1/3/2024 0 Comments Turn a photo into 8 bit![]() ![]() ![]() It can be viewed on all displays and most programs without modification. The position I've moved to after reading through and thinking about all the responses here and gdal-dev ( here) is we need to ask for both 8 & 12 bit from the vendors.Ĩ bit is for visual use. That leaves more bits to encode the data in the middle of the intensity region, which might be the important part. I say "the simplest way of converting" because other methods could be used that preserve detail in certain regions of the intensity - perhaps the highest and lowest parts of the image can be encoded with less dynamic range, taking fewer bits, by a non-linear scaling of the raw data. The same happens with binary data truncated from 12 to 8 bits. For example with decimal numbers, the ability to discriminate between values of 9 and 12 - think how if rounded to the nearest 10 they'll both be 10 in the data. Its like rounding decimal numbers to the nearest 100 or 1000. In the simplest way of converting from 12 bits to 8 bits the last four bits will be set to zero, and the remaining bits shifted right four places. ![]() It depends on the use case.Ĩ bits can encode a number from 0 to 255 (2 to the power of 8 values)ġ2 bits can encode a number from 0 to 4095 (2 to the power of 12 values) Tl dr: Maybe nothing is lost, maybe it completely breaks a workflow. Preserving the 12-bits also gives a customer the option of defining their own stretch or other radiometric transformation. It's not clear to me from the docs what stretch is performed to transform from 12-bits to 8-bits in the Primary Products. If one just wants to look at color composites (without having to stretch the images oneself), then ordering Primary Products stretched to 8-bits should be fine. If you're ordering Primary Products (as opposed to Standard Orthos), one might want to preserve the original 12-bit range in order to perform some quantitative analysis that depends on the physical units (scaled spectral radiance at sensor, in this case), or at least benefits from having as much of the original information as possible (such as some stereo photogrammetry pipelines). In the case of SPOT 6/7, you might find it helpful to review the SPOT image user guide, particularly the descriptions of different processing levels in section 2.3. Are the 12-bit numbers all in the range already? If yes, then stretching from 12-bit to 8-bit won't "lose" anything, but will make the file smaller. Are they physical units such as spectral radiance? If yes, then stretching to 8-bits may obliterate the physical meaning of the pixel values. The naive answer to the "general" question "what is lost when transforming 12-bit raster data to 8-bit?" is "4 bits of precision." This answer may not be terribly useful though because there are different ways to stretch the data from 12-bits to 8-bits and it also depends on what the 12-bit numbers represent. I know that you have tried to frame this as a general question, rather than specific to SPOT 6/7, but it's really a little of both. ![]()
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