Can RAW images be already somewhat cooked?

Tuesday March 10 2009

Sensor Insight
Fixed pattern noise correction | Noise reduction | Conclusion
Can RAW images be already somewhat cooked?

Shooting pictures in RAW gives DSLR users the opportunity to fine-tune their settings, by potentially controlling every step of the RAW conversion. Users often assume that RAW images are directly output from the sensor without any additional processing, but is this really the case? What kind of processing can be applied and what kind of influence does it have on the final image?

Fixed pattern noise correction

Most noise occurs at the pixel level and can be considered as statistically white (meaning that each individual pixel’s noise is independent from other pixels’ noise) and identically distributed. However, there is another kind of noise, usually less common, that violates this “statistically white” rule. It is noise due to irregularities on the sensor, and thus has a fixed position.

Because different sizes of transistors are shared by all the pixels in a single row or a single column, all the pixels of a row (or column) might be slightly too bright or too dark. This is called row or column noise. It is often possible to correct it by leaving a few pixels unexposed to light on the affected side of the sensor. The level of noise is measured for these pixels and the correction is applied on the whole row or column.

Some pixels can also behave almost independently of the incident light: they may stay dark (dead pixels) or overreact (hot pixels). Most DSLRs have very few dead or hot pixels. Since they are isolated, however, when they occur they are easily detected and replaced by a more adequate value. This can be done directly on the sensor.