Apparatus and method for object based rate control in a coding system

• US 6,023,296 A
• Filed: 06/15/1998
• Issued: 02/08/2000
• Est. Priority Date: 07/10/1997
• Status: Expired due to Term

• Current Assignee: MediaTek, Inc.
• Original Assignee: Sarnoff Corporation (SRI International, Inc.)
• Inventors: Lee, Hung-Ju, Chiang, Tihao
• Primary Examiner(s): LIN, BRYAN T
• Application Number:
  US09/094,645
• Time in Patent Office: 603 Days
• Field of Search: 348/384, 348/390, 348/400, 348/401, 348/402, 348/405, 348/419, 382/251
• US Class Current: 375/240.05
• CPC Class Codes: H04N 19/115 Selection of the code volume for a coding unit prior to coding
• CPC Class Codes: H04N 19/126 Details of normalisation or weighting functions, e.g. normalisation matrices or variable uniform quantisers
• CPC Class Codes: H04N 19/127 Prioritisation of hardware or computational resources
• 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 19/146 Data rate or code amount at the encoder output
• CPC Class Codes: H04N 19/152 by measuring the fullness of the transmission buffer
• CPC Class Codes: H04N 19/17 the unit being an image region, e.g. an object
• CPC Class Codes: H04N 19/172 the region being a picture, frame or field
• CPC Class Codes: H04N 19/186 the unit being a colour or a chrominance component
• CPC Class Codes: H04N 19/20 using video object coding
• CPC Class Codes: H04N 19/61 in combination with predictive coding
• CPC Class Codes: H04N 19/63 using sub-band based transform, e.g. wavelets

Abstract

A method and apparatus for selecting a quantizer scale for each object within a frame to optimize the coding rate is disclosed. A quantizer scale is selected for each region or "object" within each frame such that the target bit rate for the frame is achieved while maintaining a uniform visual quality over the entire frame.

Claims

1. A method for allocating bits to encode each frame of an image sequence, each of said frame having at least one object, said method comprising the steps of:

(a) determining a target frame bit rate for the frame; and

(b) allocating said target frame bit rate among the at least one object, wherein said allocating step comprises the step of allocating said target frame bit rate in accordance with a target object bit rate for the at least one object, wherein said target object bit rate for the at least one object is selected in accordance with a mean absolute differences (Mad) of said object in accordance with;

##EQU6## where Mad_i is the mean absolute difference (Mad) of an object i, n is a number of said objects in the frame, T_frame is said target frame bit rate and V_i is said target object bit rate for object i.

2. A method for allocating bits to encode each frame of an image sequence, each of said frame having at least one object, said method comprising the steps of:

(a) determining a target frame bit rate for the frame; and

(b) allocating said target frame bit rate among the at least one object, wherein said allocating step comprises the step of allocating said target frame bit rate in accordance with a target object bit rate for the at least one object, wherein said target object bit rate is adjusted in accordance with a measure of a buffer fullness in accordance with;

space="preserve" listing-type="equation">if (buffer.sub.-- fullness+V.sub.i >

margin) then

space="preserve" listing-type="equation">V.sub.i =Max(R.sub.s /30/number.sub.-- of.sub.-- objects, margin-buffer.sub.-- fullness)

where "margin" is defined as;

space="preserve" listing-type="equation">margin=ceil((1.0-SAFETY.sub.-- MARGIN)×

buffer.sub.-- fullness)

where buffer_-- fullness is a current buffer fullness of a buffer, where SAFTETY_-- MARGIN is a constant, where R_s is a bit rate, where V_i is said target object bit rate for object i and where number_-- of_-- objects is a number of objects in the input image.

