Question about resolutions for aspect ratio after cropping/resizing

[spiritgumm]

Trying to reduce file size of a UHD video (3840x2160). It's of an older full screen movie (1.37) with black bars, so I wanted to crop and resize. Cropping 440 each side yields 1.37 resolution. To maintain this aspect, the smaller resolution options are 1480x1080 (abit large) or 740x540 (abit small). Would prefer to find an in-between size for TV. One option is to resize to 988x720 but the DAR is a tad larger.
A similar solution is to first resize to 1280x720, then crop, but the DAR would be a tad smaller.
In both 720 encode situations, the DAR could be changed in another program to 1.37, but would that be inaccurate, or unacceptable to purists?

[jagabo]

Why are you worried about tiny AR errors like that?

[_Al_]

When it was pillarboxed, an aspect error was most likely introduced to jam it forcefully to one dimension and keep subsumpling. Or perhaps master had already a tiny ar error as well. No point to be that pedant and keep two decimal floating point as well. I would not worry. If you are lucky you actually might end up fixing that error.

[spiritgumm]

Why are you worried about tiny AR errors like that?

I'd like to get it right, and I figured there must be a method or trick to preserve AR at more resolutions. If there was, it would help with other encodes.

[_Al_]

If you want to find out best resolution with almost no AR error, you can run Python code for example, it has to include mods for subsampling and range you want to include, for example for you new height from 700 to 740 and you let it run or you decide. Or you start with a width range like 960 to 1010 to get height:

WIDTH = 3840-440-440 #(2960)
HEIGHT = 2160
MODX = 2  #for YUV420, but it can be testes as both values as 4 (mod4) or 8 (mod8)
MODY = 2
NEW_WIDTH_RANGE  = (960, 1010)
NEW_HEIGTH_RANGE = (700,740)


def boxing(w, h, W, H):
    #back check, if w/h is not too off, x or y positive values means it is quite off
    if W/H > w/h:
        w = w*H/h
        x = int((W-w)/2)
        x = x - x%MODX
        return max(0, x)
    else:
        h = h*W/w
        y = int((H-h)/2)
        y = y - y%MODY
        return max(0, y)

for h in range(*NEW_HEIGTH_RANGE, MODY):
    h = h - h%MODY
    w = WIDTH/HEIGHT *h
    w = w - w%MODX
    off_value = boxing(w, h, WIDTH, HEIGHT)
    if not off_value:
        print(f'new width = {int(w)}, new height = {int(h)} new ar = {w/h}') 
print('')   
for w in range(*NEW_WIDTH_RANGE, MODX):
    w = w - w%MODX
    h = HEIGHT/WIDTH *w
    h = h - h%MODY
    off_value = boxing(w, h, WIDTH, HEIGHT)
    if not off_value:
        print(f'new width = {int(w)}, new height = {int(h)} new ar = {w/h}')

edit: I fix code above, same results, but it is more readable, more logical to read the code

you'd get:

new width = 958, new height = 700 new ar = 1.3685714285714285
new width = 962, new height = 702 new ar = 1.3703703703703705
new width = 964, new height = 704 new ar = 1.3693181818181819
new width = 970, new height = 708 new ar = 1.3700564971751412
new width = 972, new height = 710 new ar = 1.3690140845070422
new width = 978, new height = 714 new ar = 1.3697478991596639
new width = 980, new height = 716 new ar = 1.3687150837988826
new width = 986, new height = 720 new ar = 1.3694444444444445
new width = 992, new height = 724 new ar = 1.3701657458563536
new width = 994, new height = 726 new ar = 1.3691460055096418
new width = 1000, new height = 730 new ar = 1.36986301369863
new width = 1002, new height = 732 new ar = 1.3688524590163935
new width = 1008, new height = 736 new ar = 1.3695652173913044
new width = 1010, new height = 738 new ar = 1.3685636856368564

new width = 960, new height = 700 new ar = 1.3714285714285714
new width = 962, new height = 702 new ar = 1.3703703703703705
new width = 966, new height = 704 new ar = 1.3721590909090908
new width = 968, new height = 706 new ar = 1.3711048158640227
new width = 972, new height = 708 new ar = 1.3728813559322033
new width = 974, new height = 710 new ar = 1.371830985915493
new width = 976, new height = 712 new ar = 1.3707865168539326
new width = 980, new height = 714 new ar = 1.3725490196078431
new width = 982, new height = 716 new ar = 1.3715083798882681
new width = 984, new height = 718 new ar = 1.3704735376044568
new width = 988, new height = 720 new ar = 1.3722222222222222
new width = 990, new height = 722 new ar = 1.371191135734072
new width = 996, new height = 726 new ar = 1.371900826446281
new width = 998, new height = 728 new ar = 1.370879120879121
new width = 1002, new height = 730 new ar = 1.3726027397260274
new width = 1004, new height = 732 new ar = 1.3715846994535519
new width = 1006, new height = 734 new ar = 1.3705722070844686

