bitrate and resolution

[xalien]

Hi all,
I want to compress a movie in divx and I use a bitrate calculator to determine the correct bitrate, I insert the movie duration (147M), the audio quality and the max size of result file. OK, but the resolution? My doubt is this: the final size does not depend also of the resolution? How the bitrate calculator give me the result size without know the size?! I suppose that I using 576x480 is not the same that using 1024x768!
Thanks of the aid!

[benderman]

The calculated bitrate means how many bits are used in every second. You can change the framerate, resolution or colordepth - if you don't change the duration of the movie the filesize will always be the same. If you increase the resolution (the number of pixels) and use the same bitrate there will be less bits per every pixel. If you use this formula

BitPerPixel=Bitrate[in kbps]*1000/(width*hight*Framerate)

and get a value of 0.25 or higher the movie should be nice quality. More than 0.6 is not usefull. I tested it with many movies and this formula workes fine with resolutions from 512 to 640.

[tomnhanni]

Dear Mr Benderman,

I'm sure you know what you're talking about. I just couldn't figure your formula out But I have I question for you. I used to think before that the higher the resolution the better the quality of the clip. But the I found out that if I resized my clip pretty small like no more that 400 pixels width x the proper ratio hight usually around 208, I acually got a smoother picture in my clip. With less 'pixely' display. Much more blended, and hence a better quality picture in my eyes. I'm not sure if what I'm saying here confirms or contradics your point of view but perhaps you could explain this to me.

[benderman]

sorry-for-the-style-of-typing-my-space-key-is-broken...

now-to-your-question...

if-u-use-a-bitrate-of-1000kbps-it-means-that-the-compressor-uses-1000000-bit-in-every-second.--if-the-movie-has-25-frames-per-second-and-the-resolution-is-400*300-and-use-the-formula-like-this

1000000/(400*300*25)=0,3333

that-means-that-you-have-only-the-third-part-of-a-bit-for-every-pixel.--if-u-increase-the-resolution-to-800*600-it-will-only-be-0,08333-bit-per-pixel.--uncompressed-video-uses-16-to-24-bit-per-pixel.--a-lower-res-always-makes-a-smoother-video,-but-u-will-loose-some-details.

i-will-be-able-to-tell-u-more-when-i've-fixed-my-keyboard...

[tomnhanni]

So if I understand now the lower the resolution, the more data bits are used for each pxel, but you don't get as much detail cuz there's less pixel per frame. But in a way that's ok cuz the only detail I was getting with a higher resoluton was a more 'pixely' image. Not a more clear image. For me when I watch a movie it really bothers me if I see a bunch of pixels. One thing I noticed is that when I use WMP7, as soon as I load up a clip with a resolution less than 400 pixels width, the pixels are noticable for second and then after that it kinda filckers and gets real smooth. And the playback is nice. If I load up a clip with a higher res like 688x400 or so, the pixels are noticed throughout the whole clip. Perhaps it has to do with my system not being able to render the high resolution and apply some kind of filter(?) with that high a res. I have a 800mhz Duron, with NGF2Mx 32 mb grafic card and 256 ram. So I would suppose it's enough. Beats me. All I know is that with my system with a lower res even to the hurt of some picture detail, I get a better 'quality' playback with a lower res. Not to mention a faster converting time when I backup.