CN114205628A - MP4 file encoding method, device and system - Google Patents

MP4 file encoding method, device and system Download PDF

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Publication number
CN114205628A
CN114205628A CN202010909378.9A CN202010909378A CN114205628A CN 114205628 A CN114205628 A CN 114205628A CN 202010909378 A CN202010909378 A CN 202010909378A CN 114205628 A CN114205628 A CN 114205628A
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data
moov
size
encoding
moov data
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张萍
陈云海
刘晓玲
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China Telecom Corp Ltd
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/85—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/70—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals characterised by syntax aspects related to video coding, e.g. related to compression standards
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
    • H04N21/20—Servers specifically adapted for the distribution of content, e.g. VOD servers; Operations thereof
    • H04N21/23—Processing of content or additional data; Elementary server operations; Server middleware
    • H04N21/235—Processing of additional data, e.g. scrambling of additional data or processing content descriptors
    • H04N21/2353—Processing of additional data, e.g. scrambling of additional data or processing content descriptors specifically adapted to content descriptors, e.g. coding, compressing or processing of metadata
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
    • H04N21/80—Generation or processing of content or additional data by content creator independently of the distribution process; Content per se
    • H04N21/85—Assembly of content; Generation of multimedia applications
    • H04N21/854—Content authoring
    • H04N21/85406—Content authoring involving a specific file format, e.g. MP4 format

Abstract

The invention at least relates to a method, a device and a system for encoding an MP4 file. Specifically discloses an encoding method of an MP4 file, which comprises the following steps: estimating the space required by MOOV data according to coding parameters before MDAT data coding; reserving an estimated space behind the FTYP data; coding the MDAT data, and writing the MDAT data after the reserved space; and generating MOOV data and inserting the generated MOOV data into the reserved space.

Description

MP4 file encoding method, device and system
Technical Field
The present invention relates to the field of video coding, and more particularly, to a method, an apparatus, and a system for encoding an MP4 file.
Background
MP4 is the current mainstream video packaging format, and online video websites, cloud storage, etc. often create MP4 backups of multiple encoding specifications by means of offline transcoding, so as to match users with different bandwidths to provide a smooth playing experience. To speed up the on-line playing of MP4, moov data (Movie Box) containing important decoding information such as encoding metadata, key frames, and indexes is often needed to be preceded.
According to an existing MP4 encoding method for pre-moov data, complete moov data can be generated and the size of the moov data can be known after media data (mdat) encoding is completed, then the mdat data occupying about 99% of a file is moved backwards, and about 1% of space insertion moov data is obtained.
For example, the invention patent CN1650628B entitled "method and apparatus for supporting AVC in MP 4" and the invention patent CN101803388B entitled "image decoding apparatus, image decoding method, image encoding apparatus, and image encoding method" all use a conventional encoding method of obtaining spatially inserted moov data by shifting mdat data backward after moov data is generated.
In the above conventional encoding, a large amount of data shift operations of concurrent tasks easily cause IO bottleneck of the storage disk, which causes overtime of the encoding task. For example, 700 encoding tasks are output to 2 net disks, and more than 83% of the tasks are blocked in a MOOV pre-stage.
Disclosure of Invention
In view of one or more of the above technical problems, there is a need for an MP4 encoding method capable of fast pre-moov. Therefore, the invention provides an MP4 coding method for fast pre-MOOV. The core of the method is that before mdat data are coded, the size of each frame (box) of an audio and video track (track) in moov data is estimated according to audio and video coding parameters such as a frame rate, a sampling rate, duration, a key frame and the like, the estimated size of the moov data is obtained, and a required space is reserved. Then, after the mdat data are coded, MOOV is inserted into a reserved position, and the reserved extra space is packaged into a free box (free box) which can be identified and skipped by a decoder, so that the MOOV data can be rapidly preposed without shifting the mdat data which accounts for 99% of the file, IO bottleneck is avoided, and the coding time is saved.
According to one aspect of the invention, an encoding method of an MP4 file is disclosed, which comprises the following steps: estimating the space required by moov data according to the encoding parameters before encoding mdat data; reserving an estimated space behind the ftyp data; encoding the mdat data, and writing the mdat data after the reserved space; and generating moov data and inserting the generated moov data in the reserved space.
