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Why difference between JPEG still and video still image

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hector d silva
Why difference between JPEG still and video still image
on May 15, 2011 at 8:02:16 am

Well question for CC experts, I am trying to understand why there is big difference in details and sharpness from an still image shot from DSRL in lowest quality and lowest size in JPEG and compare to video still shot in same composition in full HD and full quality. I use nikon D7000 for the test. well the smallest size of JPEG is still bigger then HD so i scaled down to HDTV preset in photoshop to make same resolution of HD video and compared both images, JPG has more details and sharper then the HD video still. I have heard about issues of moire, aliasing, skewing or is it line skipping the main issue or is it the compresion of video.


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Noah Kadner
Re: Why difference between JPEG still and video still image
on May 15, 2011 at 3:36:08 pm

All of the above- not to mention shutter speed gets optimized for stills. Stills from video can look decent but nothing beats a full res RAW or even JPEG camera still scaled down.

Noah

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hector d silva
Re: Why difference between JPEG still and video still image
on May 15, 2011 at 5:05:08 pm

True noah,nothing beats a full res RAW or even JPEG camera still scaled down. Forget RAW, this is ultimate, but even smallest JPEG is far superior, even if you keep same shutter in both. What could be the technical reason?



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Bill Davis
Re: Why difference between JPEG still and video still image
on May 15, 2011 at 5:25:12 pm

Fundamentally there are three aspects of the difference which are most important to understanding this.

A) Raster size issues
B) Progressive verses interlaced issues
and C) Temporal offset issues.

In order,

A) Nearly every camera including the modern high-rez DSLRs "dumb down" the area of the sensor that captures video. This is because trying to stream data from a full chip capture creates WAY too much data to allow full 30 frame throughput. So a modern digital still camera taking one capture will typically take a MUCH higher rez fundamental image than ANY video camera.

B) Most video systems use an interlaced field capture. The even lines are scanned, then the odd lines are filled in. This means the fundamental data of any single "capture" (one field) is essentially HALF the actual resolution of the chip producing it. Only when two fields are combined, do you get the FULL resolution the imager is capable of. (this wouldn't be AS big an issue if not for the following...)

C) Temporal offset is the reality that when you're capturing interlaced fields (and sometimes even when you're NOT!) an object in fast motion will physically change location during the time it takes to actually capture the picture. In interlaced systems, this means objects can easily change position in the time BETWEEN the capturing of the first field and the second field. Think of a race car. Even if you use a fast shutter, if you're taking TWO pictures (the two fields of interlaced video) separated by milliseconds - the race care will be in a DIFFERENT place during the second scan than it was during the first. So it's common practice to have to DE-INTERLACE still captures from video in order to remove the distortion that comes from adding together two picture fields taken at slightly different times. Most systems just take the first field and copy it and use that to replace the second field, an act that cuts the ACTUAL resolution in half.

Fundamentally, these are the basic reasons why it will be quite a while before video capture systems can come close to the resolution abilities of even inexpensive digital still cameras.

Hope that helps.

"Before speaking out ask yourself whether your words are true, whether they are respectful and whether they are needed in our civil discussions."-Justice O'Conner


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hector d silva
Re: Why difference between JPEG still and video still image
on May 15, 2011 at 5:56:09 pm

Thanx bill, this was an brilliant technical answer and made me understand the ABC points. We need to educate ourself and i was strugling to know the answer, where CC proves to be the right resource of experts. Thanx again for the time and effort.



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