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Old 20th March 2006, 22:32   #21  |  Link
hellfred
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Quote:
Originally Posted by akupenguin
That's unsigned integers or shift operators... because x/2 is not the same as x>>1 for signed x, so no compiler is allowed to optimize one into the other.
I am not sure if I understood you correct.
I wanted to express that ppl are advised to change
Code:
int x = 8;
int y = x/2;
to
Code:
unsigned int x = 8;
unsigned y = x >> 1;
So they are supposed to use unsigned int together with the shift operator.
here.
Hellfred

Small programm to test:
Code:
#include <iostream>
using namespace std;
int main() {
	unsigned int x = 8;
	x = x >> 1;                // x/2
	cout << x << endl;
	x = x >> 1;                // x/2
	cout << x << endl;
	unsigned int y = 2;
	y = y << 1;                // y*2
	cout << y << endl;
	y = y << 1;                // y*2
	cout << y << endl;
	return 0;
}
will output
Code:
haibane@LapLap:~/tmp$ g++ shift.cpp -o shift
haibane@LapLap:~/tmp$ ./shift
4
2
4
8

Last edited by hellfred; 20th March 2006 at 22:42.
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Old 21st March 2006, 00:57   #22  |  Link
akupenguin
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Quote:
Originally Posted by hellfred
I am not sure if I understood you correct.
I wanted to express that ppl are advised to change
Code:
int x = 8;
int y = x/2;
to
Code:
unsigned int x = 8;
unsigned y = x >> 1;
So they are supposed to use unsigned int together with the shift operator.
That was not quite the advice. First, since x is a constant, the math will be optimized away either way. So consider:
Code:
int munge0(int x) { return x/2; }
If x is really always nonnegative, that should be changed to any one of
Code:
int munge1(int x) { return x>>1; }
int munge2(unsigned int x) { return x/2; }
int munge3(unsigned int x) { return x>>1; }
...which are all equivalent, and faster than the 1st version.
munge1 though munge3 will be compiled into a single shift instruction. munge0 can't, because (-1)>>1 != (-1)/2

Last edited by akupenguin; 21st March 2006 at 01:08.
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Old 21st March 2006, 01:39   #23  |  Link
IgorC
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I'm not a video dev so don't push on me hard but maybe if they used signed int it will be also possible to verifire if integer is negative (complement to 2) by masking it with something like 0x80 and do optimisation only if integer >0

Nice decoder but still no information about it from on2 (they are too busy with a new generation of vp8/vp9 )

Last edited by IgorC; 21st March 2006 at 01:58.
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Old 21st March 2006, 10:53   #24  |  Link
hellfred
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OT: c programming tips

Thanks for enligthning me, akupenguing.
Can you suggest a source that will teach me lots of c/c++ tweaks like the use of the shift operator for division with 2^n on unsigneded integers. I had c / c++ lessons only for about 1 year at university, and they mainly covered the basic. So I do need to learn those tricks to be ever able to understand the sourcecode of projects like x264/mplayer/ffmpeg. And understanding is required before I am able to help with such projects.

Hellfred
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Old 21st March 2006, 13:49   #25  |  Link
bratao
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I think that its Official code, not Reversed code..
The code is So clear , commented and dosnt have Magics numbers
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Old 21st March 2006, 14:01   #26  |  Link
dimzon
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Quote:
Originally Posted by bratao
I think that its Official code, not Reversed code..
The code is So clear , commented and dosnt have Magics numbers
Or stolen...
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Old 21st March 2006, 14:29   #27  |  Link
gabest
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Quote:
Originally Posted by akupenguin
munge1 though munge3 will be compiled into a single shift instruction. munge0 can't, because (-1)>>1 != (-1)/2
Not a single shift, but it can be done with two additional instructions: (eax = -1)
(-1)>>1: sar eax, 1
(-1)/2: cdq / sub eax, edx / sar eax, 1
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Old 22nd March 2006, 13:20   #28  |  Link
Sirber
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@dizmon

you should update your post "On2 VP7 is great in quality but it is unusable for long-term video backup puposes!" since now VP62 is opensource
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Old 22nd March 2006, 13:24   #29  |  Link
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Quote:
Originally Posted by Sirber
@dizmon

you should update your post "On2 VP7 is great in quality but it is unusable for long-term video backup puposes!" since now VP62 is opensource
Done
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Old 22nd March 2006, 13:27   #30  |  Link
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anyone heard of a CLI VP6 encoder that input AVS?
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Old 22nd March 2006, 13:42   #31  |  Link
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Quote:
Originally Posted by Sirber
anyone heard of a CLI VP6 encoder that input AVS?
Just wait a little - it will be assimilated by ffmpeg
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Old 22nd March 2006, 13:43   #32  |  Link
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so let me get this right, there will soon be an ffmpeg build with the ability to produce VP6 based FLV files?

I currently use ffmpeg alot to produce FLV files, not sure what the video codec name is but it's pretty bad in comparison to equivelent sized WMV's i've produced. This would be great.
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Old 22nd March 2006, 13:45   #33  |  Link
hellfred
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Just decoder up to now

Quote:
Originally Posted by Sirber
anyone heard of a CLI VP6 encoder that input AVS?
Sirber, you are ahead of time. Though the project aims to implement both the decoder and encoder for VP6, there is only the decoder at the moment. So you have to go ask On2 for any encoder app.

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Old 22nd March 2006, 13:46   #34  |  Link
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sorry to go OFT, but are there any better codecs that can be used in FLV (and decodable by standard flash player) that are better than the one built into ffmpeg (sorry dont know name of the actual codec used ffmpeg).
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Old 22nd March 2006, 14:27   #35  |  Link
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@hellfred

Thanks for the info!
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Old 22nd March 2006, 16:25   #36  |  Link
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Looks like the project has been removed.
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Old 22nd March 2006, 16:31   #37  |  Link
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CVS still online. Get it while it's avalible
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Old 22nd March 2006, 16:44   #38  |  Link
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Here's a link to the files:

libvp62.tar.bz2
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Old 22nd March 2006, 22:29   #39  |  Link
hellfred
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Quote:
Originally Posted by Sirber
Here's a link to the files:

libvp62.tar.bz2
This file was reported as illegal material.
Access to the file is temporary blocked.

Gone, too.

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Old 22nd March 2006, 22:33   #40  |  Link
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Want another link?

Files are still on CVS in any case: http://cvs.sourceforge.net/viewcvs.py/libvp62/

[edit]

new link
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Last edited by Sirber; 22nd March 2006 at 22:41.
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