According to another aspect of the present invention, there is disclosed an apparatus for encoding an MP4 file, comprising: a memory storing computer-executable instructions; and a processor configured to implement the method as described above when executing computer-executable instructions stored in the memory.
According to another aspect of the invention, there is disclosed an encoding system for an MP4 file, comprising means configured to perform the steps of the method as described above.
According to another aspect of the invention, a non-transitory computer readable storage medium is disclosed, storing instructions that when executed by a processor implement the method as described above.
Drawings
The present disclosure may be more clearly understood from the following detailed description with reference to the accompanying drawings, in which:
fig. 1 shows the structure of an MP4 file;
fig. 2 illustrates a structure of an MP4 file in an encoding process of an MP4 file of the related art;
fig. 3 illustrates a structure of an MP4 file in an encoding process of an MP4 file according to an embodiment of the present invention;
FIG. 4 is a flowchart illustrating an encoding method of an MP4 file according to an embodiment of the present invention; and
fig. 5 is a flowchart illustrating an encoding method of an MP4 file according to another embodiment of the present invention.
Detailed Description
Various exemplary embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. The relative arrangement of the components and steps, the numerical expressions, and numerical values set forth in these embodiments do not limit the scope of the present disclosure unless specifically stated otherwise.
First, the composition of the file structure of MP4 is described with reference to fig. 1.
All data in an MP4 file is encapsulated in a box (box), which may be understood as a block of data objects, i.e. an MP4 file consists of several blocks of data objects, each box having a type and a length. The boxes may contain subframes.
As shown in fig. 1, the MP4 file is composed of three top boxes, namely, ftyp, moov, and mdat.
An MP4 file will first have a "ftyp" type box that acts as a flag in the MP4 format and contains some information about the file; then, a "moov" type Box (Movie Box) is provided, which comprises a plurality of sub-boxes respectively containing important decoding information such as encoding metadata, key frames, indexes and the like; the media Data of the MP4 file is contained in a Box of the "mdat" type (midi a Data Box), which is located after the moov Data, occupying about 99% of the space of the entire MP4 file.
As further shown in fig. 1, moov data includes two track (track) indices, an audio track and a video track. The audio track comprises a plurality of subframes, respectively: stsd, stts, stss, stsc, stsz, and stco. The video track also contains the same plurality of children boxes, namely stsd, stts, stss, stsc, stsz, and stco. The size of each subframe differs according to the content it encapsulates.
The conventional front operation of moov data is described below with reference to the structure of an MP4 file in the encoding process of a related art MP4 file shown in fig. 2.
Since moov data contains important decoding information such as encoding metadata, key frames, indexes, etc., complete moov data can be generated only after encoding of media data (mdat) is completed. However, when playing online, moov data must be inserted before mdat data to ensure normal playing of video. Therefore, conventionally, mdat data needs to be shifted backward by a shift operation to make room for moov prefix.
The left diagram in fig. 2 shows the MP4 file structure after media data encoding, in which mdat data is written in a position immediately adjacent to ftyp. At this time, moov data is generated, and the size thereof is obtained, and mdat data is shifted backward according to the size of the moov data (as shown in the middle diagram in fig. 2). Thereafter, as shown in the right diagram in fig. 2, moov data is written in the space vacated by the backward-shifted mdat data.
Hereinafter, an encoding method of an MP4 file according to an embodiment of the present invention will be described with reference to an MP4 file structure diagram shown in fig. 3 and a flowchart of fig. 4.
As shown in step 401 of fig. 4, first, the space required for moov data is estimated from the encoding parameters before mdat data is encoded.
The encoding parameters comprise parameters related to the properties of the audio and video files to be encoded and parameters related to operations such as sampling compression and the like in the encoding process. Some parameters may be set by the user as desired. Although moov data is generally generated after mdat data encoding is completed, the size of the moov data and encoding parameters are closely related in practice, and thus the moov data can be estimated based on the encoding parameters even though it has not been generated. In one embodiment, as can be seen from the analysis, the encoding parameters determining the moov data size at least include: frame rate f, sampling rate ar, key frame number k and file duration T. These parameters are common audio-video coding parameters and are not described in detail here.
