Why does frame size not affect the file size?

[David9799]

I'm converting a DVD to Divx and it don't matter what frame size i pick the file size is the same?

Shouldn't a small frame size decrease the file size?

[gadgetguy]

Filesize equals bitrate times duration. Frame size (resolution) does not matter. Smaller resolutions allow for lower bitrates but simply selecting a smaller resolution does not automatically lower the bitrate.

[redwudz]

If your bitrate and running time is the same the filesize will be the same with different framesizes. The filesize is the bitrate X the running time.

However a smaller framesize will make better use of a lower bitrate. With a higher bitrate, not much visible difference between framesizes. A good example is the 1/2 D1 format for DVD, 352 x 480/576 NTSC/PAL. With a low bitrate full D1 will look fuzzy, but the same bitrate with 1/2 D1 will look much better.

I'm sure someone here can explain it better, that's just from my observations.

[David9799]

that makes sense i am using dvd fab and it shows the bits per pixel......i guess more is better.

I get more bits per pixel with a smaller frame size so i can set a lower bitrate?

at 640x480 with a bitrate of 2,000 i get 0.217 bits per pixel and a file size of 2271mb
at 480x368 with a bitrate of 1,150 i get 0.217 bits per pixal and a file size of 1338mb


so the 480x368 file should be just as good as the 640x480 BUT with a much smaller file size...

[jagabo]

I get more bits per pixel with a smaller frame size so i can set a lower bitrate?

Yes.

But if you don't need files of a specific size you can use 1-pass quality based encoding (in Divx). Pick the quality you want and encode in a single pass. You will get exactly the bitrate needed to maintain the selected quality regardless of the frame size, frame rate, and the video content.

[bottle-necked]

so the 480x368 file should be just as good as the 640x480 BUT with a much smaller file size...

Ok, the 480x368 file will be just as good as the 640x480 file but at what size are you going to display (play) the video at... You are going to blow up the video to full screen. The problem is when you display the smaller resolution file at full screen you will show all the problems it actually contains and you will see just how much detail you have lost with the down-convert. To retain the detail you want large framesizes and high bitrates. Cut either of these and the detail the video once contained is lost forever. The idea with small framesizes is that it takes less bits to maintain the quality level since the small framesize contains less pixels per square inch, but when you blow these fewer pixels up to full screen display size these pixels are so large you can then see just how much details has been lost. To maintain the detail in a video you need to have a large enought framesize and a high enought bitrate and viewed from enought distance that you can't see the garbage. Or something like that. lol

[jagabo]

480x360 with a decent bitrate doesn't look too bad on an SD TV.

[David9799]

480x360 with a decent bitrate doesn't look too bad on an SD TV.

The TV is only a 19inch.....480x360 should be ok on that size of a TV?

[jagabo]

480x360 with a decent bitrate doesn't look too bad on an SD TV.

The TV is only a 19inch.....480x360 should be ok on that size of a TV?

Yes. It will probably be hard to see any difference on a 19" TV unless the resizing is done poorly.

It can vary depending on your player too -- some have sweet spots and sour spots. You should try a few tests with some high quality sources and see for yourself.

[David9799]

480x360 with a decent bitrate doesn't look too bad on an SD TV.

The TV is only a 19inch.....480x360 should be ok on that size of a TV?

Yes. It will probably be hard to see any difference on a 19" TV unless the resizing is done poorly.

It can vary depending on your player too -- some have sweet spots and sour spots. You should try a few tests with some high quality sources and see for yourself.

I have the Philips DVP5960 dvd player. I have a 36" TV to but i'm thinking the videos will look like crap on it lol

[jagabo]

I watch on a 32" TV and they don't look bad. A little "softer" than 640x480 but certainly watchable.