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I am trying to find references on processing carried out inside the camera before the image is stored in RAW format. I am quite sure that some denoising is performed at the hardware level after the gain (that in photography we call ISO).

Are there any reference on the possible processing carried out before RAW image are produced?

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That all depends on the specific source of the "raw" images.

"Raw" isn't a file format, or even a type of file. It's just a catchphrase we use to refer to image information recorded at the most basic level by a camera.

Since different cameras can and often will do things differently, exactly what a raw file contains and does not contain can vary from one camera to the next. Manufacturers tend to do most or all of the same things in the various models they offer, but there's a significant variation about what is and is not processed before raw information is recorded from one camera maker to the next.

Most raw image files from cameras with Bayer mask color arrays are saved without any demosaicing or gamma correction applied. So each "pixel" contains a single, linear, monochromatic luminance value that tells how much light energy was recorded by each photosite on the sensor.

  • Almost all do statistical noise reduction prior to analog-to-digital conversion.
  • Some do little else, other than basic noise reduction, before converting the analog signal to digital information and recording the values at that point.
  • Other do things as extensive as lens correction for things such as distortion and vignetting, and maybe even chromatic aberration, before saving the file.

For more about what information raw image files tend to contain, please see these related questions and their answers here at Photo.SE:

RAW files store 3 colors per pixel, or only one?
What does an unprocessed RAW file look like?
Why don't mainstream sensors use CYM filters instead of RGB?
Why can software correct white balance more accurately for RAW files than it can with JPEGs?
RAW in ACR vs JPG in ACR
What is RAW, technically?

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  • Sony is notorious for doing more than asked for, but even they don't really 'apply' lens correction (distortion and chromatic aberrations) to RAW (ARW). They just attach the correction information to EXIF and 'encourage' raw software developers to always use it. But open software can ignore it, and it is possible to remove this info from EXIF. I don't think it's possible (or at least practical) to do any geometric correction (incl. chromatic aberration) without demosaicing first, but this would defeat the purpose of RAW... – Zeus Feb 25 at 0:14
  • "I don't think it's possible (or at least practical) to do any geometric correction (incl. chromatic aberration) without demosaicing first, but this would defeat the purpose of RAW.." Canon's DLO apparently does just that (or at least the earliest versions of it, which could only be applied to a raw file using DPP after the fact, did). – Michael C Feb 25 at 4:20
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You are right. RAW is pretty raw, but of course the camera manufacturers are incorporating different strategies on optimizing signal to noise ratio.

While I am sure that most of these are secret, one commonly incorporated feature is a statistical de-noising process which, when enabled on long exposures, does another secondary exposure with closed shutter to receive a noise-only shot which then is used to lessen the noise from the original shot.

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Yes, raw images are really raw (with few other pieces of information). You may have hardware processing (nano optics, physics, electronic amplification, etc), but then the result is stored in a raw file. In there you won't have pixels yet, but the photosites (RGBG in a bayer pattern usually) information, and the raw development consists in creating those pixels from it. This demosaicking (google scholar demosaicking) can happen in different ways, as well as the resampling, denoising, etc.

Raw files may contain more than just photosites and parameters, it can combine multiple images, or sensor information useful for denoising.

Few references on raw processing: google scholar raw image processing

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