Encoding a Transparency (ALPHA) Channel in a Video Bitstream

• US 20110064142A1
• Filed: 11/22/2010
• Published: 03/17/2011
• Est. Priority Date: 03/31/2005
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Singer, David William, Haskell, Barin Geoffry
• Granted Patent: US 8,830,262 B2
• Time in Patent Office: Days
• US Class Current: 375/240.25
• CPC Class Codes: G09G 2340/02 Handling of images in compressed format, e.g. JPEG, MPEG
• CPC Class Codes: G09G 2340/125 wherein one of the images is motion video
• CPC Class Codes: G09G 5/377 for mixing or overlaying two or more graphic patterns G09G5/02, G09G5/397 take precedence
• CPC Class Codes: H04N 19/103 Selection of coding mode or of prediction mode
• CPC Class Codes: H04N 19/188 the unit being a video data packet, e.g. a network abstraction layer [NAL] unit
• CPC Class Codes: H04N 19/21 with binary alpha-plane coding for video objects, e.g. context-based arithmetic encoding [CAE]
• CPC Class Codes: H04N 19/44 Decoders specially adapted therefor, e.g. video decoders which are asymmetric with respect to the encoder

Abstract

Disclosed herein is a technique for delimiting the alpha channel at the NAL layer in codecs like H.264 to facilitate the optional nature of the alpha channel. In coded video sequences that include alpha, there is one alpha picture for every primary coded (e.g., luma-chroma) picture, and the coded alpha picture is contained in the same access unit as its corresponding primary coded picture. The alpha coded slice NAL units of each access unit are sent after the NAL units of the primary coded picture and redundant coded pictures, if any. The presence or absence of the alpha NAL units does not affect the decoding of the remaining NAL units in any way.

Claims

1. A method to decode an image data stream, the method comprising:

receiving, by a decoder, an image data stream having one or more syntax structures corresponding to an encoded foreground image and one or more syntax structures corresponding to an encoded transparency mask, wherein the one or more syntax structures corresponding to the encoded transparency mask are delimited from and located after the one or more syntax structures corresponding to the foreground image in the image data stream;

decoding, by the decoder, the one or more syntax structures corresponding to the foreground image to produce a decoded foreground image;

decoding, by the decoder, the one or more syntax structures corresponding to the transparency mask to produce a decoded transparency mask; and

developing a display image as a function of the decoded foreground image and the decoded transparency mask.

2. The method of claim 1, wherein the act of receiving comprises receiving, by the decoder, an image data stream encoded in accordance with ITU Recommendation H.264.

3. The method of claim 2, wherein the image data stream comprises a NAL unit format.

4. The method of claim 2, wherein the image data stream comprises a byte stream format.

5. The method of claim 3, further comprising receiving, by the decoder, one or more syntax structures corresponding to an access unit delimiter NAL, wherein the one or more syntax structures corresponding to the access unit delimiter NAL is located before the one or more syntax structures corresponding to the encoded foreground image in the image data stream.

6. The method of claim 1, wherein the act of developing further comprises developing a display image as a function of the decoded foreground image, the decoded transparency mask and a background image.

7. The method of claim 6, wherein the background image is received, by the decoder, separately from the encoded foreground image and the encoded transparency mask.

8. The method of claim 6, wherein the background image is created by the decoder and is not received as part of the image data stream.

9. The method of claim 1, wherein there is one and only one transparency mask for each foreground image.

10. A method to decode an image data stream, the method comprising:

extracting, by a decoder, a syntax structure corresponding to an encoded foreground image from an image data stream;

extracting, by the decoder, a syntax structure corresponding to an encoded transparency mask from the image data stream, wherein the syntax structure corresponding to the transparency mask is delimited from and located after the syntax structure corresponding to the foreground image in the image data stream;

decoding, by the decoder, the syntax structure corresponding to the encoded foreground image to generate a decoded foreground image;

decoding, by the decoder, the syntax structure corresponding to the encoded transparency mask to generate a decoded transparency mask; and

developing, by the decoder, a display image as a function of the decoded foreground image and the decoded transparency mask.

11. A method to encode images, the method comprising:

encoding, by an encoder, a foreground image into one or more syntax structures;

encoding, by the encoder, a transparency mask into one or more syntax structures;

combining, by the encoder, the one or more syntax structures corresponding to the encoded foreground image and the one or more syntax structures corresponding to the encoded transparency mask into an image data stream, wherein the one or more syntax structures corresponding to the encoded transparency mask are delimited from and located after the one or more syntax structures corresponding to the encoded foreground image in the image data stream.

12. The method of claim 11, wherein the image data stream is encoded, by the encoder, in accordance with ITU Recommendation H.264.

13. The method of claim 12, wherein the image data stream is formatted, by the encoder, in a NAL unit stream format.

14. The method of claim 12, wherein the image data stream is formatted, by the encoder, in a byte stream format.

15. The method of claim 13, further comprising:

generating, by the encoder, a SEI NAL unit; and

placing, by the encoder, the SEI NAL unit into the image data stream before the one or more syntax structures corresponding to the encoded foreground image.

16. The method of claim 11, further comprising:

encoding, by the encoder, a background image into one or more syntax structures; and

placing, by the encoder, the one or more syntax structures corresponding to the encoded background image into the image data stream after the one or more syntax structures corresponding to the encoded foreground image and before the one or more syntax structures corresponding to the transparency mask.

17. The method of claim 11, wherein the encoder encodes one and only one transparency mask for each foreground image.

18. A method to decode an image comprising:

receiving, by a decoder, an image data stream encoded in accordance with ITU Recommendation H.264, the image data stream including a primary encoded picture, a delimiter following the primary encoded picture and an encoded transparency picture following the delimiter;

decoding, by the decoder, the encoded primary picture to produce a decoded primary picture;

decoding, by the decoder, the encoded transparency picture to produce a decoded transparency picture; and

developing, by the decoder, a display image as a function of the decoded primary picture and the decoded transparency picture.

19. The method of claim 18, wherein the act of receiving further comprises receiving, by the decoder, an encoded redundant picture following the encoded primary picture and before the encoded transparency picture.

20. The method of claim 19, further comprising decoding, by the decoder, the redundant picture.

21. The method of claim 20, wherein the act of developing further comprises developing, by the decoder, the display image as a function of the redundant picture.

22. The method of claim 18, wherein the act of developing comprises:

generating, by the decoder, a background picture; and

developing, by the decoder, the display image as a function of the decoded primary picture and the background picture and the decoded transparency picture.

23. The method of claim 18, wherein the act of receiving further comprises receiving, by the decoder, a unit delimiter NAL before the encoded primary picture.