Does avi.net change the image size?

[simonsonjh]

avi.NET appears to reduce the image size of the video. A video originally sized 720x480 seems to be reduced to 640xY. The aspect ratio is not changed.

Is the size really being changed by avi.NET? Is there some sort of cropping occurring?

Can I control this function? Is it a good idea to use the resizing arrows to return to 720x480?

[simonsonjh]

Hi,
Yes it does. avi.NET will reduce sizes to predetermined sizes depending on
input size. The reason for this is because, well, 99% of all conversions to
AVI from DVD were always at 640 width. It just a size that nearly eveyone
has got used to using/setting.

You can just resize it back to 720 if you wish but just bear in mind:
1. Don't resize the height to 480, unless it's a full screen, the 480 height
for an NTSC movie also includes the 'black bars', so if you resize to 720,
just make a mental note of the DAR value beforehand and after resizzing the
width resize the height until you get the same, or roughtly the same DAR
value. Probaly more around the 320, 352 or 360 mark, or whatever, depends on
input.
2. A dvd of 720 x 480 might not actually be 720 wide. avi.NET, and other
programs, will do an 'autocrop' to get rid of the black bars. We don't need
or want to convert black bars so the auto cropping will remove them and
leave us with just the picture area. You'll find most movies are more like
704 wide, or 696, or 716 once the duff black pixels at the edge are removed,
so using 720 wide would actually be increasing your source video size which
is never a good idea in conversions.

Drop me a line if there's any more questions.
CloneAD

[simonsonjh]

I would prefer avi.NET not change the size, but only crop black borders. Why throw away good video information by reducing the size?

[mats.hogberg]

Because, by making it smaller (resolutionwise) you can get away with lower bitrate.
Also, A DVD is always (mostly) 720x480 resolution. However, this is either displayed as 4:3 or 16:9, meaning the pixels aren't square. AVI always (again, mostly) has square pixels, so some resizing is inevitable to make it look good (with the correct aspect ratio) as AVI.

/Mats
PS! Are you suffering from a split personality? DS

[simonsonjh]

I would rather choose a bigger file if I need a higher bit-rate. There should be a "Do not re-size" check box. I just want to be assured there is black-border cropping, and no changes to the image size or aspect ratio.

[mats.hogberg]

Let me try to explain how I think.
A 16:9 DVD movie looks like this.

However, the mpg at it's 720x480 resolution looks like this:

It's slightly stretched vertically, compared to how it should be displayed. The DVD player will resize the mpg to fit the 16:9 format.
An AVI however, is normally not resized by most players - it's just displayed as it is, assuming square pixels. So, if you'd keep the 720x480 res from the mpg to the AVI, it's be displayed as the second pic, and we wouldn't want that, would we?
Instead, the mpg to AVI encoding process has to resize the source to make the AVI resolution fit the DAR of the mpg, 16:9 in this case. Assuming you keep it 720 wide, 16:9 would make it 405 high.

So, in this case you can't both keep image size and aspect ratio. It's either.

/Mats

[simonsonjh]

Your example is correct; there is a vertical resizing to compensate for the anamorphic distortion. So the final size in order to preserve the correct aspect ration is 720x405.

I am referring to the type of resizing that actually does remove image information. In this example, I believe avi.NET reduces the size to 640 x 360.

I want to be assured I am getting 720 x 405 without having to manually click the sizing arrows and take a chance of changing the aspect ratio.

[mats.hogberg]

Ok, you don't want it cropped! Well I wasn't aware avi.net cropped off part of the picture - just that it resized.

/Mats

[simonsonjh]

I do want black borders cropped. I think avi.NET is reducing the size of the real image, thus throwing away useful information. If the size of the image after cropping is 720x405, then avi.NET always reduces the x-axis to 640, and changes the y-axis to preserve the aspect ratio. I want 720x405, not 640x360.

[mats.hogberg]

I think avi.NET is reducing the size of the real image

What makes you think that?

/Mats

[BinaryJustin]

I've had problems with the resolution after cropping too.

Basically, I'm archiving a load of laserdiscs which are in 4:3 ratio.

I realise that regardless of the initial resolution, the program is going to resize them to 640*480, unless I choose otherwise. I don't have a problem with this as it obviously compresses the filesize further.

Now... My problem is with playback on a standalone DVD Player. No, it isn't DivX-certified. It's a cheapo Yamada which otherwise works brilliantly.

The auto-cropping is set at 40 pixels in avi.NET, so the image goes from being 640*480 to 640*440.



I don't have a problem with this, in fact I quite like the more rectangular resolution... On my ancient 4:3 television and using a Lite-On (DivX-certified) DVD player, I see the black bars at the top and bottom of the screen.

HOWEVER... On the cheapo Yamada DVD Player, it's stretching the image to fill the screen vertically. I realise that it's the player which is at fault, but I thought that the whole point of avi.NET was to produce MPEG4 files which are fully-compliant with standalone players - not just DivX-certified standalone players.

The best way I can explain what it's doing is to demonstrate pictorially...

Here's the original video from the laserdisc.


Here's the cropped DivX-encoded video as displayed through a DivX-certified player.


Here's the cropped DivX-encoded video as displayed through a non-DivX-certified player.


Not only is a lot of the image missing from the left and right of the television screen but it's quite fuzzy. It seems that the Yamada's software can't understand that the DivX file is 640*440, not 640*480 and uses the resolution which matches most closely.

So this got me thinking if I could somehow add "flags" to the avi file to make the DVD Player realise that the image isn't in 4:3 resolution.

I found this webpage - http://www.forret.com/tools/megapixel.asp?width=640&height=440 which calculated that the actual resolution was no longer 4:3 but 16:11.

I used MPEG4Modifier to add a custom Display Aspect Ratio of 16:11 to the file, burned it to a disc and played it on the Yamada DVD Player. It's exactly the same. It appears that the Yamada won't recognise non-standard video resolutions.

My request is to allow a "zero" integer to the auto-border-cropping option within avi.NET. In the meantime, I'm going to have to encode the file again with the 20 pixels cropping option - but even this isn't ideal. I'd rather that there was no cropping whatsoever.

To recap, here's the original video, the cropped video and the expanded cropped video...



I hope this all makes sense. I can't explain it any better than this.

p.s. I haven't tried amending the Pixel Aspect Ratio yet (only the Display Aspect Ratio), so I'll give it a go and let you all know what happens.

[BinaryJustin]

Amending the Display and Pixel Ratio flags with MPEG4Modifier didn't help...

I then took a closer look at the actual file and it isn't 640*440 as I thought but 640*480!

I guess it's the player and there's nothing I can do about it. Even so, avi.NET has cropped the top and bottom of the picture somewhat. I just can't get my head around it all.