Transcoder from first MPEG stream to second MPEG stream

1. A system for transcoding an input first MPEG stream to a second MPEG stream, the system comprising:

a first MPEG decoder configured to:

decode the first MPEG stream;

identify metadata indicating how the first MPEG stream was encoded, the identified metadata determined during the decoding of the first MPEG steam; and

output the identified metadata; and

a second MPEG encoder configured to encode the second MPEG stream, wherein the second MPEG encoder is further configured to

maintain a decoded picture type of I, P, or B;

maintain a decoded picture structure of frame or field;

receive the identified metadata from the first MPEG decoder;

determine whether to re-encode a macroblock (MB) of a picture in the second MPEG stream using one of a frame or a field mode based on the identified metadata indicating how the first MPEG stream was encoded; and

re-encode the MB using the determined frame or field mode.

2. The system of claim 1 , wherein the first MPEG stream is an MPEG-2 stream and the second MPEG stream is an MPEG-4 part 10 stream.

3. The system of claim 1 , wherein the second MPEG encoder is configured to:

select the field mode to re-encode the MB of the picture in the second MPEG stream if the identified metadata indicates the picture was encoded in the field mode in the first MPEG stream; and,

if the picture type is an I field picture, select an intra mode to re-encode the MB in the second MPEG stream;

if the picture is one of a B field picture and a P field picture and the identified metadata indicates the MB was encoded in the intra mode in the first MPEG stream, determine a coding cost of intra modes allowed under the second MPEG format including intra mode 4×4, intra mode 8×8 and intra mode 16×16, and select a lowest cost mode to re-encode the MB in the second MPEG stream; and

if the picture is one of a B field picture and a P field picture and the identified metadata indicates the MB was encoded in an inter mode in the first MPEG stream, determine a coding cost of inter modes allowed under the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16, and select a lowest cost mode to re-encode the MB in the second MPEG stream.

4. The system of claim 3 , wherein the second MPEG encoder is configured to:

if inter mode is used to re-encode the MB, identify a decoded motion vector (MV) in the first MPEG stream wherein the decoded MV is in ½ pel resolution;

use the decoded MV as a starting MV;

determine a refined MV by performing motion estimation (ME) refinement on the starting MV for each partition of inter modes allowed in the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16; and

select the refined MV to re-encode the MB into the second MPEG stream.

5. The system of claim 1 , wherein the second MPEG encoder is configured to:

select the frame mode to re-encode the MB in the second MPEG stream if the identified metadata indicates the picture was encoded in the frame mode in the first MPEG stream and a macroblock adaptive frame/field (MBAFF) is off, and

if the MB was encoded in an inter mode in the first MPEG stream, determine a coding cost of inter modes allowed under the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16, and select a lowest cost mode to re-encode the MB in the second MPEG stream; and

if the MB was encoded in the intra mode in the first MPEG stream, determine a coding cost of intra modes allowed under the second MPEG format including intra mode 4×4, intra mode 8×8 and intra mode 16×16, and select a lowest cost mode to re-encode the MB in the second MPEG stream.

6. The system of claim 5 , wherein the second MPEG encoder is configured to:

if inter mode is used to re-encode the MB and the MB was encoded in the frame mode in the first MPEG stream, identify a decoded MV in ½ pel resolution in the first MPEG stream;

use the decoded MV as a starting MV;

determine a refined MV performing ME refinement on the starting MV for each partition of inter modes allowed under the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16; and

re-encode the MB into the second MPEG stream using the refined MV.

7. The system of claim 5 , wherein the second MPEG encoder is configured to:

if inter mode is used to re-encode the MB and the identified metadata indicates the MB was encoded in the field mode in the first MPEG stream, identify decoded MVs in ½ pel resolution for two 8×16 MVs in the first MPEG stream;

use the decoded MVs to derive a starting MV;

determine a refined MV by performing ME refinement on the starting MV for each partition of inter modes allowed in the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16; and

select the refined MV to re-encode the MB into the second MPEG stream.

8. The system of claim 1 , wherein, if the identified metadata indicates the picture was encoded in frame mode in the first MPEG stream and a (macroblock adaptive frame/field (MBAFF) in the second MPEG stream is on, the second MPEG encoder is configured to:

select the frame mode to re-encode two MBs if the identified metadata indicates the two MBs were encoded in the frame mode in the first MPEG stream;

select the field mode to re-encode the two MBs if the identified metadata indicates the two MBs were encoded in the field mode in the first MPEG stream; and

determine a coding cost of the field mode and determine a coding cost of the frame mode for the two MBs and select a lowest cost mode to re-encode the two MBs if the identified metadata indicates the two MBs were encoded in the frame mode for one MB and the field mode for the other MB in the first MPEG stream.

