9/11/2023 0 Comments Slimraw comparisonLogarithmic tonal compression is also used in ARRIRAW (although Arri prefer to call their raw "uncompressed") and other raw formats. The same idea was used in the 10-bit Cineon log format for negative film scans (film density itself is a logarithmic function of captured light intensity) and, more recently, in 10-bit log raw from the Canon C500 cinema camera. The curve maps the original linear signal into a 10-bit coding space. The log conversion is transparent for the user: the log-to-linear inverse transform is described in the DNG metadata and the curve is reversed by the raw processing software on import. The curve is designed so that only the brightest few stops of the captured dynamic range are tonally compressed and use less coding values than the linear distribution darks are left untouched. This is an entropy reducing step which allows for further space savings compared to Lossless. It utilizes the same lossless entropy coding as the Lossless mode, but applies a non-linear (logarithmic) tone mapping curve before entropy coding. This is where the "Lossless 10-bit log" mode is grounded. This distribution of coding values is quite ineffective, because eye response to light intensity is non-linear (approximately logarithmic): we don't really benefit from the excess of tonal info in the brightest captured stops. For example, in 12-bit linear raw the highest stop uses 2048 values, the stop below uses 1024 values, the next stop down uses 512 values, and so on. When these light levels are digitized, the highest stop of the captured dynamic range occupies half of the digital coding space, the next stop below the highest takes half of what is left, etc. Double the amount of light leads to double the photons captured. Sensors capture light in a linear fashion. Lossless compression is included in the DNG specification and is widely supported by video and photo raw processing applications.Ī little bit of background first. But if your storage throughput (and not the CPU) is the bottleneck, you may leave this option enabled with no processing speed hit. Enabling the option "Maximum compression" (in File->Other options) can often squeeze a little bit more space savings in exchange for slower execution. For example, Sony FS-series CinemaDNG will often get compressed to 2.5:1 or better, and Blackmagic lossless CinemaDNG will be around 1.6:1. This can go up or down depending on image complexity and tonal specifics. Typical compression achieved is around 2:1 (or down to around 50% of the original size). Lossless compression is variable bit rate by nature. In the raw processor the original data is fully reconstructed from the compressed data: the decompressed image is exactly the same as the original uncompressed image. DNG/CinemaDNG lossless compression is truly lossless as opposed to "visually lossless" (which is a euphemism for "lossy" outside the strict scientific community). The purpose of lossless compression is creating a smaller representation of the original data with no loss of information.
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