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IDProjectCategoryView StatusLast Update
0001251DCP-o-maticBugspublic2023-12-22 22:33
Reportercarl Assigned To 
PrioritynormalSeveritymajorReproducibilityalways
Status acknowledgedResolutionopen 
Summary0001251: Subtitles are too small in preview
Description

If you measure the height in pixels in the preview window it comes out too small: it looks like the calculation in marked_up() may be wrong? It assumes 96dpi which the pango docs suggest is the default.

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Carsten

2018-04-24 12:22

manager   ~0002396

Last edited: 2018-04-24 16:16

The question is, how do you actually calculate the size, over aspects like variable preview window size, possible different j2k decoding resolutions? And then why is the burn-in consistent with preview, but not timed text? I can only help as much as trying to test this on our Sony and Barco... and a friend may test it on his Doremi/Nec,

  • Carsten

Carsten

2018-04-24 18:21

manager   ~0002397

Last edited: 2018-04-27 03:37

Trying to do some test - I remember seeing something on the project progress bar (DCP-o-matic main page left) about having two Load Font tags in XMLs - did I dream that? I can't find it there, on Mantis, or in the git?

Ah, there... https://dcpomatic.com/mantis/view.php?id=1273

  • Carsten

Carsten

2018-04-27 03:08

manager   ~0002404

Last edited: 2018-04-27 23:54

Hmm. Do you mean pt numbers calculate too small chars?

I created a test with two subtile lines, using their default sizes. I created a DCP with them. Played back in player, they appear at the same size as in preview. I exported this DCP to an MP4 file and added it to the subtitle test project - the rendered subtitles in the background appear at exactly the same size and position as the overlaid timed text and burn-in titles.

Is it possible that different OS create different pt-dpi references, hence different char sizes?

What about different screen resolutions/preview sizes - something similar to the scope cropping issue in https://dcpomatic.com/mantis/view.php?id=1274 ?

  • Carsten

Carsten

2018-05-06 01:02

manager   ~0002409

Last edited: 2018-05-06 01:09

Hmm. There is a bug. When I add a 4:3 image as background, I set subtitle scale to 66, the subtitles appear very small in preview. However, when I watch the final DCP with burnt-in subtitles, they stretch nearly from left to right visible area.

When I reopen the project, under video set 'scale' to flat (stretching the image), and then create the DCP, the burnt-in subtitles appear at the same size as in preview.

It looks as if the video-scale setting has some effect on subtile scaling as well?

On a side note, when I select a subtitle file, all controls under 'audio' are disabled, but under 'video', I can still adjust settings like 2D, scale to, cropping etc.. Maybe irrelevant, but I guess these video controls should all be disabled as well for subtitle content.

  • Carsten

edit: It seems to be just the scale factor, not the actual image processing - the subtitle comes out much larger than in preview when scale to is set to 1.33. However, when I set ScaleTo at 'no scale', the background image is displayed identically - but the burn-in subtitle is displayed as small as in preview.
So, it appears that the ScaleTo is somehow having an impact on subtitle scaling that is not reflected in preview. I guess the subtitle burn should always happen with respect to target container size/resolution, but it seems it happens based on source resolution?

carl

2018-05-17 23:54

administrator   ~0002435

Last edited: 2018-05-18 00:09

I was initially talking about the preview size not matching what should be / is displayed by a projector. i.e. if you make a DCP with non-burnt subs does the preview look the same as the projected DCP.

72pt should be 1/11 of the frame height, so for a 2K DCP the size of some T point text in pixels should be

(T / 72) * 1080 / 11

e.g. in burnt_subtitle_test_dcp we are burning a 39pt font into a DCP so it should be 53 pixels high but in the test it's only about 49 (taken as the distance between the descender of g and the ascender of h). Which isn't all that bad actually.

Carsten

2018-05-18 19:55

manager   ~0002440

Hmm. I guess I still don't get it. When I use the same simple SRT file twice in a project, place them in parallel (not sequentially). I offset one file vertically to create some space between them. I set one of these files to burn-in, the second to timed text.

Both appear in the same size in preview. Both appear in the same size on our two different DCI machines. Then what is wrong?

I may need to create a bitmap backdrop (bounding box) where the subtitles should fit in precisely. Then see what happens between DCP-o-matic preview and DCI projector.

