Specifications

CD-i FAQ 2000 Edition Revised February 21, 2001
Latest version and more CD-i info: http://www.icdia.org
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used for a single-colored background or for MPEG full motion video (or to display video from an
external source on some players). The content of each of both middle planes is placed in one of
the two memory banks (see: 3.9 How much RAM does CD-i incorporate?).
Parts of an image on one of the middle two planes can be transparant, so that the underlying
plane becomes visible. This can for example be used to show subtitles or menubars on an image.
Both planes can also be used to dissolve from one image to the other, using an unlimited
variations of wipes, including professional fades in a way that's virtually impossible to achieve
with today's modern VGA adapters.
There are various encoding techniques for video that can be used in CD-i:
• DYUV
DYUV or Delta YUV is used for the encoding of high quality photographs and other natural
images. It is based on the fact that the human eye is more sensible for differences in brightness
then for differences in color. Therefore, it stores one color for a set of pixels, and a brightness
value for each pixel. The result is an image of slightly more than 100 KB, leaving enough space in
memory for other information. Due to the complexity of a DYUV image it must be stored on the
disc in advance, a DYUV image can not be created nor modified in the player. DYUV is used
most often in CD-i titles because of its high quality and efficiant storage.
• RGB 555
RGB 555 is a professional image format which allows for 5 bits per R, G and B value, resulting in
a picture with a maximum of over 32,000 colors. Since RGB 555 uses both planes to display the
image, it can not be used in combination with other graphics. An RGB 55 image is roughly 200
KB in size. The image can be altered by the player at runtime. RGB 555 is actually never used in
regular CD-i titles because of its inefficiency and limitations in usage.
• CLUT
CLUT, or Color Look-Up Table, is a way of encoding simple graphics. The colors that should be
used in a certain picture are stored in a CLUT-table, which reduces the size of the image
dramatically because color values refer to the appropriate CLUT-entry instead of indicating for
example a 24-bit color value. In CD-i, a CLUT image can have a 8 bit (256 color), 7 bit (128
color), 4 bit (16 color) or 3 bit (8 color). The latter two can be used in double resolution (see:
3.13.1 What is the screen resolution of a CD-i player?).
• Run Lenght Encoding
RLE or Run Lenght Encoding is a variation of CLUT. Besides storing the CLUT-color table in an
image, it further reduces the image size by storing certain 'run lenghts' of repeating horizontal
pixels with the same color. The results are usually pictures between 10 and 30 KB in size. This
makes RLE ideal for animations. Most of the animations you see on Base Case CD-i titles are
RLE.
• QHY
QHY or Quantized High Y is an encoding technique that is in essence not a part of the Base
Case, but it is described in the Green Book and supported by all CD-i players. Its encoding
technique is made up of the combination of DYUV and RLE, resulting in a very sharp high quality
natural image, that is displayed in CD-i's high resolution mode (see: 3.13.1 What is the screen
resolution of a CD-i player?). A QHY image is usually about 130 KB in size. Since it consists of a
DYUV component, it cannot be modified by the player. QHY is for example used to display the
images of a Photo-CD in high resolution on a CD-i player.
Besides these Base Case video encoding options, a player equiped with a Digital Video cartridge
can also display MPEG motion video, and normal and high resolution MPEG still images. The
encoding teqniques for MPEG are far more advanced than the ones defined for Base Case CD-i,
but images in these formats cannot be viewed on players without a Digital Video cartridge.