Adobe’s DNG specifications may seem like a rather esoteric and arcane document to go digging around in, if you’ve only got experience of developing pictures, rather than software. However, the latest changes, that allow for ‘corrections and enhancements’ might be of interest, because they give something of an insight into the kinds of processing and corrections that camera manufacturers are applying to their RAW data.

In order to provide a non-proprietary way of storing RAW data, DNG files have to include all the processing instructions necessary to produce a final image the way the camera manufacturer wanted. Consequently, to act as a universal format, the DNG specification has to have the capability to define all the different types of processing that those manufacturers might want to apply to their RAW data to accurately produce a finished image.

So the addition of correction and enhancement parameters (known as ‘opcodes’) into the DNG v1.3 specification gives us some clues about the behind-the-scenes work manufacturers are doing to produce the best possible images from the RAW data their cameras are capturing.

Tom Hogarty, senior product manager at Adobe, has this to say about opcodes on the Lightroom Blog: “’Opcode Lists’ allow complex processing to be moved off the camera hardware, which can have limited processing power, and into the DNG reader, which is often running on more powerful hardware. This also allows processing steps to be specified, such as lens corrections, which ideally should be performed on the image data after it has been demosaiced, while still retaining the advantages of a raw mosaic data format.”

The changes mean that the kinds of distortion correction being applied in to the JPEGs of Micro Four Thirds cameras (for example), which are known to correct for lens distortion and aberrations, can be embedded along with the RAW data so that all DNG compliant software will recognise and consistently apply these corrections.

Among other things, the parameters defined in the latest specification allow not only for the correction of geometric distortion in rectilinear lenses (using 4th degree polynomial corrections that can correct complex distortions), but also the remapping of essentially spherical projections (fisheye lenses for example) to a rectilinear image. Lateral chromatic aberrations and vignetting are also accounted for.

However, the available corrections are not simply limited to lens corrections - there are also parameters such as ‘TrimBounds’ that would allow the edges to be trimmed-off during development processing – which would be useful in processing images from cameras that use different crops from the sensor to provide final images in different aspect ratios. Parameters for addressing pattern noise have also been included.

Another new parameter is ‘GainMap,’ which allows the use of a ‘map’ image that defines which areas of an image should have additional ‘gain’ applied to them. This would allow the inclusion, alongside the RAW data, of a map that defines the areas brightened by the kinds of localised dynamic range compression techniques such as DRO, D-Lighting and Shadow Adjustment technology that are applied selectively across images. This would mean the effect of these features would still be available when processing RAW files.

In addition to the new parameters that appear as opcodes, the latest DNG v1.3 gains several new options for a tag that helps define the colour filter array pattern. These 'CFALayout' values, combined with some of the new opcode parameters would appear to make it possible to describe an interleaved sensor design with different exposure values in different rows or columns. This would potentially allow the correct demosaicing and rendering of RAW data from sensors with unusual layouts or exposure modes.

We’re sure that further analysis from the more technical end of the dpreview community will reveal more of what the new DNG specification allows. However, it should be remembered that these changes merely allow processing parameters to be stored in DNG files, they don’t demand that all DNG converters automatically apply them.