The different preview sizes depending on image aspect ratio I mentioned above bother me, though. But I need to do some more tests...

  • Carsten

Carsten

2018-05-19 02:00

manager   ~0002441

Here is my test project and DCP - a bitmap with two bounding boxes, a single line subtitle file used as timed text and burn-in within these boxes.

  • Carsten

Carsten

2018-05-19 02:01

manager   ~0002442

Last edited: 2018-05-19 02:02

project file including background image with bounding box and subtitle SRT file.

Carsten

2018-05-19 02:02

manager  

2SubSize.zip (6,901 bytes)

carl

2018-05-20 00:58

administrator   ~0002443

Last edited: 2018-05-20 00:58

The aspect-ratio-dependency thing is a bug, I think, somewhere... possibly related to 0001246.

The nub of this bug report is that preview looks different to playback on a DCI projector for non-burnt subs. If you don't see that, it's interesting: is it even possible to change the size of subs in the configuration of a DCI system? I know there used to be the option to get the projector or the playback server to make the subtitles...

carl

2019-01-24 22:52

administrator   ~0002934

Not really sure if there is a bug here.

Carsten

2019-01-25 01:19

manager   ~0002938

Last edited: 2019-01-25 01:28

At least not within the scope that I did my testing in. Maybe someone fell about that aspect ratio scaling issue I mentioned in my 4th post above, but that seems to be fixed. Something may happen with specific scaling properties, but in general, when I create two video based bounding boxes of the same size as a background, and put a timed text subtitle line into one, and a burn-in into the second, and both appear same size within that bounding box on our DCI projector - then, at least in principle, DCP-o-matic is doing it right. Or whatever DCP-o-matic is doing wrong is equally compensated by our Sony ;-)

The Doremi servers, when doing server side subtitle rendering, only allow repositioning, but not scaling of subtitles. On projectors, there is no (accessible) option to set the size of timed text subtitles.

I would close this bug for now. You remember there are some users which are quite picky about subtitle appearance. If there'd be something as obvious as size mismatch, I guess they'll let us know.

  • Carsten

Bug History

Date Modified Username Field Change
2018-03-22 11:58 carl New Bug
2018-04-10 23:42 carl Severity minor => major
2018-04-24 12:22 Carsten Note Added: 0002396
2018-04-24 16:16 Carsten Note Edited: 0002396
2018-04-24 18:21 Carsten Note Added: 0002397
2018-04-27 03:08 Carsten Note Added: 0002404
2018-04-27 03:37 Carsten Note Edited: 0002397
2018-04-27 11:20 Carsten Note Edited: 0002404
2018-04-27 23:53 Carsten Note Edited: 0002404
2018-04-27 23:54 Carsten Note Edited: 0002404
2018-05-06 01:02 Carsten Note Added: 0002409
2018-05-06 01:09 Carsten Note Edited: 0002409
2018-05-06 01:09 Carsten Note Edited: 0002409
2018-05-17 23:54 carl Note Added: 0002435
2018-05-18 00:08 carl Note Edited: 0002435
2018-05-18 00:09 carl Note Edited: 0002435
2018-05-18 19:55 Carsten Note Added: 0002440
2018-05-19 02:00 Carsten File Added: Bildschirmfoto 2018-05-19 um 02.56.19.png
2018-05-19 02:00 Carsten Note Added: 0002441
2018-05-19 02:01 Carsten Note Added: 0002442
2018-05-19 02:02 Carsten File Added: 2SubSize.zip
2018-05-19 02:02 Carsten Note Edited: 0002442
2018-05-20 00:58 carl Note Added: 0002443
2018-05-20 00:58 carl Note Edited: 0002443
2019-01-24 22:52 carl Target Version 2.14.0 =>
2019-01-24 22:52 carl Note Added: 0002934
2019-01-25 01:19 Carsten Note Added: 0002938
2019-01-25 01:20 Carsten Note Edited: 0002938
2019-01-25 01:24 Carsten Note Edited: 0002938
2019-01-25 01:27 Carsten Note Edited: 0002938
2019-01-25 01:28 Carsten Note Edited: 0002938
2023-12-22 22:33 carl Status new => acknowledged