Global vs lo­cal process ing

Lo­calised pro­cess­ing is very pow­er­ful, but use it with care...

NPhoto - - Competition -

■ In film-based pho­tog­ra­phy, most of the ways of ad­just­ing how a pic­ture looks af­fect the en­tire im­age. Dig­i­tal pro­cess­ing, in con­trast, en­ables us to ad­just in­creas­ingly spe­cific at­tributes. Many of these meth­ods are dis­tinctly non-tra­di­tional, in that they can ad­just bright­ness and colour ac­cord­ing to what neigh­bour­ing pix­els look like.

This is called tone map­ping, a dig­i­tal pro­ce­dure for map­ping one set of tones and colours to another. And there are two kinds of tone map­ping: global op­er­a­tors and lo­cal op­er­a­tors. The first ap­plies its cal­cu­la­tions across a whole im­age, the sec­ond works only at a cer­tain dis­tance from each pixel. This kind of pro­cess­ing can al­ter the im­age – by teas­ing out shadow de­tail, for ex­am­ple – to a de­gree that would just be im­pos­si­ble us­ing meth­ods such as Curves. How­ever, if used with­out care, the re­sult will look less ‘pho­to­graphic’. It pays to treat lo­calised ad­just­ments like High­lights, Shad­ows and Clar­ity with cau­tion.

A de­tail of an im­age pro­cessed with High­lights and Shad­ows re­cov­ery, for a lo­cal ef­fect. The slid­ers are highly ef­fec­tive,

but tra­di­tion­al­ists might not like the over-pro­cessed look

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