9. The system of claim 8 , wherein the second MPEG encoder is further configured to:

select the inter mode to re-encode the two MBs if the identified metadata indicates the two MBs were encoded in the inter mode in the first MPEG stream;

select the intra mode to re-encode the two MBs if the identified metadata indicates the two MBs were encoded in the intra mode, in the first MPEG stream; and

determine a coding cost of the intra mode and determine a coding cost of the inter mode for each MB of the two MBs and select a lowest cost mode to re-encode the MB if the identified metadata indicates the two MBs were encoded in the intra mode for one MB and the inter mode for the other MB in the first MPEG stream.

10. The system of claim 9 , wherein if re-encoding a frame MB in inter mode in the second MPEG stream, the second MPEG encoder is configured to:

if the two MBs were encoded in the frame mode in the first MPEG stream:

identify at least one decoded MV in ½ pel resolution for the two MBs in the first MPEG stream;

scale the at least one decoded MV of the first MPEG stream to a current reference frame pointed by ref Idx to form at least one starting MV;

determine a refined MV by performing ME refinement around the at least one starting MV to a quarter pel resolution for each partition of inter modes allowed in the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16;

if the two MBs were encoded in the field mode in the first MPEG stream:

identify at least one decoded MV in ½ pel resolution for the two MBs in the first MPEG stream;

determine at least one starting MV from the at least one decoded field MV;

determine a refined MV by performing ME refinement around the at least one starting MV to a quarter pel resolution for each partition of inter modes allowed in the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16;

if the identified metadata indicates the two MBs were encoded in the frame mode for one MB and the field mode for the other MB in the first MPEG stream:

identify at least one decoded MV in ½ pel resolution for the two MBs in the first MPEG stream;

determine at least one starting MV from the at least one decoded MV; and

determine a refined MV by performing ME refinement around the at least one starting MV to a quarter pel resolution for each partition of inter modes allowed in the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16.

11. The system of claim 9 , wherein if re-encoding a field MB in inter mode in the second MPEG stream, the second MPEG encoder is configured to:

determine whether the two MBs were encoded in both the frame mode, both the field mode, or one the field mode and the frame mode in the first MPEG stream;

identify at least one decoded MV in ½ pel resolution for the two MBs in the first MPEG stream;

determine at least one starting MV from the at least one decoded MV;

determine a refined MV by performing ME refinement around the at least one starting MV to a quarter pel resolution for each partition of inter modes allowed in the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16.

12. The system of claim 9 , wherein if one MB is in intra mode and the other MB is in inter mode in the first MPEG stream, the second MPEG encoder is configured to set the starting MV of the MB in the intra mode to the MV of the other MB; and

if the two MBs are in intra mode in the first MPEG stream, set the starting MVs to zero.

13. The system of claim 1 , wherein, if a macroblock adaptive frame/field (MBAFF) is on, the second MPEG encoder is configured to determine the coding cost of the field mode and determine the coding cost of the frame mode and to select the lower cost mode to re-encode the MB pair.

14. A method of transcoding an input first MPEG stream to a second MPEG stream, the method comprising:

at a first MPEG decoder,

decoding the first MPEG stream;

identifying metadata indicating how the first MPEG stream was encoded, the identified metadata determined during the decoding of the first MPEG steam; and

outputting the metadata; and

at a second MPEG encoder,

maintaining a decoded picture type of I, P, or B;

maintaining a decoded picture structure of frame or field;

receiving the outputted metadata from the first MPEG decoder;

determining whether to re-encode a macroblock (MB) of a picture into the second MPEG stream using one of a frame or a field mode based on the identified metadata indicating how the first MPEG stream was encoded; and

re-encoding the MB into the second MPEG stream using the determined frame or field mode.

15. The method of claim 14 , wherein determining whether to re-encode the MB into the second MPEG stream further comprises:

selecting the field mode to re-encode the MB of the picture in the second MPEG stream if the identified metadata indicates the picture was encoded in the field mode in the first MPEG stream; and

if the picture type is an I field picture, selecting an intra mode to re-encode the MB in the second MPEG stream;

if the picture is one of a B and a P field picture and the identified metadata indicates the MB was encoded in the intra mode in the first MPEG stream, determining a coding cost of intra modes allowed under the second MPEG format including intra mode 4×4, intra mode 8×8 and intra mode 16×16 and selecting a lowest cost mode to re-encode the MB in the second MPEG stream; and

if the picture is one of a B and a P field picture and the identified metadata indicates the MB was encoded in an inter mode in the first MPEG stream, determining the coding cost of inter modes allowed under the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16 and selecting a lowest cost mode to re-encode the MB in the second MPEG stream.

16. The method of claim 15 , further comprising:

if inter mode is used to re-encode the MB, identifying at least one decoded motion vector (MV) in the first MPEG stream wherein the at least one decoded MV is in ½ pel resolution;

using the at least one decoded MV as at least one starting MV;

performing motion estimation (ME) refinement performing ME refinement around the at least one starting MV to a quarter pel resolution for each partition of inter modes allowed in the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and intra mode 16×16; and

re-encoding the MB into the second MPEG stream using the refined MV.

