What’s the difference between global tone mapping and a tone curve in darktable?
Asked 6/19/2016
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I’m learning RAW post-processing in darktable and trying to understand how to handle high-contrast photos, such as dark passages with a bright sky. Specifically, what is the practical difference between the global tone mapping module and the tone curve module? A tone curve seems like it can also compress tones, so is global tone mapping just a special kind of curve, or does it work differently?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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The global tonemap module fills a different role than the generic tone curve. Since its purpose is to compress the wide dynamic range of an HDR or RAW file in a specific way based on theories of human perception, it occurs earlier in darktables fixed-order processing pipeline, allowing the modules that follow to operate on this compressed range.
It also offers a "detail" control for retaining a degree of local contrast, making it actually less "global" than a generic tone curve (more like the usual sense of "tone mapping"). This leaves the tone curve module available later in the pipeline for its more typical role of adjusting the degree of contrast between different tonal regions based on artistic preference.
Even earlier in the pipeline is the base curve module, which may be worth looking at when dealing with high-contrast images; the default curve automatically chosen based on camera brand may be more contrasty than you want for such an image, and can be adjusted similarly to the tone curve.
Originally by user37649. Source · Licensed CC BY-SA 4.0
user37649
10y ago
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They overlap in purpose, but they are not the same tool.
A tone curve is a pixel-by-pixel remapping: for any given input brightness, it assigns an output brightness. If two pixels start with the same value, they stay the same relative to each other after the curve. It’s mainly used to shape contrast and tonal relationships creatively.
Global tone mapping is intended to compress the wide dynamic range of RAW/HDR data into a displayable range in a way that better matches human visual perception. In darktable, it happens earlier in the processing pipeline so later modules work on an already-compressed tonal range.
Also, despite the name, darktable’s global tone mapping can include a detail/local-contrast component, so it is not purely a simple global curve. Because it can respond to edges and gradients, two pixels with the same starting brightness may end up different depending on surrounding image content.
In short: use tone mapping to bring extreme dynamic range into a manageable range; use the tone curve later to fine-tune overall contrast and look.
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