so you can grab resolution that is closest to your 1.37037

[_Al_]

with all mod as 4, you'd get:

new width = 964, new height = 704 new ar = 1.3693181818181819
new width = 968, new height = 708 new ar = 1.3672316384180792
new width = 980, new height = 716 new ar = 1.3687150837988826
new width = 992, new height = 724 new ar = 1.3701657458563536
new width = 996, new height = 728 new ar = 1.3681318681318682
new width = 1008, new height = 736 new ar = 1.3695652173913044

new width = 960, new height = 700 new ar = 1.3714285714285714
new width = 968, new height = 704 new ar = 1.375
new width = 972, new height = 708 new ar = 1.3728813559322033
new width = 976, new height = 712 new ar = 1.3707865168539326
new width = 984, new height = 716 new ar = 1.3743016759776536
new width = 988, new height = 720 new ar = 1.3722222222222222
new width = 1000, new height = 728 new ar = 1.3736263736263736
new width = 1004, new height = 732 new ar = 1.3715846994535519

all mods as 8 it would be:

new width = 952, new height = 696 new ar = 1.367816091954023
new width = 960, new height = 704 new ar = 1.3636363636363635
new width = 984, new height = 720 new ar = 1.3666666666666667

new width = 960, new height = 696 new ar = 1.3793103448275863
new width = 968, new height = 704 new ar = 1.375
new width = 976, new height = 712 new ar = 1.3707865168539326
new width = 992, new height = 720 new ar = 1.3777777777777778
new width = 1000, new height = 728 new ar = 1.3736263736263736

So as best mod 8, and closest to your AR, it seams to be that the winner might be:
976 x 712 ,new ar = 1.3707865168539326
or to keep conventions 720p:
992x720 is not bad as well

[jagabo]

I'd like to get it right

There is no "right". For example to get exactly 1.37:1 AR with a 720 pixel height requires you resize to a horizontal size of 986.4 (720 * 1.37). Since you are probably working with YUV 4:2:0 video the nearest you can get is a mod 2 frame size of 986x720. But that is mod 2. Some players have problems with mod 2 frame sizes so the software is saving you from yourself and using mod 4 sizes. The nearest mod 4 size is 988x720 (1.37222...:1). It knows you cannot see a ~0.162 percent AR error.

[ProWo]

Mod 4:
988x720
1480x1080

[spiritgumm]

Since there is no perfect fit in the 720 range, any comment about changing its DAR in mkvtoolnix (mmg.exe) to 1.370?

[jagabo]

Not all players will respect MKV DAR (and other) flags.

[spiritgumm]

So if I want to maintain the old "Academy ratio" (1.370) which the film was released in, from a purist point of view, I'm limited to the few exact resolutions, correct?

[poisondeathray]

A purist would check to see if the 1.37 transfer was correct . Often there are slight AR errors to begin with

[spiritgumm]

The movie was painstakingly remastered, and is listed on imdb as being 1.37:1. I'm not dismissing that the AR could be wrong, but I don't know how to check it.

[poisondeathray]

You'd have to measure some known objects (such as a circular wall clock), shot straight on ( preferably at the center of the frame to reduce lens curvature issues) . There are several threads discussing this. Otherwise there is a possibility that you're jumping through these hoops worrying about 0.2% errors and just propagating the upstream AR errors which might be more than 0.2% in the first place

[spiritgumm]

Thanks for all the replies.

[poisondeathray]

If you make the assumption the transfer was correct without small AR errors - the "perfect fit" at 720 is 1280x720. ie. don't crop it.

[spiritgumm]

I suppose that's one idea, but I also wanted to compress the UHD video to a smaller file, and it seems a waste to keep the black bars.

[poisondeathray]

Constant clean black bars do not "cost" much bitrate to encode, probably <0.5% . If they are not clean (unlikely for a remastered release), you can crop and add clean ones back.

[spiritgumm]

I assumed (incorrectly) that any large border would take up significant space.

[_Al_]

That is my approach as well downsizing with black bars as well, I actually do exactly that. And only after resizing you can crop to the mod, possibly leave tiny black bars, if you do not want to crop to live image - I do not recommend that, who cares that there is 2 pixels left or right.
You end up exactly with 100% same aspect ratio.

