Switch-select single frame reference
Coding techniques for a video image compression system involve improving an image quality of a sequence of two or more bi-directionally predicted intermediate frames, where each of the frames includes multiple pixels. One method involves determining a brightness value of at least one pixel of each bi-directionally predicted intermediate frame in the sequence as an equal average of brightness values of pixels in non-bidirectionally predicted frames bracketing the sequence of bi-directionally predicted intermediate frames. The brightness values of the pixels in at least one of the non-bidirectionally predicted frames is converted from a non-linear representation.
1. A method for inter-frame prediction decoding of a bitstream, the method comprising:
receiving a bitstream at a decoder, the bitstream including a plurality of inter-frame predicted frames, a plurality of intra-frame predicted frames, and a 2-bit switch-select code for a macroblock of a first of the inter-frame predicted frames, wherein the 2-bit switch-select code designates a single previous frame as a reference frame of choice;
selecting, at the decoder, a single reference frame from a set of candidate reference frames for forward inter-frame prediction for the macroblock of the first inter-frame predicted frame based on the 2-bit switch-select code, the set of candidate reference frames comprising at least three referenceable frames; and
in a mode without interpolation between reference frames, decoding, by the decoder, the macroblock of the first inter-frame predicted frame by frame reference to the single selected reference frame,
wherein the set of candidate reference frames for forward inter-frame prediction includes:
(i) a most recent previous frame in display order from the first inter-frame predicted frame, and
(ii) an intra-frame predicted frame more distant in previous display order than the most recent previous frame from the first inter-frame predicted frame.
2. The method of claim 1 wherein the bitstream includes a 2-bit code for changing the reference frame of choice for each macroblock of the first inter-frame predicted frame.
3. The method of claim 1 wherein the most recent previous frame is an inter-frame predicted frame.
4. The method of claim 1 wherein the most recent previous frame is an intra-frame predicted frame.
5. A system for inter-frame prediction decoding of a bitstream, the system comprising:
one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising:
receiving a bitstream at a decoder, the bitstream including a plurality of inter-frame predicted frames, a plurality of intra-frame predicted frames, and a 2-bit, wherein the 2-bit code designates a previous frame as a reference frame of choice;
selecting, at the decoder, a reference frame from a set of candidate reference frames for forward inter-frame prediction based on the 2-bit code, the set of candidate reference frames comprising at least three referenceable frames;
decoding, by the decoder, a macroblock of a first inter-frame predicted frame by frame reference to the selected reference frame; and
applying a displacement filter to pixel values of the selected reference frame for sub-pixel interpolation, the displacement filter having negative lobes,
wherein the set of candidate reference frames for forward inter-frame prediction includes:
(i) a most recent previous frame in display order from the first inter-frame predicted frame, and
(ii) an intra-frame predicted frame more distant in previous display order than the most recent previous frame from the first inter-frame predicted frame.
6. The system of claim 5 wherein the bitstream includes a 2-bit code for changing the reference frame of choice for each macroblock of the first inter-frame predicted frame.
7. The system of claim 5 wherein the most recent previous frame is an inter-frame predicted frame.
8. The system of claim 5 wherein the most recent previous frame is an intra-frame predicted frame.
9. A method for inter-frame prediction decoding of a bitstream, the method comprising:
receiving a bitstream at a decoder, the bitstream including inter-frame predicted frames, intra-frame predicted frames, and a switch-select code for a macroblock of a first of the inter-frame predicted frames for reference frame selection of a single reference frame;
selecting, at the decoder, a single reference frame from a set of candidate reference frames for forward inter-frame prediction for the macroblock of the first inter-frame predicted frame based on the switch-select code, the set of candidate reference frames comprising at least three referenceable frames; and
in a mode without interpolation between reference frames, decoding, by the decoder, the macroblock of the first inter-frame predicted frame by frame reference to the single selected reference frame,
wherein the set of candidate reference frames for forward inter-frame prediction includes:
(i) a most recent previous frame in display order from the first inter-frame predicted frame, and
(ii) two frames more distant in previous display order than the most recent previous frame from the first inter-frame predicted frame.
10. The method of claim 9 wherein the bitstream includes a code for changing the reference frame of choice for each macroblock of the first inter-frame predicted frame.
11. The method of claim 9 wherein the most recent previous frame is an inter-frame predicted frame.
12. The method of claim 9 wherein the most recent previous frame is an intra-frame predicted frame.
13. The method of claim 9 wherein the switch-select code is a 2-bit switch-select code.