Extended bit-range in HEIF
Improved image coding techniques for high-bit depth images includes deriving two or more separate lower-bit-depth main and extension images from one high-bit-depth source image, and then encoding them separately as lower-bit-depth images. At a decoder, the separate main and extension images are decoded, and then combined to create a single reconstructed source image. The improved techniques may use lossless or lossy codecs, and may provide some backward compatibility with legacy decoders.
1 . An image encoding method, comprising:
deriving a main image from most significant bits of pixel values of a source image, pixel values of the main image having a main bit depth determined by a selected upshift number;
encoding the main image according to a coding protocol;
decoding the encoded main image as a reconstructed main image;
deriving an extension image from the reconstructed main image, the source image and the selected upshift number, wherein pixel values of the extension image have an extension bit depth;
encoding the extension image according to the coding protocol;
storing, according to a syntax of a coded media file, the encoded main image, the encoded extension image, and coding parameters indicating the extension bit depth and the selected upshift number.
2 . The image encoding method of claim 1 , wherein the upshift number is selected based on a bit depth of pixel values in the source image and a bit depth constraint of the coding protocol.
3 . The image encoding method of claim 1 , wherein the extension bit depth is selected as the selected upshift number plus zero or more.
4 . The image encoding method of claim 1 , wherein the coding protocol supports the main bit depth and does not support a bit depth of pixel values in the source image.
5 . The image encoding method of claim 1 , wherein
the deriving of the extension image is based on a selected extension algorithm; and
the stored coding parameters further indicate the selected extension algorithm.
6 . The image encoding method of claim 1 , wherein, when the extension bit depth is greater than a bit depth constraint of the coding protocol, the extension image is a temporary image that is recursively encoded by:
deriving a first extension image from most significant bits of pixel values of the temporary image, pixel values of the first extension image having a first extension bit depth determined by a selected second upshift number;
encoding the first extension image according to the coding protocol;
decoding the encoded first extension image as a reconstructed first extension image;
deriving a second extension image from the reconstructed first extension image, the temporary image, and the selected second upshift number, wherein pixel values of the second extension image have a second extension bit depth;
encoding the second extension image according to the coding protocol;
storing, according to the syntax of the coded media file, the encoded first extension image, the encoded second extension image, and a representation of the second extension bit depth and the selected second upshift number.
7 . The image encoding method of claim 1 , wherein, when the main bit depth is greater than a bit depth constraint of the coding protocol, the main image is a temporary main image that is recursively encoded by:
deriving a second main image from the most significant bits of pixel values of the temporary main image and a second selected upshift number;
encoding the second main image according to the coding protocol;
decoding the second main image as a reconstructed second main image;
deriving a second extension image from the reconstructed second main image, the temporary main image, the second selected upshift number, wherein the pixel values of the second extension image have a selected second extension bit depth; and
encoding the second extension image according to the coding protocol,
storing, according to the syntax of the coded media file, the encoded second main image, the encoded second extension image, and a representation of the second selected upshift number and the selected extension bit depth.
8 . The image encoding method of claim 1 , wherein the coding parameters are stored in a derived image syntax element indicating the derived image is based on the encoded main image and the encoded extension images.
9 . The image encoding method of claim 1 , wherein the coding parameters are stored in a group syntax element for a group including the encoded main image and the encoded extension images.
10 . The method of claim 1 , wherein the syntax represents the coded media file as:
a main image element that encapsulates the encoded main image,
a derived image element that encapsulates the encoded extension image, and
a metadata element that contains:
a pointer to the main image element,
a pointer to the derived image element, and
the coding parameters indicating the extension bit depth and the selected upshift number.
11 . An image decoding method, comprising:
extracting, from a syntax in a coded media file, an encoded main image, an encoded extension image and coding parameters including a selected upshift number of bits and an extension bit depth;
decoding the encoded main image according to a coding protocol to produce a reconstructed main image having pixels with a main bit depth;
decoding the encoded extension image according to the coding protocol to produce a reconstructed extension image having pixel with the extension bit depth;
combining the reconstructed main image with the reconstructed extension image based on the selected upshift number and the extension bit depth to produce a reconstructed source image.
12 . The image decoding method of claim 11 , wherein the extension bit depth is zero or more greater than selected upshift number.
13 . The image decoding method of claim 11 , wherein the method is performed by an image decoder that uses a coding protocol that supports the main bit depth and does not support a bit depth of the reconstructed source image.
14 . The image decoding method of claim 11 , wherein the combining is based on a selected extension algorithm, and the extracted coding parameters further indicate the selected extension algorithm.
15 . The image decoding method of claim 11 , wherein the coding parameters are extracted from a derived image syntax element indicating the derived image is based on the encoded main image and the encoded extension images.
16 . The image decoding method of claim 11 , wherein the coding parameters are extracted from a group syntax element for a group including the encoded main image and the encoded extension images.
17 . The method of claim 11 , wherein the syntax represents the coded media file as:
a main image element that encapsulates the encoded main image,
a derived image element that encapsulates the encoded extension image, and
a metadata element that contains:
a pointer to the main image element,
a pointer to the derived image element, and
coding parameters indicating the extension bit depth and the selected upshift number.
18 . An image decoder, comprising:
a parser configured for parsing, from syntax in a coded media file, an encoded main image, an encoded extension image, and coding parameters including a selected upshift number of bits and an extension bit depth;
an image decoder for decoding an encoded image according to a coding protocol;
an extension combiner for combing a main image with an extension image; and
a memory device storing program instructions that, when executed by a processor, cause the processor to:
control decoding, by the image decoder, the encoded main image to produce a reconstructed main image having pixels with a main bit depth;
control decoding, by the image decoder, the encoded extension image to produce a reconstructed extension image having pixel with the extension bit depth;
control combining, by the extension combiner, the reconstructed main image with the reconstructed extension image based on the selected upshift number and the extension bit depth to produce a reconstructed source image.
19 . The image decoder of claim 18 , wherein the upshift number is selected based on a bit depth of pixel values in the source image and a bit depth constraint of the coding protocol.
20 . The image decoder of claim 18 , wherein the extension bit depth is selected as the selected upshift number plus zero or more.
21 . The image decoder of claim 18 , wherein the coding protocol supports the main bit depth and does not support a bit depth of pixel values in the source image.
22 . The image decoder of claim 18 , the combining of the reconstructed images is based on a selected extension algorithm; and the stored coding parameters further indicate the selected extension algorithm.