- FourCC: IV41
- Company: Intel, then Ligos
- Samples: http://samples.mplayerhq.hu/V-codecs/IV41/
Indeo Video Interactive (further IVI) is a proprietary video compression algorithm developed by Intel. IVI is a completely different technology compared to the previous Indeo releases were based on vector quantization (see Indeo 2 and Indeo 3). IVI is a block-based interframe compression algorithm similar to MPEG with the exception that it uses block wavelet transforms instead of the common DCT transform. IVI offers a set of "interactive" features which go beyond the traditional services provided by codecs. These include transparency (chroma-keying), local decoding, scalabilty and media protection.
There are currently two main versions of IVI: version 4 (further Indeo4) and version 5 (further Indeo5). Version 5 is very similar to 4, but uses a different more compactly bitstream format. For a description of the IVI version 5 see here: Indeo 5.
Although IVI codec software is currently available for Windows, Macintosh (QuickTime) and Linux (Xanim player), it is not well suited to cross-platform playback due to several incompabilities and speed differencies.
The following links provide a good user-level information about IVI:   
This article focuses on technical details of Indeo4 necessary to build a decoder. IVI is considered an undocumented algorithm. The author of this article was able to find several patents issued by Intel describing different parts of IVI. You can find all them in the references below. Unfortunately the most important parts of the codec like bitstream format or decoding tables were not described in the patents (or it was difficult to find any). The detailed information was obtained through reverse engineering of binary codecs.
Indeo Video Interactive Version 4 (Indeo4)
Brief description of the coding techniques
Indeo4 operates internally in the YUV color space. The color format used is YVU 4:1:0 aka YVU9.
Transform coding is used to convert images from the spatial domain to the frequency domain. Indeo4 decomposes an image into square blocks and performs block wavelet transform in order to obtain frequency coefficients. Further quantization, run-length coding and huffman coding are performed in order to compress these coefficients. Indeo4 uses the block Haar and block Slant transforms as block transforms. Available block sizes are 8x8 and 4x4 pixels. Compared to the DCT, both Haar and Slant transforms can be programmed to use only shifts and adds without multiplies. It makes the coding much faster compared to the DCT-based algorithms (MPEG, H.263). Further these transforms be easily parallelized using a SIMD instruction set.
Indeo4 uses motion estimation in order to perform temporal compression. It supports intra-coded frames (frames without prediction) and inter-coded frames (delta frames). Delta frames can be coded relatively to a previous frame (P frame) or relatively to previous and subsequent frames (bi-directional or B frame).
Brief description of the interactive features
The Indeo4 supports a form of transparency analogous to chroma-keying or blue-screening, allowing foreground video objects to be composited dynamically over a different background - a bitmap or possibly even another video. Encoder analyses each frame and generates a 1-bit transparency bitmask: a pixel is either transparent or not. This bitmask is then encoded as an additional plane into the bitstream using huffman coding.
Sometimes an application (a game, for example) needs to display only a part of the decoded video image. In this case, much of the source image doesn't need to be displayed. Indeo4 provides a capability called local decode that saves processor resources by decoding images partially. The playback application can tell Indeo4 to decode only a rectangular subregion, called the view port, from the source video image. The minimum possible size of the local decode viewport is defined during compression, but the display size and location of the viewport can be changed dynamically during playback. During compression, Indeo4 breaks each frame into small pieces called slices (tiles). Each slice is then encoded into the bitstream as self-containing section that can be easily skipped at decoding time. The decoder decides if a particular slice should be decoded or not.
This feature allows Indeo4 to adapt playback to the processor power of the particular machine being used for playback. This works by dividing the image into a number of frequency bands using wavelet decomposition. These bands represent the image at a different level of sharpness. All bands are necessary to perfectly recreate the original image. But if there is not enough processor power available, the decoder can decompress fewer bands of each frame, rather than simply dropping frames. This produces blurry images, but preserves the motion.
Media protection through access keys
Indeo4 allows video to be compressed with an embedded "password" (access key) which must be supplied by the player software for the video to be usable. If no valid password was specified the decoder should not decode a protected video clip. Nevertheless no encryption of the frame data is performed.
Annex A: Huffman coding
Huffman codes used in Indeo4 are of the form [string of k 1's][some additional bits]. The [string of k 1's] is the code prefix and the additional bits are the code bits. The same form of huffman coding was already used in Indeo 2 codec. Below an example of such a codebook:
0x 10xx 110xxx 1110xxxx 11110xxxxx 111110xxxxxx 111111xxxxxxx*
Codebooks like this one can be completely specified by saying for each k how many "additional bits x" there are. Thus the codebook above can be defined using the following short descriptor:
numRows = 7 xbits[numRows] = 1, 2, 3, 4, 5, 6, 7*
The asterisk denotes that the "0" at the end of the prefix in the last row is replaced with an "x" bit.
All Indeo4 codebooks are specified using short descriptors described above. Due to the high compactly nature of those, it's possibly to change codebooks several times per frame (for example, each band can have its own codebook representing signals more compactly as the predefined one).