Tutorial - RAW-processing.

This term refers to the operation of transforming a RAW file into a JPG or TIF file. By analogy with film photography, it is also referred to as RAW development.

In digital photography, derawtisation is compulsory. This is also why it is often compared to developing film. Of course, it is always possible to request a JPG shot, which does not require development, but this simply means that the development has been done automatically by the camera. In fact, all sensors, including those in mobile phones, deliver information in RAW format. Entry-level cameras or mobile phones immediately translate this RAW into JPG to make life easier for the user.

Graphic file formats.

• The RAW format.
  Digital camera sensors all deliver a file in a format called RAW. Unfortunately, this format is anything but standardised: all manufacturers have their own variant(s). There are even differences between cameras of the same brand.
  The term RAW is therefore a generic term for a file from a sensor, but covers many different formats. Most manufacturers use a different file extension: NEF at Nikon, CRW or [CR2} at Canon, etc.
  A RAW file contains the raw data from the sensor, and information about the settings chosen during shooting (EXIF). It may also contain information such as the GPS coordinates of the location where the photo was taken.
• The DNG format.
  Defined by Adobe, the DNG specification is public, unlike manufacturers' RAW formats. The advantage of this format would of course be to simplify file processing by software: there would be a single format to process instead of the multitude that currently exists. But very few camera manufacturers have adopted this format to date, including Hasselblad, Leica, Pentax, Samsung and a few others.
• The JPG or JPEG format.
  This format is fully standardised and recognised by all image processing software and printing systems. It is also the preferred format for publishing photos on websites.
  Unfortunately, it is destructive, meaning that each recording in this format alters the image. This is due to the desire to reduce the size of files, in particular to enable them to be distributed via the Internet. The compression factor is adjustable, which means that by accepting larger files, the alteration of the image can be limited to an imperceptible level.
• The TIF ou TIFF format.
  The TIF format is also very standard. Compared with JPG, it has the advantage of not altering images but (as a consequence) it produces much larger files. For this reason, it is rarely used on websites. It can be used to store processed images for printing, for example.

Automatic Raw processing by the camera or manual in post-processing.

Raw-processing by the camera has the advantage of being quick and easy: you get your photo straight out of the camera, in JPG format. Another advantage is that JPG files are much smaller than RAW files, so you can store many more photos on your memory card.

Of course, there is one major drawback to this solution: if you have made a mistake in the camera settings, the JPG will be generated with this error. The most common case is a white balance error. It will then be tricky to correct this error in the JPG format, and in all cases this will result in a deterioration in image quality.
On the other hand, with a RAW file, reasonable adjustments to exposure, white balance, contrast, etc. do not alter the quality of the final image. You can therefore correct shooting errors to a large extent.

Another advantage is that it is often necessary to make several variants of the same photo (one for printing, another for publication on social networks or a website, etc.). As already mentioned, modifications to JPGs are less easy and more destructive.

Finally, it's important to bear in mind that software improves every day. A treatment done 2 years ago can be redone today with a better result, but that means starting from the original file, i.e. the RAW file.

Distinguishing between raw-processing and retouching.

Be careful also not to confuse raw-processing and retouching. If raw-processing is mandatory, retouching is optional. The operations carried out during these two phases of photographic post-processing are not the same:

Raw-processing :

• Brightness and contrast adjustment.
• Adjusting highlights and lowlights.
• White balance.
• Etc.

Retouching :

• Masking pimples, small scars, etc. on a face.
• Removal of unsightly background elements.
• Skin smoothing.
• Etc.

What exactly is raw-proessing ?

Let's take a closer look at what happens during raw-processing. This requires looking at the sensor itself. This one is composed of microscopic elements photosensitive to red, green or blue. These elements are arranged as follows:

We see that the elements sensitive to green are twice as numerous as those sensitive to red or blue. This layout was chosen because it approximates the sensitivity of the human eye which is much more sensitive to green than to other colors.

Dematrixing.

The diagram below illustrates the dematrixing process.

Each raw-processing software has its own technique for dematrixing, and this is a jealously guarded secret. In fact, good color rendering is directly linked to the quality of this processing.

This is not a simple operation, since in JPG each pixel is made up of three colors, whereas on the sensor we have seen that green-sensitive elements are more numerous. We therefore need to find a logic for mixing the original signals (RAW), considering that, as we've seen, the sensor contains more green, whereas the JPG format is made up of pixels, each with a red, green and blue component.

Transposition into a color space.

JPG or TIF formats don't store true colors, but only numerical values corresponding to the basic components: red, green and blue. Each of these three values ranges from 0 to 255. These values must be interpreted in relation to the chosen color space: for example, a value of 255 for red corresponds to the reddest (most saturated) red that the color space can represent.

For example, if the color space indicates that the reddest red that can be obtained is this one , and that we need to memorize this red one , a value of approximately 127 is recorded for red.

The most common color spaces are :

• sRGB : the most widely recognized standard. It suffers from weaknesses in the rendering of the most saturated colors.
• Adobe RGB : more extensive, it better translates saturated colors. But as it is poorly recognized by most peripherals,
• ProfotoRGB : the most ambitious, it can represent colors that are even invisible to the human eye.
• Manufacturer-specific spaces such as Nikon RGB.

Application of ICC camera profile.

Nothing is perfect, and neither are digital cameras. A sensor, for example, may tend to emphasize red to the detriment of other basic colors. It is then necessary to correct this dominance; this is the role of the camera's ICC profile. These errors can be more or less significant, depending on luminosity: the correction coefficients included in the ICC profile are therefore quite numerous.

Application of user-selected settings.

Whether raw-processing is done manually or automatically by the camera, choices are made: white balance, over- or under-exposure, color rendering (neutral, pastel, vivid colors, etc.). These parameters are applied when the JPG or TIF file is generated.

The RAW format is only influenced by the camera's physical settings: aperture, shutter speed, focus and so on (parameters that already existed in film photography). JPG or TIF files, on the other hand, are affected by ALL the settings you may have made (or that the automatic system may have made) at the time of shooting, or by all the adjustments you make in post-processing.

An amusing analogy between raw-processing and cocktail preparation.

We found this comparison interesting, as it helps us understand the differences between RAW and JPG.

The RAW file contains the raw data from the sensor, i.e., in the case of a cocktail, the ingredients: rum, orange juice, grenadine, etc. To make a cocktail, apply a recipe: 1/4 rum, 1/3 orange juice, a dash of grenadine, and top up with sparkling water. The resulting cocktail is the JPG file. If it's not a good one - too sweet, for example - it's easy to make another by modifying the recipe (with the advantage that, unlike the ingredients the RAW file is inexhaustible).

Now suppose you're given a ready-made cocktail, without being given the ingredients. If it's to your liking, all is well, but if you find it mediocre, it will be difficult to improve. Some faults are easy to correct, for example, if it's not sweet enough, it's easy to add a little grenadine or sugar. But if it's too sweet, you won't be able to correct the defect.

It's exactly the same for rw-processing: if you've only recovered the JPG file, it will be difficult, if not impossible, to correct certain defects. Our advice is therefore to always set your camera to record RAW files, or at the very least to record both RAW and JPG files. Of course, this will take up more space on the memory card, but it will enable you to rework photos that may have had a defect when you took them, while using directly, without post-processing, those that come out right the first time.

This solution can be a good compromise for souvenir photos, but for the photos you take for your pleasure, there will necessarily be a step manual post-processing.