In estimating moov data, a suitable estimation algorithm may be chosen such that the estimated size is very close to the actual moov data size.
A preferred accurate estimation process will be described later.
Then, in step 402, the estimated space is reserved after the ftyp data. Next, in step 403, mdat data is encoded, and mdat data is written after the reserved space. The file structure at this time is shown in the left diagram in fig. 3.
The space may be reserved and the mdat data may be encoded to a position behind the reserved space in various ways. For example, according to an embodiment of the present invention, it is possible to write ftyp data when creating the encoded file, and then record the starting point reserved _ header _ pos of the reserved location. Then, the writing of mdat data is started after skipping the bytes corresponding to the reserved space from the start of the reserved position.
Then, the process proceeds to step 404, moov data is generated, and the generated moov data is inserted in the reserved space. For example, moov data may be written from a reserved position start reserved _ header _ pos immediately after ftyp data. The file structure at this time is shown in the right diagram in fig. 3.
By the encoding method, the MP4 file with the moov data in the front can be directly obtained without performing backward shift operation on mdat data after encoding is completed, so that the problem of IO bottleneck of a storage disk caused by a large amount of data shift operation is solved when encoding is saved.
Although the estimated moov data size is very close to the size of the actual generated moov data, there is still a deviation between the two. The processing method when such an estimation deviation exists according to one embodiment will be described below with reference to fig. 5. The flow shown in fig. 5 is a further refinement of step 404 of the flow shown in fig. 4.
In step 501, moov data is generated and the actual size of the moov data is obtained.
Then, in step 502, the reserved space of the moov data is compared with the actual size of the moov data, thereby determining whether the reserved space is larger than the moov data. The comparison may be made by comparing the number of bytes occupied by the reserved space with the number of bytes actually occupied by the moov data.
If it is determined in step 502 that the reserved space is larger than the generated moov data, proceeding to step 503, moov data is inserted in the reserved space. Next, in step 504, the remaining space after the moov data is inserted in the reserved space is packed as a free box (free box). A free frame is a special type of frame in the MP4 package structure. The data in the free box is typically all 0's, which act as placeholders. The free box can be recognized and ignored by the decoder when decoding. Therefore, the insertion of the free frame does not affect neither the encoding speed nor the decoding of the file. The operation of step 504 enables the encoding method of the present invention to achieve the technical effect of obtaining the MP4 file of the previous moov data quickly and efficiently even if the estimated value is large.
If it is judged in step 502 that the reserved space is smaller than the generated moov data, it proceeds to step 505. In step 505, mdat data is shifted back. The amount of shift is equal to the difference between the reserved space and the moov data size. Then, at step 506, moov data is inserted in the reserved space and the space vacated by the mdat shift.
Steps 505 and 506 increase the reliability of the encoding method according to the invention as a solution in the case of insufficient headroom. Even if the estimation value is small and the reserved space is insufficient, the MP4 file can be encoded by moving the mdat data backwards.
However, according to an embodiment of the present invention, the estimation formula is preferably selected such that the estimated headroom is larger than the actual size of moov data. In this case, the above steps 505 and 506 can be omitted, thereby eliminating the step of shifting the mdat data backward, ensuring that the encoding time can be saved.
Next, a method of accurately estimating the moov data size according to an embodiment of the present invention is described. The method has the advantages that the size of moov data can be accurately estimated (namely, the estimation error is small), and the estimated space is basically ensured to be larger than the actual size of the moov data (so that the situation that the estimated value is small and the mdat data needs to be moved backwards is eliminated).
Specifically, the estimation method of this embodiment derives an estimation formula in which the size of each sub-box is strongly associated with the encoding parameter by statistically analyzing the sizes of the main 6 sub-boxes of the moov box in a large number of MP4 video sample files and combining the data structure of each sub-box.
The first step is as follows: obtaining encoding parameters including, but not limited to: frame rate f, sampling rate ar, key frame number k and file duration T. The above encoding parameters are well known to those skilled in the art and will not be described in detail here. The above-mentioned encoding parameters may be set by a user.
