Reproduction device, integrated circuit, reproduction method, program, and computer-readable recording medium

An integrated circuit mountable in a playback device for playing back a subtitle stream in synchronization with a video stream, the integrated circuit comprising:

• a video decoder operable to decode the video stream;
• a subtitle decoder operable to decode the subtitle stream to obtain a bitmap of a subtitle;
• and
• a color conversion unit operable to convert bit values of the bitmap into display colors based on a color lookup table, the display colors including a character color, a background color, and one or more gradation colors for gradation between the character color and the background color, and the color lookup table including pieces of color information corresponding one-to-one to indexes of the bit values of the bitmap,
• wherein, prior to the conversion of the bit values of the bitmap into the display colors, the subtitle decoder:
• detects a period on a time axis of the video stream, in which a pair of (i) an index corresponding to a piece of color information of the character color and (ii) an index corresponding to a piece of color information of the background color is continuously used;
• and
• updates the color lookup table by assigning a piece of color information of each of the one or more gradation colors to one of the indexes other than the indexes used in the period, and
• wherein a number N of gradation colors used for the gradation in the period is calculated according to the following formula:
• the number N of gradation colors=(2n−M)/a number of pairs of character colors and background colors, where M represents a number of used indexes in the period and n represents a bit width of each index.

A playback method for playing back a subtitle stream in synchronization with a video stream, the playback method comprising the steps of:

• decoding the video stream;
• decoding the subtitle stream to obtain a bitmap of a subtitle;
• and
• converting bit values of the bitmap into display colors based on a color lookup table, the display colors including a character color, a background color, and one or more gradation colors for gradation between the character color and the background color, and the color lookup table including pieces of color information corresponding one-to-one to indexes of the bit values of the bitmap,
• wherein, prior to the conversion of the bit values of the bitmap into the display colors, the step of decoding the subtitle stream to obtain the bitmap:
• detects a period on a time axis of the video stream, in which a pair of (i) an index corresponding to a piece of color information of the character color and (ii) an index corresponding to a piece of color information of the background color is continuously used;
• and
• updates the color lookup table by assigning a piece of color information of each of the one or more gradation colors to one of the indexes other than the indexes used in the period, and
• wherein a number N of gradation colors used for the gradation in the period is calculated according to the following formula:
• the number N of gradation colors=(2n−M)/a number of pairs of character colors and background colors, where M represents a number of used indexes in the period and n represents a bit width of each index.

A non-transitory computer-readable recording medium having a program recorded thereon, the program for playing back a subtitle stream in synchronization with a video stream, and the program causing a computer to execute a playback method comprising the steps of:

• decoding the video stream;
• decoding the subtitle stream to obtain a bitmap of a subtitle;
• and
• converting bit values of the bitmap into display colors based on a color lookup table, the display colors including a character color, a background color, and one or more gradation colors for gradation between the character color and the background color, and the color lookup table including pieces of color information corresponding one-to-one to indexes of the bit values of the bitmap,
• wherein, prior to the conversion of the bit values of the bitmap into the display colors, the step of decoding the subtitle stream to obtain the bitmap:
• detects a period on a time axis of the video stream, in which a pair of (i) an index corresponding to a piece of color information of the character color and (ii) an index corresponding to a piece of color information of the background color is continuously used;
• and
• updates the color lookup table by assigning a piece of color information of each of the one or more gradation colors to one of the indexes other than the indexes used in the period, and
• wherein a number N of gradation colors used for the gradation in the period is calculated according to the following formula:
• the number N of gradation colors=(2n−M)/a number of pairs of character colors and background colors, where M represents a number of used indexes in the period and n represents a bit width of each index.