

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Apple ProRes

The quality-preserving capability of the various Apple ProRes codecs can be

expressed in both quantitative and qualitative terms. In the field of image and video

compression, the most widely used quantitative measure of image fidelity is peak

signal-to-noise ratio (PSNR). PSNR is a measure of how closely a compressed image

(after being decompressed) matches the original image handed to the encoder. The

higher the PSNR value, the more closely the encoded image matches the original. The

graph below plots the PSNR value for each image frame in a test sequence for three

different codecs: Apple ProRes 422 (HQ), Avid DNxHD, and Panasonic D5.

PSNR Comparison - Apple ProRes, DNxHD, and D5

Apple ProRes 422 (HQ) (163 Mbps)

DNxHD 175X (175 Mbps)

(

)

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Apple ProRes

In addition to indicating visual fidelity, the difference in PSNR values also denotes

headroom. For example, if you were to view the original sequence used in the graph

above, and then view the Apple ProRes 422 (HQ) and Apple ProRes 422 encoded

versions of the same stream, all three would look visually identical. However, the

higher PSNR value for Apple ProRes 422 (HQ) indicates greater quality headroom. This

increased headroom means that an image sequence can be decoded and reencoded

over multiple generations and still look visually identical to the original, as shown in

the graph below:

Multigeneration PSNR

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Performance

The Apple ProRes family of codecs is designed for speed, and high speed of both

encoding and decoding is essential to avoid workflow bottlenecks.

Fast decoding is especially critical for multistream, real-time editing in Final Cut Pro.

The Apple ProRes codec family performs exceptionally well in this regard. For each

Apple ProRes codec type, the following chart shows the number of full-width,

1920 x 1080 HD streams that can be edited simultaneously in real time on a

MacBook Pro computer. In practice of course, you may not often need to edit five, six,

or more streams simultaneously, but this chart gives an idea of how much processing

time will be available for real-time titling, effects, and so on, when just one, two, or

three streams are being used.

MacBook Pro – Final Cut Pro Multicam Streams at 1920 x 1080, 23.976 fps

ProRes 422 (Proxy)*

ProRes 422 (LT)*

ProRes 422

ProRes 422 (HQ)

ProRes 4444

(w/o alpha)

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Apple ProRes

Today’s Mac notebook and desktop machines rely on multicore processing, so the

speed of a fast editing decoder must scale up—meaning that decoding time per

frame should decrease—as the number of processing cores increases. Many industry

codec implementations “hit the wall” and do not realize further performance gains as

more processors are added, but Apple ProRes codecs continue to get faster as more

cores are added, as the following chart shows.

Multiprocessor Scaling – Apple ProRes 422 (HQ) at 1920 x 1080

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The chart below shows that half-size decoding is substantially faster than already-fast

full-size decoding, especially for the higher-quality Apple ProRes codecs. The faster

decoding speed means more CPU time is available for decoding more streams or

more real-time effects.

Reduced Resolution Decoding Speed at 1920 x 1080

1.4

0.7

1.9

1.0

2.5

1.2

3.4

1.3

5.1

1.9

024

ProRes 422 (Proxy)

Full Size

Half Size

ProRes 422 (LT)

ProRes 422

ProRes 422 (HQ)

ProRes 4444

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Apple ProRes

Fast encoding and decoding also benefits rendering and exporting. Rendering effects,

as part of a creative process or the final step before output, is basically a decode

of the source media and a reencode to the chosen final output format. During the

rendering process, all of the decoding, blending, and compositing steps must be

precomputed before encoding to the compressed format defined in your Final Cut Pro

project. Although you can choose any Apple ProRes codec as a rendering format—

from Apple ProRes 422 (LT) to Apple ProRes 4444—and change it at any time during

post-production, Final Cut Pro X defaults to rendering in Apple ProRes 422.

When rendering to Apple ProRes, the total rendering time is determined by the

speed of both the decoding and encoding steps, which can be significantly quicker

compared to other, more complex and slower codecs. The speed advantage of

Apple ProRes is also beneficial when exporting a file at the end of a project. If

you need to deliver to the web, DVD, or Blu-ray disc, you can speed up the export

process by choosing to edit in Apple ProRes instead of other professional formats,

including uncompressed.

Apple ProRes 4444 Alpha Channel Support

In addition to supporting Y’CC or RGB 4:4:4 pixel data, the Apple ProRes 4444 codec

type supports an optional alpha channel. The sampling nomenclature for such Y’CCA

or RGBA images is 4:4:4:4, to indicate that for each pixel location, there is an alpha—

or A—value in addition to the three Y’C

C or RGB values. An alpha value specifies the

proportion of its associated RGB or Y’C

C pixel that should be blended with the pixel

at the corresponding location of a background image, creating the effect of varying

transparency for use in compositing workflows. Unlike Y’C

C or RGB pixel values,

alpha values do not represent samples of a real-world image, or even samples of a

computer-generated image, both of which are intended for human viewing.