The second step is that: calculating the number of audio and video samples by the following formula:
number of audio samples
Figure BDA0002662706150000061
Number of video samples vs＝f×T。
The third step: the coefficients associated with the number of audio samples and the number of video samples (hereinafter simply referred to as association coefficients) are calculated by the following formula:
min＝min(as,vs)；
max＝max(as,vs) (ii) a And
r＝max％min。
the above correlation coefficient will be used to calculate the size of each sub-box in moov data in the following estimation formula.
The third step: and estimating the size of the audio and video track.
The moov data box includes two tracks: audio tracks and video tracks. Each track includes 6 children boxes, namely stsd, stts, stss, stsc, stsz, and stco. The relationship between the size of each sub-box obtained by analyzing the samples and the coding parameters is described in detail below:
1、stsd
the audio and video stsd only has 1 item, the size is basically fixed, and 256 bytes are preset.
2、stts
The audio/video stts has compression of different degrees, the upper limit does not exceed the number of audio and video samples, each single item occupies 8 bytes, and the obtained video stts is 8vs+ 16; audio stts 8as+16。
3、stss
The video stss is proportional to the number of key frames, and each single entry takes 4 bytes, resulting in stss being 4k +16, where k is the number of key frames. The audio has no stss, so the byte is 0.
4、stsc
The audio/video stsc is influenced by the audio/video sampling quantity, the derivation is complex, and an estimation formula related to the correlation coefficients max, min and r is obtained.
5、stsz
The audio/video stsz is determined by the audio/video sampling quantity, a small amount of compression exists, each single term occupies 4 bytes, and the video stsz is obtained by taking 4v as the videos+20, audio stsz ═ 4as+20。
6、stco
The audio and video stco calculation method is basically consistent, each single item occupies 4 bytes, and the number of the items is related to the minimum value of the number of the adopted audio and video.
Summarized, the video track estimation formula and the audio track estimation formula (unit: byte) shown in the following table 1 are obtained:
Figure BDA0002662706150000071
TABLE 1
The fourth step: the reserved space of moov data is estimated.
The following overall estimation formula is obtained by adding the estimation formulas of the subframes of the audio and video tracks:
Figure BDA0002662706150000081
wherein moovsizeI.e., the size of moov data.
The moov data size estimated according to the above-described total estimation formula is very high in accuracy. This is because the above-mentioned overall estimation formula is analyzed in conjunction with moov data of a large number of real MP4 files, and thus the size of the obtained moov data is very close to the size of the actual moov data.
Table 2 shows moov data size estimates and errors for four duration videos. It can be seen that the estimated error of the moov data size is less than 0.5%, that is, less than 0.5% of reserved space is added.
Video duration moov size Pre-estimated size Error (%)
10 minutes 268819 269700 0.33％
30 minutes 803611 805380 0.22％
1 hour 1605835 1608900 0.19％
2 hours 3169307 3175324 0.19％
TABLE 2
Further, the moov data size estimated according to the above-described total estimation formula does not substantially occur to be smaller than the actual moov data size. In practical application of the encoding method of the present invention, a case where the estimated moov data size is smaller than the actual moov data size has not yet occurred. This is because each box is already left with a suitable margin when designing the estimation formula. For example, the stsd subframe of an audiovisual track is typically less than 256 bytes, so 256 bytes are sufficient for the stsd subframe. Therefore, the estimation formula according to the invention basically eliminates the need of backward-shifting mdat data, greatly saves the coding time and improves the resource utilization rate.
The above-described method may be a method that can be implemented by a computer or a processor, and may be performed by software, hardware, or a combination of software and hardware. For example, the above-described methods may be performed by respective software modules of the encoding apparatus, respectively. The above-described methods may also be implemented as program code stored in a storage medium for execution by a processor. The processor may be a special purpose processor or a general purpose processor.
Hardware or software modules for performing the above methods or various other implementations of the above methods are also included within the scope of the present invention. This is described below.