But I had some heated discussion about that, supposedly resize is not correct at the border etc. , folks usually love to crop -resize - and actually possibly crop again, that makes no sense at all. So supposedly one downsizes image, which translates to downgrading and then persisting about not correct resize that actually brings AR error. Sometimes one has to think straight, what actually makes sense.

[poisondeathray]

But I had some heated discussion about that, supposedly resize is not correct at the border etc. , folks usually love to crop -resize - and actually possibly crop again, that makes no sense at all. So supposedly one downsizes image, which translates to downgrading and then persisting about not correct resize that actually brings AR error. Sometimes one has to think straight, what actually makes sense.

A resampler will technically be incorrect at border of black pixels, because the black pixels add contamination. For example , if you take bicubic or spline, it uses 16 pixels (4x4 grid)

Eitherway there are tradeoffs . Best is do nothing. Or decide which tradeoff you're willing to accept

[_Al_]

Yes, but getting clean lines for that resize mostly folks cut the live footage anyway and loose that border anyway, that is happening all the time. I was going thru this long time ago when setting auto process of backing up BD's. Problem is most of the time this logic is used, crop-resize - (and possible crop again), not realizing that that border is lost anyway and aspect ration is lost as well. For example op's case, , did he actually got exactly 440 pixels from sides and end up exactly cutting black when the only thing left was live image? Was not edge already screwed up when during authoring, not sure if edges are already exact clean so to speak.

[hello_hello]

spiritgumm,
maybe have a play with CropResize (assuming you use Avisynth). By default it outputs a mod4 width and/or height when it's making the decisions, but you can change that. Enabling a cropping preview and Info=true will show you exactly what it's going to do.

I upscaled an image and added borders to use as the starting point, based on your description. You probably won't be able to resize to exactly 1.37 dimensions, so the script will do a bit of extra cropping if need be to prevent aspect error. You can specify just the width or the height if you prefer and the script will take care the other.

Source


[Attachment 68746 - Click to enlarge]

Cropping and resizing based on an output height of 720, as an example.
Well, it's not actually cropped or resized in the screenshot because the preview is enabled. The script will crop an extra 1.458 pixels from the top and bottom to output 988x720, which results in an aspect ratio of 1.3722.

CropResize(0,720, 440,0,-440,0, CPreview=2, Info=true)


[Attachment 68747 - Click to enlarge]

Or you can try different heights. A height of 692 will get you much closer to the original DAR and the script only has to crop half a pixel more each side to prevent aspect error. The output will be 948x692 with a DAR of 1.3699.

CropResize(0,692, 440,0,-440,0, CPreview=2, Info=true)


[Attachment 68748 - Click to enlarge]

As it's so close you can always specify the original DAR for the output. Info=true will tell you the SAR to use for encoding (9601:9598 in this case). If a player pays attention to that the DAR will be correct, but if it doesn't the aspect error will be too small to matter. The correct display dimensions would be 948.3 x 692, but if a player displays it as 948 x 692 instead, you won't notice.

CropResize(948,692, 440,0,-440,0, OutDAR=2960.0/2160.0, CPreview=2, Info=true)


[Attachment 68749 - Click to enlarge]

If it was me, I'd just sacrifice a pixel or two and not worry about making it anamorphic.

CropResize(0,692, 440,0,-440,0)


[Attachment 68750 - Click to enlarge]

[spiritgumm]

Thanks, this will take me alittle time to digest.

[hello_hello]

In case it's not obvious, the script uses the resizer to apply any extra non-mod or sub-pixel cropping required, so for the last example

CropResize(0,692, 440,0,-440,0)

would be the same as

CropResize(948,692, 440,0,-440,0)

and would be the same as this (using the numbers shown by Info=true which are rounded to 3 decimal places for cropping when they're displayed)

CropResize(948,692, 440.462,0,-440.462,0)

and the script is actually doing this

Crop(440,0,-440,0).Spline36Resize(948,692, 0.462,0,-0.462,0)

So it's nothing you can't do yourself, but the script makes it easier by doing the math for you, and by default it'll crop the least amount of extra picture required to prevent aspect error, whether it be from the width or the height. For the example above:

3840 - 440 - 440 - 0.462 - 0.462 = 2959.076

2959.076 / 2160 = 1.369942

948 / 692 = 1.369942

If you want to use a different resizer for CropResize other than the default of Spline36Resize, you can.

CropResize(948,692, 440,0,-440,0, Resizer="BicubicResize")

If you specify both an output width and height, CropResize will crop as much extra as required to prevent aspect error, so if in doubt, but you know the output width or height you want, just specify one of them.

CropResize(0,720, 440,0,-440,0) or CropResize(1280,0, 440,0,-440,0) etc.