Alpha values are essentially numeric data that specify how to blend, or composite,

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Appendix

Target Data Rates

Dimensions Frame ProRes 422 ProRes 422 ProRes 422 ProRes 422 ProRes 4444

Rate (Proxy) (LT) (HQ) (excl. alpha)

Mb/s GB/hr Mb/s GB/hr Mb/s GB/hr Mb/s GB/hr Mb/s GB/hr

720 x 486 24p 10 4 23 10 34 15 50 23 75 34

60i, 30p 12 5 29 13 42 19 63 28 94 42

720 x 576 50i, 25p 12 6 28 13 41 18 61 28 92 41

960 x 720 24p 15 7 35 16 50 23 75 34 113 51

25p 16 7 36 16 52 24 79 35 118 53

30p 19 9 44 20 63 28 94 42 141 64

50p 32 14 73 33 105 47 157 71 236 106

60p 38 17 87 39 126 57 189 85 283 127

1280 x 720 24p 18 8 41 18 59 26 88 40 132 59

25p 19 9 42 19 61 28 92 41 138 62

30p 23 10 51 23 73 33 110 49 165 74

50p 38 17 84 38 122 55 184 83 275 124

60p 45 20 101 46 147 66 220 99 330 148

1280 x 1080 24p 31 14 70 31 101 45 151 68 226 102

60i, 30p 38 17 87 39 126 57 189 85 283 127

1440 x 1080 24p 31 14 70 31 101 45 151 68 226 102

50i, 25p 32 14 73 33 105 47 157 71 236 106

60i, 30p 38 17 87 39 126 57 189 85 283 127

1920 x 1080 24p 36 16 82 37 117 53 176 79 264 119

50i, 25p 38 17 85 38 122 55 184 83 275 124

60i, 30p 45 20 102 46 147 66 220 99 330 148

50p 76 34 170 77 245 110 367 165 551 248

60p 91 41 204 92 293 132 440 198 660 297

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Target Data Rates

(continued)

Dimensions Frame ProRes 422 ProRes 422 ProRes 422 ProRes 422 ProRes 4444

Rate (Proxy) (LT) (HQ) (excl. alpha)

Mb/s GB/hr Mb/s GB/hr Mb/s GB/hr Mb/s GB/hr Mb/s GB/hr

2048 x 1080 24p 41 19 93 42 134 60 201 91 302 136

25p 43 19 97 154 140 63 210 94 315 142

30p 52 23 116 185 168 75 251 113 377 170

50p 86 39 194 308 280 126 419 189 629 283

60p 103 46 232 369 335 151 503 226 754 339

2048 x 1556 24p 56 25 126 57 181 81 272 122 407 183

25p 58 26 131 59 189 85 283 127 425 191

30p 70 31 157 71 226 102 340 153 509 339

50p 377 170 567 255 850 382 117 52 262 118

60p 452 203 679 306 1019 458 140 63 314 141

3840 x 2160 24p 471 212 707 318 1061 477 145 65 328 148

25p 151 68 342 154 492 221 737 332 1106 498

30p 182 82 410 185 589 265 884 398 1326 597

50p 983 442 1475 664 2212 995 303 136 684 308

60p 363 163 821 369 1178 530 1768 795 2652 1193

4096 x 2160 24p 155 70 350 157 503 226 754 339 1131 509

25p 162 73 365 164 524 236 786 354 1180 531

30p 194 87 437 197 629 283 943 424 1414 636

50p 323 145 730 328 1049 472 1573 708 2359 1062

60p 388 174 875 394 1257 566 1886 848 2828 1273

5120 x 2160 24p 194 87 437 197 629 283 943 424 1414 636

25p 202 91 456 205 655 295 983 442 1475 664

30p 243 109 546 246 786 354 1178 530 1768 795

50p 405 182 912 410 1311 590 1966 885 2949 1327

60p 1571 707 2357 1061 3535 1591 485 218 1093 492

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Glossary

alpha channel: An additional channel of information that may optionally be included

with RGB and Y’C

Cimages. If included with an RGB image, for each R, G, and B value

that defines a pixel, there is an “A” value that specifies how the RGB pixel should be

blended with a background image. Typically, one extreme value of A indicates 100%

transparency and the other extreme value indicates 100% opacity. Values in between

the extremes indicate the degree of opacity.

Apple ProRes format: An Apple ProRes-encoded bitstream, typically in the form of

a .mov file, for which the Apple ProRes codec type and video format are specified.

For example, an “Apple ProRes 422 (HQ) 1920 x 1080i 29.97 format.”

codec: Abbreviation for compressor/decompressor. A general term referring to both

encoder and decoder.

decoder: An algorithm or processing system that takes a compressed bitstream as input

and delivers a sequence of images or video frames as output. For Apple ProRes, this term

refers to a QuickTime decompressor component that converts an Apple ProRes-encoded

.mov file to a sequence of images, for further processing or display.

encoder: An algorithm or processing system that takes uncompressed images as input

and delivers a compressed bitstream as output. For Apple ProRes, this term refers to a

QuickTime compressor component that generates an Apple ProRes-encoded .mov file.

image sequence: An ordered set of image frames that, when displayed at a specified

frame rate, is perceived by the viewer as a real-time motion image sequence. If

not referred to as “video,” an image sequence is often a set of RGB images (with an

optional alpha channel), such as the DPX, TIFF, and OpenEXR file formats.

lossless: A type of codec for which putting an image frame through encoding

followed by decoding results in an image that is mathematically guaranteed to have

exactly the same pixel values as the original.

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