According to an embodiment of the present invention, there is provided an apparatus for encoding an MP4 file, including: a memory storing computer-executable instructions; and a processor configured to implement any of the methods described above when executing computer-executable instructions stored in the memory. Memory herein should be understood to include, but not be limited to, solid state memory, as well as optical and magnetic media. Specific examples include non-volatile memory, including, for example, semiconductor memory devices (e.g., electrically programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM)) and flash memory devices; magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks
According to another embodiment of the invention, there is provided an encoding system of MP4 files, comprising means configured to perform the steps of any of the methods as described above. It should be understood that the components herein are merely logical blocks divided according to the specific functions implemented thereby, and are not intended to limit the specific implementation. In actual implementation, the devices may be implemented as separate physical entities, or may also be implemented by a single entity, e.g., a processor (CPU or DSP, etc.), an integrated circuit, etc.
According to another embodiment of the invention, there is provided a non-transitory computer readable storage medium storing instructions which, when executed by a processor, implement any of the methods described above. The term "computer-readable storage medium" shall be taken to include any tangible medium (non-transitory medium) that is capable of storing, encoding or carrying instructions for execution by the processor and that cause the processor to perform any one or more of the methodologies of the present disclosure, or that is capable of storing, encoding or carrying data structures utilized by or associated with such instructions.
The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Claims (12)

1. An encoding method of an MP4 file, comprising:
estimating the space required by moov data according to the encoding parameters before encoding mdat data;
reserving an estimated space behind the ftyp data;
encoding the mdat data, and writing the mdat data after the reserved space; and
and generating moov data, and inserting the generated moov data into the reserved space.
2. The method of claim 1, wherein a space required for moov data is estimated according to a moov data size estimation formula obtained by analyzing a relationship of a size of moov data in a plurality of MP4 sample files with an encoding parameter.
3. The method of claim 2, wherein analyzing the moov data size versus encoding parameters in the plurality of MP4 sample files comprises:
analyzing the relationship between the size of the audio track of moov data in a plurality of MP4 sample files and the encoding parameters to obtain an audio track estimation formula;
analyzing the relationship between the size of the video track of moov data in a plurality of MP4 sample files and the encoding parameters to obtain a video track estimation formula; and
and obtaining a moov data size estimation formula based on the audio track estimation formula and the video track estimation formula.
4. The method of claim 3, wherein,
obtaining an audio track estimation formula by analyzing the relation between the size of each subframe of the audio track and the coding parameters; and
and obtaining a video track estimation formula by analyzing the relation between the size of each subframe of the video track and the coding parameters.
5. The method of claim 2, wherein estimating the space required for moov data from the encoding parameters comprises:
respectively calculating the number of audio samples and the number of video samples according to the coding parameters;
calculating a sample number correlation coefficient based on the number of audio samples and the number of video samples; and
and obtaining a moov data size estimation formula by analyzing the relation between the size of the moov data in a plurality of MP4 sample files and the correlation coefficients of the number of audio samples, the number of video samples and the number of samples.
6. The method of claim 5, wherein the encoding parameters include at least: frame rate f, sampling rate ar, key frame number k and file duration T; and the sample number correlation coefficient is min-min (a)s，vs)，max＝max(as，vs) And r max% min.
7. The method of claim 2, wherein the moov data size estimation formula is:
Figure FDA0002662706140000021
wherein r is max% min, min is min (a)s，vs)，max＝max(as，vs)，
Wherein moovsizeSpace occupied by mox data, vsIs the number of samples of video data, asK is the number of samples of the audio data and k is the number of key frames.
8. The method of claim 1, further comprising:
comparing the size of the generated moov data with the reserved space; and
and if the reserved space is larger than the generated moov data, inserting the moov data into the reserved space, and packaging the rest space into a free box.
9. The method of claim 7, further comprising: if the reserved space is smaller than the generated moov data, the mdat data is shifted backward, and then the generated moov data is inserted after the ftyp data.
10. An apparatus for encoding an MP4 file, comprising:
a memory storing computer-executable instructions; and
a processor configured to implement the method of any one of claims 1-9 when executing computer-executable instructions stored in a memory.
11. An encoding system of MP4 files, comprising means configured to perform the steps of the method according to any of claims 1-9.
12. A non-transitory computer readable storage medium storing instructions which, when executed by a processor, implement the method of any one of claims 1-9.
CN202010909378.9A 2020-09-02 2020-09-02 MP4 file encoding method, device and system Pending CN114205628A (en)

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