4 Introduction to Picture Grading
Picture grading an edited programme refers to the process of manipulating the colour balance and brightness of video footage to achieve the best results from both technical and artistic standpoints. There are two main purposes of picture grading a video image, sequence or entire programme. The first, Colour Grading, is adjusting the ranges of colours, or specific colours, to generate a uniform look to the images. The second, Display-Type Grading, is to ensure that chroma (colour) and luminance levels are within the limits or range of the intended display types – such as CRT (Cathode Ray Tube), Plasma, PC screen, etc.
4.1 Colour Grading
Colour grading is the process of altering the colour of a video sequence or image. This can be achieved with either specific hardware tools or, more commonly, with software. Software colour grading tools are included with many nonlinear editing programs, but there are also more sophisticated external ‘plug- ins’ (programs that work in conjunction with nonlinear editing programs) or stand-alone programs. Colour grading can be further divided into two separate areas – corrective and effects.
4.2 Corrective Colour Grading
Corrective colour grading consists of altering or correcting the colour balance. Whilst this can achieve satisfactory results, any processing of the image in post production will have an impact on the overall quality or tonal values, so it is important to shoot material with the correct colour balance.
However, when editing together shots from different times of day or different cameras, corrective colour grading helps to smooth edits from one shot to another by matching tonal values of items within the shots.
4.3 Effects Colour Grading
Colour grading is the process of altering the colour of a video sequence or image. This can be achieved with either specific hardware tools or, more commonly, with software. Software colour grading tools are included with many nonlinear editing programs, but there are also more sophisticated external ‘plug- ins’ (programs that work in conjunction with nonlinear editing programs) or stand-alone programs. Colour grading can be further divided into two separate areas – corrective and effects. >
4.4 Display-Type Grading
With the advent of LCD and plasma screens, video on the web, high definition video shown on computers, and the phasing out of traditional CRT television sets, care must be taken in making video look its best when moved from TV screen to computer screen, or vice versa.
In general, video created for TV screens tends to look dark and murky on computer screens. Video created for computer screens (especially with clean white backgrounds) will probably be too bright for TV sets, and often manifests in unstable images and audio interference. Furthermore, differences in display type (and individual setup) can radically affect image quality.
Each display type will have a specific ‘gamut’ (range of colour levels) which it can work within or reproduce. Even though we are moving towards a digital domain, broadcasters still specify ‘broadcast-legal limits’ for video pictures based on analogue CRT specifications. Providing broadcasters with video which is outside of these specifications might result in distorted pictures and sound for the viewer.
While many nonlinear editing systems (NLEs) include filters and plug-ins to ensure colours don’t extend beyond the legal limits of broadcast technology, they can be a bit ambiguous – even on high end systems. A full colour grade would end with the final video passed through a hardware ‘legaliser’ set for the limits of a specific broadcaster (which can vary). It’s this final output that is recorded to tape and sent to the broadcaster – but careful monitoring is necessary. It’s not uncommon for colour rich sequences to look very different having gone through the legaliser!
Material supplied for quick reaction news, or supplied as ‘stock’ for documentaries, will usually just be accepted and legalised as part of the broadcaster’s processes. However, material that is supplied as a completed programme will go through a ‘Quality Control’ (QC) prior to acceptance, and failure of that QC due to legal limits being exceeded (broadcaster-dependent) may cause the programme master tape to be rejected. Video that is graded for analogue broadcast or CRT-type monitors will not be ideal for material generated for LCD or Plasma screens. These screens use backlight to generate the emitted light, which in turn governs the spectrum they can produce.
With higher quality LCD screens this spectrum is generally wider than that produced by a CRT monitor. The result of grading a picture for CRT display, which is subsequently displayed on an LCD monitor, will be pictures that do not contain their full dynamic range of colours and brightness for that display type. To obtain the optimum potential for your video images, consider grading different versions of the video for display on specific display-types, at least versions for video and data. Obviously, this is not always practical and you might not be aware of what types of display your programme will end up being displayed on. The best means of ensuring a wide compliance is to take this into consideration at the planning and shooting stages of your production.