3. A method for allocating bits to encode each frame of an image sequence, each of said frame having at least one object, said method comprising the steps of:

(a) determining a target frame bit rate for the frame; and

(b) allocating said target frame bit rate among the at least one object, wherein said allocating step comprises the step of allocating said target frame bit rate in accordance with a target object bit rate for the at least one object, wherein said target object bit rate is adjusted in accordance with a measure of a buffer fullness in accordance with;

space="preserve" listing-type="equation">if (buffer.sub.-- fullness-B.sub.pp +V.sub.i ≦

SAFETY.sub.-- MARGIN×

buffer.sub.-- size) then

space="preserve" listing-type="equation">V.sub.i =B.sub.pp -V.sub.i -buffer.sub.-- fullness+SAFETY.sub.-- MARGIN×

buffer.sub.-- size,

where buffer_-- fullness is a current buffer fullness of a buffer, where SAFTETY_-- MARGIN is a constant, where buffer_-- size is a size of said buffer, where V_i is said target object bit rate for object i and B_pp is a channel output rate.

4. A method for allocating bits to encode each frame of an image sequence, each of said frame having at least one object, said method comprising the steps of:

(a) determining a target frame bit rate for the frame, wherein said target frame bit rate, T_frame, is derived in accordance with;

##EQU7## where R is a remaining number of bits for the image sequence, Nf is a number of remaining frames in the image sequence, T_previous frame is a number of bits used for encoding a previous frame, and past_-- percent is a constant; and

(b) allocating said target frame bit rate among the at least one object.

5. Apparatus for encoding each frame of an image sequence, said frame having at least one object, said apparatus comprising:

a motion compensator for generating a predicted image of a current frame;

a transform module for applying a transformation to a difference signal between the current frame and said predicted image, where said transformation produces a plurality of coefficients;

a quantizer for quantizing said plurality of coefficients with at least one quantizer scale; and

a controller for selectively adjusting said at least one quantizer scale for a current frame in response to a target object bit rate for the at least one object, wherein said target object bit rate for the at least one object is selected in accordance with a mean absolute differences (Mad) of said object in accordance with;

##EQU8## where Mad_i is a mean absolute difference (Mad) of an object i, n is a number of said objects in the frame, T_frame is said target frame bit rate and V_i is said target object bit rate for object i.

6. The apparatus of claim 5, wherein said target object bit rate is adjusted in accordance with a measure of a buffer fullness.

7. The apparatus of claim 6, wherein said target object bit rate is adjusted in accordance with:

space="preserve" listing-type="equation">if (buffer.sub.-- fullness+V.sub.i >

margin) then

space="preserve" listing-type="equation">V.sub.i =Max(R.sub.s /30/number.sub.-- of.sub.-- objects, margin-buffer.sub.-- fullness)

where "margin" is defined as;

space="preserve" listing-type="equation">margin=ceil((1.0-SAFETY.sub.-- MARGIN)×

buffer.sub.-- fullness)

where buffer_-- fullness is a current buffer fullness of a buffer, where SAFTETY_-- MARGIN is a constant, where R_s is a bit rate, where V_i is said target object bit rate for object i and where number_-- of_-- objects is a number of objects in the image.

8. The apparatus of claim 6, wherein said target object bit rate is adjusted in accordance with:

space="preserve" listing-type="equation">if (buffer.sub.-- fullness-B.sub.pp +V.sub.i ≦

SAFETY.sub.-- MARGIN×

buffer.sub.-- size) then

space="preserve" listing-type="equation">V.sub.i =B.sub.pp -V.sub.i -buffer.sub.-- fullness+SAFETY.sub.-- MARGIN×

buffer.sub.-- size,

where buffer_-- fullness is a current buffer fullness of a buffer, where SAFTETY_-- MARGIN is a constant, where buffer_-- size is a size of said buffer, where V_i is said target object bit rate for object i and B_pp is a channel output rate.

