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• Tone-mapping

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— HDRI: Tone-mapping —

Tone-mapping

Tone-mapping can be compared to the adjustment of brightness, contrast and gamma for conventional LDRI (Low Dynamic Range Image) pictures. However, different tone-mapping operators come with many more controls. There are global and local operators. Global operators work on the whole picture while local ones also adjust the particularly bright or dark parts without changing the rest of the picture considerably. At the time being, I am aware of the following tone-mappers:

• Adaptive Logarithmic, global; 5 parameters.
• Ashikhmin, global; 3 parameters. After Michael Ashikhmin.
• Cipher, local; 10 parameters.
• Display, global; 7 parameters.
• Drago, global; 2 parameters. After Frédéric Drago.
• Durand, global; 4 parameters. After Frédo Durand.
• EriKate, local; 4 parameters. After Erik Reinhard und Kate Devling.
• ExpLog, global; 0 parameters: Exponential oder Logarithmical.
• Exponential, global; 8 parameters.
• Fattal, local; 10 parameters. After Raanan Fattal.
• Greg, global; 1 parameter. After Greg Ward.
• Linear, global; 0 parameters.
• Lock05, local; 10 parameters. After Thomas Lock.
• Lock06, local; 10 parameters. After Thomas Lock.
• Mantiuk, global; 3 parameters. After Rafal Mantiuk.
• Pattanaik, local; 6 parameters. After Sumanta Pattanaik.
• Photoreceptor, global; 9 parameters. Approach to film response.
• Photoreceptor Physiology, global; 5 parameters.
• Reinhard, global; 7 parameters. After Erik Reinhard.
• Scan, global; 0 parameters. Francesco Banterle.
• Schlick, local; 10 parameters. After Christophe Schlick.
• Stockham, local; 10 parameters. After T.G. Stockham.

Tone-mapping Examples

After Erik Reinhard	After Erik Reinhard und Kate Devling
After T.G. Stockham	After Raanan Fattal

Both Reinhards and Stockham are near the reality, Fattal is dramatised. All tone-mappings show the full brightness range: the light from the window and the fluorescent lamps are not «burned-out» and even the dark places are not completely black. The picture on top right was tone-mapped in HDRShop with one of Banterle's plugins, the other three in Artizen HDR 2.5.14. The top left image is nearest to reality but it appears bland, poor in contrast and therefore a bit foggy. The one on top right is pleasingly bright, also down the corridor, but gives the impression of being in a hospital. Below left features a high contrast and good illumination. I would choose this picture to advertise for this place. Below at right finally is far away from reality but it enhances the shadows and gives this place a dramatic touch. Some parts are brightened up even though no light falls on them. The picture appears hard and the place unclean. This is no ranking of tone-mapping operators. You select the tone-mapper according to the picture at hand and what you want to express with it. Besides, there are so many parameters to adjust that the result can be fine-tuned to one's desires. There is an almost infinite number of variations. << Mind the detail at left in its original size. The fluorescent tube is not overexposed and the faint neon light in the light switch still glows. Neither is the wall blindingly bright as it would be if a flashlight was used. This detail was extracted from the Stockham tone-mapped picture. These examples are meant to show the potential of HDRI photography and tone-mapping, and encourage to start experimenting with it. The effort is well worth it! This concludes the introduction into the topic but it is not yet exhausted, not at all. The basics explained here ought to suffice to start experimenting with HDRI photography, though.