Bit rate control mechanism for digital image and video data compression

• US 5,995,151 A
• Filed: 09/18/1997
• Issued: 11/30/1999
• Est. Priority Date: 12/04/1995
• Status: Expired due to Term

• Current Assignee: France Brevets SAS
• Original Assignee: Tektronix Incorporated (Fortive Corp.)
• Inventors: Tabatabai, Ali, Eifrig, Robert O., Naveen, Thumpudi
• Primary Examiner(s): BRITTON, HOWARD W
• Application Number:
  US08/932,681
• Time in Patent Office: 803 Days
• Field of Search: 348/420, 348/394, 348/405, 348/415, 348/419, 382/239, 382/243
• US Class Current: 375/240.24
• CPC Class Codes: H04N 19/124 Quantisation
• CPC Class Codes: H04N 19/14 Coding unit complexity, e.g. amount of activity or edge presence estimation H04N19/146 takes precedence
• CPC Class Codes: H04N 19/142 Detection of scene cut or scene change
• CPC Class Codes: H04N 19/146 Data rate or code amount at the encoder output
• CPC Class Codes: H04N 19/15 by monitoring actual compressed data size at the memory before deciding storage at the transmission buffer
• CPC Class Codes: H04N 19/172 the region being a picture, frame or field
• CPC Class Codes: H04N 19/179 the unit being a scene or a shot
• CPC Class Codes: H04N 19/196 being specially adapted for the computation of encoding parameters, e.g. by averaging previously computed encoding parameters processing of motion vectors H04N19/513
• CPC Class Codes: H04N 19/198 including smoothing of a sequence of encoding parameters, e.g. by averaging, by choice of the maximum, minimum or median value
• CPC Class Codes: H04N 19/503 involving temporal prediction adaptive coding with adaptive selection between spatial and temporal predictive coding H04N19/107; adaptive coding with adaptive selection among a plurality of temporal predictive coding modes H04N19/109
• CPC Class Codes: H04N 19/61 in combination with predictive coding
• CPC Class Codes: H04N 19/87 involving scene cut or scene change detection in combination with video compression

Abstract

A bit rate control mechanism for a digital image or video compression system estimates a complexity parameter for a current picture, or block of samples, of a video signal as a function of parameters for a prior picture of the video signal, which parameters include a bit rate. From the complexity parameter a quality factor for the current picture is determined and applied to a quantizer to compress the current picture. A complexity pre-processor may also be used to detect scene changes in the video signal prior to estimating the complexity parameter. If there is a scene change detected, then the rate control mechanism is reset prior to estimating the complexity parameter for the first picture in the new scene. Also a video buffer verifier is controlled so that the buffer occupancy at the end of a specified image sequence is at a target value so that looping and editing applications are facilitated.

Claims

1. A bit rate controller for a moving image data compression encoder comprising:

means in response to an actual bit rate for a prior coded block of the moving image data for generating a complexity factor as a function of a prior complexity factor and a prior quality factor for the prior coded block;

means for computing a quality factor for a present block of the moving image data compression decoder as a function of the complexity factor and a target bit rate for the present block; and

means for controlling a virtual buffer occupancy so that a target occupancy value is achieved at the end of a specified sequence of images encompassed by the moving image data.

2. A bit rate controller as recited in claim 1 wherein the controlling means comprises:

means for initializing parameters including the complexity factor;

means for determining a number of bits for a window that includes a subset of the specified sequence based on the remaining pictures in the specified sequence and the target occupancy value;

means for computing a target rate for a current picture within the window;

means for measuring the actual bits generated for the current picture;

means for updating the parameters including the complexity factor; and

means for moving to a next picture in the window and repeating the computing, measuring and updating means until all the pictures in the window are processed.

3. The bit rate controller as recited in claim 2 wherein the complexity factor updating means comprises means for detecting whether a scene change has occurred from one picture in the specified sequence to the next based upon an expected deviation of the complexity factor.

4. The bit rate controller as recited in claim 2 wherein the complexity factor updating means comprises means for maintaining state information from one picture to the next in the specified sequence using vector representations in order to adjust the complexity factor when a scene change is detected.

5. A method of compressing moving image data comprising the steps of:

initializing a bit rate controller by selecting a complexity value and a size for a processing window in terms of a given number of blocks of the moving image data;

determining a number of bits for the window as a function of the size of the processing window and an average bit rate;

computing by the bit rate controller a quality factor for a block of the moving image data as a function of the complexity value, a remaining size of the processing window and the number of bits for the processing window, and computing a target bit rate for the block of the moving image data as a function of the quality factor and the complexity value;

coding the block of the moving image data in an encoder using the quality factor;

updating the number of bits in the processing window as a function of an actual bit rate from the encoder for the block of the moving image data, the size of the processing window to determine the remaining size, and the complexity value for a next block of the moving image data within the processing window as a function of the quality factor and complexity value for the block of the moving image data;

repeating the determining, computing, coding and updating steps for each block within the processing window;

moving the processing window by the size of the processing window and repeating the above steps for a next group of blocks of the moving image data; and

controlling a virtual buffer occupancy so that a target occupancy value is achieved at the end of a specified sequence of images encompassed by the moving image data prior to the initializing step.