[opendtv] Re: Panasonic's AG-HVX200

At 10:16 AM -0400 4/27/05, Mark Schubin wrote:
>  >Now a question. Is not sharpness directly related to the ability to
>>perceive detail?
>> 
>>
>No.
>
>Sharpness is proportional to the square of the area under an MTF curve. 
>An MTF curve plots contrast ratio against detail.
>
>In my PBS presentation, I extracted two sections from a contrast
>sensitivity grating.  One, taken from the low-detail high-contrast
>section clearly appears to be sharp.  The other, taken from a
>moderate-detail moderate-contrast section does not.  I didn't choose
>high-detail low-contrast for the second, because then the audience
>wouldn't have been able to distinguish it from a grey block.

Thanks again Mark. This has always been a bit confusing to me. I 
suspect that the situation has been amplified a bit as my eyes have 
aged and I have been forced to use reading glasses and seek brighter 
light in order to read "the fine print."

What it seems to boil down to is the contrast ratio of the detail 
that one wants to perceive. In your grating examples the contrast 
ratio keeps getting lower as the the level of detail increases.

While you properly point out that there are many contributing 
factors, I  believe that one of the major issues is the need to 
properly filter images to obey the laws of physics and sampling 
theory. For video imagery we cannot allow unfiltered transitions from 
black to white, as is common with the non-Nyquist  limited samples 
you are looking at as you read this message. So while the text on 
this page may have a black sample next to a white sample, in a video 
image we need to ramp from black to white - through grey - or to 
limit the contrast of the transition.

At the lower frequencies of the grating there is good contrast 
because the peak intensity of the samples reaches full white and full 
black. There are grey samples in between, but the frequency of the 
detail is low enough to be represented faithfully. As the frequency 
of the detail increases, however, we reach a point where the 
intensity of the stimulus can no longer be represented with full 
black or white samples. Filtering becomes a factor causing the 
samples to be limited in contrast as the frequency increases, until 
we reach the point where the grating becomes a grey blob.  Obviously 
filtering is not the only contributing factor. Most cameras employ 
optical filtering, and the lens itself limits what the sensor can 
"see." Hence, the MTF curve does a good job of predicting the 
relationship of the original stimulus to the intensity of the 
stimulus that we see in the sampled image. More area under the curve 
is another way of saying that a system can pass full contrast 
transitions at higher frequencies, thus enhancing our ability to 
perceive higher levels of detail.

And then there is the reality that the display ALSO has an MTF curve, 
and that it can impact the ability to perceive detail based on the 
contrast, brightness and sharpness controls. Joe Kane makes his 
living in this space, teaching people how to control ambient light 
and adjust a display so as to maximize perceived contrast and detail.

Does all of this make sense Mark?

Regards
Craig
 
 
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