17. The method of claim 14 , further comprising:

selecting the frame mode to re-encode the MB if the identified metadata indicates the picture was encoded in the frame mode in the first MPEG stream and macroblock adaptive frame/field (MBAFF) is off, and

if the MB is in an intra mode in the first MPEG stream, selecting the intra mode to re-encode the MB in the second MPEG stream;

if the MB indicates the MB was encoded in an inter mode in the first MPEG stream, determining a coding cost of inter modes allowed under the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16 and selecting a lowest cost mode to re-encode the MB in the second MPEG stream; and

if the decoded information of the MB indicates the MB was encoded in the intra mode in the first MPEG stream, determining the coding cost of intra modes allowed under the second MPEG format including intra mode 4×4, intra mode 8×8 and intra mode 16×16 and selecting the lowest cost mode to re-encode the MB in the second MPEG stream.

18. The method of claim 17 , further comprising:

if the inter mode is used to re-encode the MB, identifying at least one decoded motion vector (MV) in the first MPEG stream in ½ pel resolution;

using the at least one decoded MV as at least one starting MV;

determining a refined MV by performing motion estimation (ME) refinement around the at least one starting MV to a quarter pel resolution for each partition of inter modes allowed in the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16; and

re-encoding the MB into the second MPEG stream using the refined MV.

19. The method of claim 14 , further comprising,

if the identified metadata indicates the picture was encoded in frame mode in the first MPEG stream and a macroblock adaptive frame/field (MBAFF) in the second MPEG stream is on, and if the identified metadata indicates two MBs were encoded in the frame mode in the first MPEG stream, selecting the frame mode to re-encode the two MBs;

if the identified metadata indicates the two MBs were encoded in the field mode in the first MPEG stream, selecting the field mode to re-encode the two MBs; and

if the identified metadata indicates the two MBs were encoded in the frame mode for one MB and the field mode for the other MB in the first MPEG stream, determining a coding cost of the field mode and determining a coding cost of the frame mode for the two MBs and selecting a lowest cost mode to re-encode the two MBs.

20. The method of claim 19 , further comprising,

selecting the inter mode to re-encode the two MBs if the identified metadata indicates the two MBs were encoded in the inter mode in the first MPEG stream;

selecting the intra mode to re-encode the two MBs if the identified metadata indicates the two MBs were encoded in the intra mode, in the first MPEG stream; and

determining a coding cost of the intra mode and determining a coding cost of the inter mode for each MB of the two MBs and selecting a lowest cost mode to re-encode the MB if the identified metadata indicates the two MBs were encoded in the intra mode for one MB and the inter mode for the other MB in the first MPEG stream.

21. The method of claim 20 , further comprising,

if re-encoding a MB in inter mode in the second MPEG stream,

identifying at least one decoded motion vector (MV) in ½ pel resolution in the first MPEG stream;

determining at least one starting MV from the at least one decoded MV; and

determining a refined MV by performing motion estimation (ME) refinement around the at least one starting MV to a quarter pel resolution for each partition of inter modes allowed in the second MPEG format including inter mode 8×8, inter mode 8×16, inter mode 16×8, and inter mode 16×16.

22. The method of claim 14 , further comprising:

determining a reference picture per MB partition in the second MPEG encoder by using the reference picture in the first MPEG stream;

wherein determining the reference picture per MB partition in the second MPEG encoder by maintaining the reference picture in the first MPEG stream for a P picture comprises allowing a frame MB to refer only to the last coded I or P frame in List 0, and allowing a field MB to refer only to two fields of the last coded I or P frames in List 0; and

wherein determining the reference picture per MB partition in the second MPEG encoder by maintaining the reference picture in the first MPEG stream for a B picture comprises allowing a frame MB to refer to at least one of a last coded I or P frame in List 0 and a last coded I or P frame in List 1, and allowing a field MB to refer to at least one of two fields of the last coded I or P frame in List 0 and two fields of the last coded I or P frame in List 1.

23. A non-transitory computer readable storage medium storing at least one computer program that when executed performs a method for transcoding an input first MPEG stream to a second MPEG stream, the method comprising:

at a first MPEG decoder,

decoding the first MPEG stream;

identifying metadata indicating how the first MPEG stream was encoded, the identified metadata determined during the decoding of the first MPEG steam; and

outputting the identified metadata; and

at a second MPEG encoder,

maintaining a decoded picture type of I, P, or B;

maintaining a decoded picture structure of frame or field;

receiving the identified metadata from the first MPEG decoder;

determining whether to re-encode a macroblock (MB) of a picture into the second MPEG stream using one of a frame or a field mode based on the identified metadata indicating how the first MPEG stream was encoded; and

re-encoding the MB into the second MPEG stream using the determined frame or field mode.