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Adobe Inc., has published a white paper
http://www.adobe.com/products/photoshop/pdfs/linear_gamma.pd f that tries to explain the colorimetric response of the human vision, film and digital image sensors and how they relate to digital image capture and -coding.
In this paper the chapter ‘Raw Capture, Linear Gamma,and Exposure’ is totally incorrect, it is incomprehensible that this kind of misconception has passed the editorial staff at Adobe.
The author (Mr. Bruce Fraser) explains that:
"Film responds to light the same way our eyes do, but silicon does not.".
Now, anyone who has ever used "film", be it a chrome or a paper exposed from a negative, knows too well that the "film" will severely compress both the dark end and the bright end of the tonal reproduction range. Shadows on "film" appear way too dark and highlights are washed out for the vision.
He continues with the claim:
"Film mimics the eye’s response to light, which is highly nonlinear."
and explains "our sensory mechanisms" taking analogies from golf balls, spoonfuls of sugar, etc. The author is clearly trying to explain the Light Adaptation of the human vision, as he tries to rationalize:
"This built-in compression allows your senses to function over an immense range of stimuli. You can go from subdued room lighting to full daylight without your eyeballs catching fire, even though you may have suddenly increased the stimulus reaching those eyeballs by a factor of 10,000 or so. But the sensors in digital cameras lack the compressive nonlinearity typical of human perception. They just count photons in a linear fashion."
But the Light Adaptation of the vision is pretty much fixed when we view a still scene or a picture of it. Light Adaptation does not explain how we perceive the surface reflectances from the scene under unchanged illumination level. Yes, light (photons) do convey that information into our eyes but the Light Adaptation level of the vision is unchanged in this situation since the lightness level does not change.
When we go e.g. from subdued room lighting to full daylight then, between these two viewing situations, the Light Adaptation of the human vision is fully working. So our perception of these two lightness levels (the lightness level in the subdued room and the lightness level under the full daylight) is about logarithmic, so behaves according to the Weber’s law. But we do not perceive the surface reflectances in either of the above viewing situations according to the Weber’s law, we perceive the surface reflectances about linearly.
The author then throws in an example image with the caption:
"Linear processed raw captures look very dark". But all the data is there in the image."
Sure the image appearance is very dark since that image is totally incorrectly color-managed! He is showing the linear image data in a steep gamma RGB working-space (or in a native gamma space of the monitor, like it is also shown in the Acrobat reader).
He follows that with another example image with the caption:
"The same linear processed capture with a tone curve appears normal."
Yes, the image appearance is now much better since the steep gamma transfer function of the viewing space is taken into account by his curve, he explains:
"This is the curve required to apply a gamma correction tone to the linear capture."
It looks that the author is trying to explain that linear image data would not appear properly for the human vision *because* the Light Adaptation of the human vision is non-linear. This is nonsense.
The situation that he is confronted is:
Linear image data appears properly for our vision when the monitor is linearly calibrated (or the image data is shown in a linear working-space of a color-managed application).
And gamma compressed image data appears properly for our vision when the monitor is gamma calibrated to that gamma space (or the image data is shown in such gamma compressed working-space of a color-managed application).
Timo Autiokari http://www.aim-dtp.net
http://www.adobe.com/products/photoshop/pdfs/linear_gamma.pd f that tries to explain the colorimetric response of the human vision, film and digital image sensors and how they relate to digital image capture and -coding.
In this paper the chapter ‘Raw Capture, Linear Gamma,and Exposure’ is totally incorrect, it is incomprehensible that this kind of misconception has passed the editorial staff at Adobe.
The author (Mr. Bruce Fraser) explains that:
"Film responds to light the same way our eyes do, but silicon does not.".
Now, anyone who has ever used "film", be it a chrome or a paper exposed from a negative, knows too well that the "film" will severely compress both the dark end and the bright end of the tonal reproduction range. Shadows on "film" appear way too dark and highlights are washed out for the vision.
He continues with the claim:
"Film mimics the eye’s response to light, which is highly nonlinear."
and explains "our sensory mechanisms" taking analogies from golf balls, spoonfuls of sugar, etc. The author is clearly trying to explain the Light Adaptation of the human vision, as he tries to rationalize:
"This built-in compression allows your senses to function over an immense range of stimuli. You can go from subdued room lighting to full daylight without your eyeballs catching fire, even though you may have suddenly increased the stimulus reaching those eyeballs by a factor of 10,000 or so. But the sensors in digital cameras lack the compressive nonlinearity typical of human perception. They just count photons in a linear fashion."
But the Light Adaptation of the vision is pretty much fixed when we view a still scene or a picture of it. Light Adaptation does not explain how we perceive the surface reflectances from the scene under unchanged illumination level. Yes, light (photons) do convey that information into our eyes but the Light Adaptation level of the vision is unchanged in this situation since the lightness level does not change.
When we go e.g. from subdued room lighting to full daylight then, between these two viewing situations, the Light Adaptation of the human vision is fully working. So our perception of these two lightness levels (the lightness level in the subdued room and the lightness level under the full daylight) is about logarithmic, so behaves according to the Weber’s law. But we do not perceive the surface reflectances in either of the above viewing situations according to the Weber’s law, we perceive the surface reflectances about linearly.
The author then throws in an example image with the caption:
"Linear processed raw captures look very dark". But all the data is there in the image."
Sure the image appearance is very dark since that image is totally incorrectly color-managed! He is showing the linear image data in a steep gamma RGB working-space (or in a native gamma space of the monitor, like it is also shown in the Acrobat reader).
He follows that with another example image with the caption:
"The same linear processed capture with a tone curve appears normal."
Yes, the image appearance is now much better since the steep gamma transfer function of the viewing space is taken into account by his curve, he explains:
"This is the curve required to apply a gamma correction tone to the linear capture."
It looks that the author is trying to explain that linear image data would not appear properly for the human vision *because* the Light Adaptation of the human vision is non-linear. This is nonsense.
The situation that he is confronted is:
Linear image data appears properly for our vision when the monitor is linearly calibrated (or the image data is shown in a linear working-space of a color-managed application).
And gamma compressed image data appears properly for our vision when the monitor is gamma calibrated to that gamma space (or the image data is shown in such gamma compressed working-space of a color-managed application).
Timo Autiokari http://www.aim-dtp.net
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