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Old Feb 3, 2005, 9:23 PM   #1
sdh
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This was debated a while back butI thought ofsomething that I don't think was ever asked, so Ithought Id rehash it. If you were toshoot a movie of an absolutelymotionless object in 640x480 resolution, under very good light with the camera on a tripod, couldn't you pause the replay on your computer and compare it with a 640x480 still image ofidentical composition?

I did a quick and dirty try of this experiement and thought the paused moviewas not too much poorer than the still image.Then I took the 640x480 still image and reduced it to 320x240, then interpolated it back up to 640x480, and did a 3 way comparison between the 2 still images and the paused movie, and the movie looked definetely more like the original 640x480 still image than the reduced-and-re-enlarged image. All of which makes me think the 640x480 movie is not really interpolated, as some had speculated or concluded. Either that or I don't have afrigging clue about evaluating images!!:G

What do you think? Is this a valid comparison?

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Old Feb 3, 2005, 10:18 PM   #2
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I'm not too sure how accuratethat comparison would be, but it may be a good way of evaluating it. I've always believed the rumors about the "VX" 640x480 to be interpolaled because older (F717, P9, P7) cameras had what was called MPEG Movie HQX which is 320x240 at 15fps and the newer cameras have MPEG Movie VX 640x480 and when set to standard or 15fps, they both use the exact same memory per second. BUT, the VX 15fps videos look better than the HQX 15fps videos.
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Old Feb 3, 2005, 11:26 PM   #3
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I agree, that does sound suspicious. Ive been downloading sample videos from reviews of other cameras to try to compare which isobviouslyhighly apples-to-oranges but still... Steve's reviews of the W1 and Canon SD300 have some short but nice video samples, both at 640x400 @ 30fps. I swear the picture looks sharper from the SD300 but again... apples to oranges. It could be telling though, that the SD300 video uses ~2MB/sec whileSonys video is ~1.25MB/sec? Or maybe the current Sonys just use more aggressive compression than other makers (as well as the older Sonys)? ...Im at a loss!
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Old Feb 9, 2005, 9:12 AM   #4
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According to the dcresource.com review of the P150, the duration of video you can fit on a 1GB card is:

12 mins of VX Fine (640x480 at 30 fps)
44 mins of VX Standard (640x480 at 16 fps)

Converting that to the number of frames stored, we get:

21600 frames of VX Fine
42240 frames of VX Standard

I.e. each frame of VX Fine uses about double the storage of each frame of VX Standard.

Does anyone know whether these formats use temporal compression? If so, this will complicate the maths!

I'm still waiting for my P200 to arrive. Can anyone comment on whether a frame of VX Fine looks to have double the resolution of a frame of VX Standard, as the figures suggest?

Dave
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Old Feb 9, 2005, 2:51 PM   #5
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Some further evidence, m'lud... I took a frame from Steve's sample movies for the V1 (VX Standard) and P150 (VX Fine).

The VX Fine frame is definitely better quality, with considerably better resolution and fewer artifacts. Zooming in on each of them, I noticed some strangeness: you can see a pattern of 8x8 pixel squares, as if each image has been encoded in terms of 80x60 such squares. It looks as though the compression tries to get away with as few colours as possible within each square, so that some squares, e.g. in sky areas, are completely uniform, with the adjacent squares also being uniform but a slightly different shade.

This is all very obvious in the VX Standard frame when viewed at 400%, but much harder to spot in the VX Fine frame. It appears that a similar type of compression is used in both, but that it's much heavier in VX Standard - by a factor of 2, judging by my previous post. VX Fine doesn't clamp down on the number of colours in each square nearly as much, so that in most parts of the image you can't even tell the squares are there.

** Steve / Forum Admin **
I have some crops of 400% zooms of the two frames that show this very well. Would it be OK to post these?

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Old Feb 9, 2005, 4:30 PM   #6
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This problem also comes to me when i was deciding to buy a camera that has 640*480 30 FPS movie option.after hearing from few of the fellows about interpolation of Sony movies resulting in less video quality,I just downloaded movies from Nikon 3700 and sony W1 videos from dcresource and stevedigicams sites and after comparing both videos side by side, Nikon quality was far much better than Sony W1 even in bright sunlight.U can check urself by downloading the videos from sony and nikon and see urself the difference.i buy nikon 3700 and quite happy with it so far to capture videos.
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Old Feb 9, 2005, 5:53 PM   #7
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If I had bothered to find out a bit about how JPEG compression works before my previous posts, I would have realised that the 8x8 squares I was talking about are just how JPEG operates. I just hadn't looked at such heavy compression so close-up before. I realised it was some sort of compression - not interpolation - that I was seeing, but didn't realise it was JPEG (or something very similar).

So, to emphasise, my ramblings don't provide any evidence of interpolation in Sony's VGA movie modes, just heavy JPEG-type compression in VX Standard, and more moderate JPEG-type compression in VX Fine.

Perhaps the explanation for the similar data rates of VX Standard and HQX is that Sony had to crank up the compression on VX Standard to keep the data rate within what a standard Memory Stick could handle. With the development of MS Pro, they were able to double the frame rate and halve the compression.

I looked at a frame from Steve's Nikon 3700 movie and saw exactly the same 8x8 squares, with a level of compression looking roughly similar to Sony's VX Fine. I can't evaluate the quality of the movies until I replace my ageing PC, it can't deal with 30fps VGA!

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