9. Apparatus for encoding each frame of an image sequence, said frame having at least one object, said apparatus comprising:

a motion compensator for generating a predicted image of a current frame;

a transform module for applying a transformation to a difference signal between the current frame and said predicted image, where said transformation produces a plurality of coefficients;

a quantizer for quantizing said plurality of coefficients with at least one quantizer scale; and

a controller for selectively adjusting said at least one quantizer scale for a current frame in response to a target object bit rate for the at least one object, wherein said target object bit rate is derived from a target frame bit rate, wherein said target frame bit rate, T_frame, is derived in accordance with;

##EQU9## where R is a remaining number of bits for a sequence of frames, Nf is a number of remaining frames in the sequence, T_previous frame is a number of bits used for encoding a previous frame, and past_-- percent is a constant.

10. A computer-readable medium having stored thereon a plurality of instructions, the plurality of instructions including instructions which, when executed by a processor, cause the processor to perform the steps comprising of:

(a) determining a target frame bit rate for the frame; and

(b) allocating said target frame bit rate among the at least one object, wherein said allocating step comprises the step of allocating said target frame bit rate in accordance with a target object bit rate for the at least one object, wherein said target object bit rate for the at least one object is selected in accordance with a mean absolute differences (Mad) of said object in accordance with;

##EQU10## where Mad_i is the mean absolute difference (Mad) of an object i, n is a number of said objects in the frame, T_frame is said target frame bit rate and V_i is said target object bit rate for object i.

11. A computer-readable medium having stored thereon a plurality of instructions, the plurality of instructions including instructions which, when executed by a processor, cause the processor to perform the steps comprising of:

(a) determining a target frame bit rate for the frame; and

(b) allocating said target frame bit rate among the at least one object, wherein said allocating step comprises the step of allocating said target frame bit rate in accordance with a target object bit rate for the at least one object, wherein said target object bit rate is adjusted in accordance with a measure of a buffer fullness in accordance with;

space="preserve" listing-type="equation">if (buffer.sub.-- fullness+V.sub.i >

margin) then

space="preserve" listing-type="equation">V.sub.i =Max(R.sub.s /30/number.sub.-- of.sub.-- objects, margin-buffer.sub.-- fullness)

where "margin" is defined as;

space="preserve" listing-type="equation">margin=ceil((1.0-SAFETY.sub.-- MARGIN)×

buffer.sub.-- fullness)

where buffer fullness is a current buffer fullness of a buffer, where SAFTETY_-- MARGIN is a constant, where R_s is a bit rate, where V_i is said target object bit rate for object i and where number of objects is a number of objects in the input image.

12. A computer-readable medium having stored thereon a plurality of instructions, the plurality of instructions including instructions which, when executed by a processor, cause the processor to perform the steps comprising of:

(a) determining a target frame bit rate for the frame; and

(b) allocating said target frame bit rate among the at least one object, wherein said allocating step comprises the step of allocating said target frame bit rate in accordance with a target object bit rate for the at least one object, wherein said target object bit rate is adjusted in accordance with a measure of a buffer fullness in accordance with;

space="preserve" listing-type="equation">if (buffer.sub.-- fullness-B.sub.pp +V.sub.i ≦

SAFETY.sub.-- MARGIN×

buffer.sub.-- size) then

space="preserve" listing-type="equation">V.sub.i =B.sub.pp -V.sub.i -buffer.sub.-- fullness+SAFETY.sub.-- MARGIN×

buffer.sub.-- size,

where buffer fullness is a current buffer fullness of a buffer, where SAFTETY_-- MARGIN is a constant, where buffer size is a size of said buffer, where V_i is said target object bit rate for object i and B_pp is a channel output rate.

13. A computer-readable medium having stored thereon a plurality of instructions, the plurality of instructions including instructions which, when executed by a processor, cause the processor to perform the steps comprising of:

(a) determining a target frame bit rate for the frame, wherein said target frame bit rate T_frame, is derived in accordance with;

##EQU11## where R is a remaining number of bits for the image sequence, Nf is a number of remaining frames in the image sequence, T_previous frame is a number of bits used for encoding a previous frame, and past percent is a constant; and

(b) allocating said target frame bit rate among the at least one object.