Apparatus and method for encoding and decoding image containing gray alpha channel image

An apparatus to encode an image containing a gray alpha channel image, the apparatus encoding the image including gray alpha components and brightness and hue components in units of predetermined blocks, the apparatus comprising:

• a block data reception unit which receives image data of a block currently being input to the apparatus and classifies the current block either as a foreground image portion or as a background image portion according to the values of the gray alpha components included in the current block;
• a foreground image encoding unit which sequentially encodes the gray alpha components and the brightness and hue components of the current block if the current block is classified as the foreground image portion;
• and
• a background image encoding unit which encodes the gray alpha components of the current block without encoding initial brightness and hue components of the current block if the current block is classified as the background image portion,
• wherein the foreground image encoding unit comprises:
• a gray prediction error generation unit which generates gray prediction errors of the current block;
• a gray prediction error encoding unit which decides whether to encode the generated gray prediction errors of the current block based on a result of comparing a sum of absolute values of the generated gray prediction errors of the current block with a predetermined critical value;
• a brightness and hue prediction error generation unit which generates brightness and hue prediction errors of the current block;
• a brightness and hue prediction error encoding unit which decides whether to encode the generated brightness and hue prediction errors of the current block based on the result of the comparing of the absolute values of the gray prediction errors of the current block with the predetermined critical value;
• and
• a bitstream generation unit which generates a bitstream based on the comparing results obtained by the gray prediction error encoding unit and the comparing results obtained by the brightness and hue prediction error encoding unit.

A method of encoding an image containing a gray alpha channel image, which encodes an image including gray alpha components and brightness and hue components in units of predetermined blocks, and which is performed by an image encoding apparatus, the method comprising:

• classifying a current block which is received by the image encoding apparatus either as a foreground image portion or as a background image portion according to values of the gray alpha components included in the current block;
• sequentially encoding the gray alpha components and the brightness and hue components of the current block if the current block is classified as the foreground image portion;
• and
• encoding the gray alpha components of the current block without encoding initial brightness and hue components of the current block if the current block is classified as the background image portion,
• wherein the gray alpha components is included in the current block in advance for serving as a mask for classifying the foreground image portion and the background image portion in the image and the values of the gray alpha components indicate a degree of opacity of the current block and serve as weights for the brightness and hue components of the current block when synthesizing a plurality of images.

An apparatus to decode an image including a gray alpha channel image, which decodes a bitstream into which an image including gray alpha components and/or brightness and hue components is encoded, the apparatus comprising:

• a bitstream interpretation unit which interprets the bitstream in units of predetermined blocks and classifies a current block obtained as one of the interpretation results either as a foreground image portion or as a background image portion;
• a foreground image decoding unit which generates a first restored gray alpha channel image and a restored brightness and hue image by sequentially decoding the gray alpha components and the brightness and hue components of the current block if the current block is classified as the foreground image portion;
• and
• a background image decoding unit which generates a second restored gray alpha channel image by decoding the gray alpha components of the current block without decoding initial brightness and hue components of the current block if the current block is classified as the background image portion,
• wherein the gray alpha components are included in the current block in advance for serving as a mask for classifying the foreground image portion and the background image portion in the image and values of the gray alpha components indicate a degree of opacity of the current block and serve as weights for the brightness and hue components of the current block when synthesizing a plurality of images.

A method of decoding an image including a gray alpha channel image, which decodes a bitstream into which an image including gray alpha components and brightness and hue components is encoded, and which is performed by an image decoding apparatus, the method comprising:

• interpreting the bitstream which is received by the image decoding apparatus in units of predetermined blocks and classifying a current block obtained as one of the interpretation results either as a foreground image portion or as a background image portion;
• generating a first restored gray alpha channel image and a restored brightness and hue image by sequentially decoding the gray alpha components and the brightness and hue components of the current block if the current block is classified as the foreground image portion;
• and
• generating a second restored gray alpha channel image by decoding the gray alpha components of the current block without decoding initial brightness and hue components of the current block if the current block is classified as the background image portion,
• wherein the gray alpha components are included in the current block in advance for serving as a mask for classifying the foreground image portion and the background image portion in the image and values of the gray alpha components indicate a degree of opacity of the current block and serve as weights for the brightness and hue components of the current block when synthesizing a plurality of images.

A non-transitory computer-readable recording medium storing a computer program for executing a method of encoding an image containing a gray alpha channel image, which encodes an image including gray alpha components and brightness and hue components in units of predetermined blocks, the method comprising:

• classifying a current block either as a foreground image portion or as a background image portion according to values of the gray alpha components included in the current block;
• sequentially encoding the gray alpha components and the brightness and hue components of the current block if the current block is classified as the foreground image portion;
• and
• encoding the gray alpha components of the current block without encoding initial brightness and hue components of the current block if the current block is classified as the background image portion,
• wherein the gray alpha components is included in the current block in advance for serving as a mask for classifying the foreground image portion and the background image portion in the image and the values of the gray alpha components indicate a degree of opacity of the current block and serve as weights for the brightness and hue components of the current block when synthesizing a plurality of images.

A non-transitory computer-readable recording medium storing a computer program for executing a method of decoding an image including a gray alpha channel image, which decodes a bitstream into which an image including gray alpha components and brightness and hue components is encoded, the method comprising:

• interpreting the bitstream in units of predetermined blocks and classifying a current block obtained as one of the interpretation results either as a foreground image portion or as a background image portion;
• generating a first restored gray alpha channel image and a restored brightness and hue image by sequentially decoding the gray alpha components and the brightness and hue components of the current block if the current block is classified as the foreground image portion;
• and
• generating a second restored gray alpha channel image by decoding the gray alpha components of the current block if the current block without decoding the brightness and hue components of the current block is classified as the background image portion,
• wherein the gray alpha components are included in the current block in advance for serving as a mask for classifying the foreground image portion and the background image portion in the image and values of the gray alpha components indicate a degree of opacity of the current block and serve as weights for the brightness and hue components of the current block when synthesizing a plurality of images.