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By choosing compress option, tvg tries to compress the data to reduce the binary size. Since the compression has the double-edges sword, we provides an option to users to select it by their demand. Basically, compression is better than non-compression. After profiling, we decided to use the encoder/decoder of Guilherme R. Lampert's. Here is the profiling result: test.tvg: 296037 -> 243411 (-17%) tiger.tvg: 54568 -> 50622 (-7%) image-embedded.tvg: 2282 -> 1231 (-46%) @Issue: https://github.com/Samsung/thorvg/issues/639 About compression method: Lempel–Ziv–Welch (LZW) encoder/decoder by Guilherme R. Lampert This is the compression scheme used by the GIF image format and the Unix 'compress' tool. Main differences from this implementation is that End Of Input (EOI) and Clear Codes (CC) are not stored in the output and the max code length in bits is 12, vs 16 in compress. EOI is simply detected by the end of the data stream, while CC happens if the dictionary gets filled. Data is written/read from bit streams, which handle byte-alignment for us in a transparent way. The decoder relies on the hardcoded data layout produced by the encoder, since no additional reconstruction data is added to the output, so they must match. The nice thing about LZW is that we can reconstruct the dictionary directly from the stream of codes generated by the encoder, so this avoids storing additional headers in the bit stream. The output code length is variable. It starts with the minimum number of bits required to store the base byte-sized dictionary and automatically increases as the dictionary gets larger (it starts at 9-bits and grows to 10-bits when code 512 is added, then 11-bits when 1024 is added, and so on). If the dictionary is filled (4096 items for a 12-bits dictionary), the whole thing is cleared and the process starts over. This is the main reason why the encoder and the decoder must match perfectly, since the lengths of the codes will not be specified with the data itself. |
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| thorvg.h | ||