[Cin] DAR, PAR and SAR, again

[Andrea paz]

I did the following test:

NOTE: I use SAR as pixel aspect ratio i.e. SAR= Sample aspect ratio. I
use PAR as frame aspect ratio i.e. PAR= Picture aspect ratio. I use
PAR=W/H.

I took the following 4 videos:

Source_1:
bd.m2ts; 1440x1080; SAR 1:1; DAR 4:3
Compositor: Display stretched vertically; square px.

Source_2:
5sec_dv01_03.dv; 720x576; SAR 16:15; DAR 4:3
Compositor: Display corrected; px nearly square.

Source_3:
5sec_hdv01_04.m2t.ts; 1440x1080; SAR 4:3; DAR 16:9
Compositor: Correct display; rectangular px.

Source_4:
bd_2.m2ts; 1920x1080; SAR 1:1; DAR 16:9;
Compositor: Correct display; square px.

I created a project (empty) 1440x1080; DAR=16:9  [SAR can be
calculated from: SAR = 16:9 / 4:3 = 1.3333 i.e. 4:3]
I imported 4 source files into Resources window.
What I notice in the Compositor is:

source_1 (same WxH; different SAR; different DAR) ==> non-square
Pixels (px); correct display; no crop/pad.
         Since WxH are the same the display remains correct. But since
the DARs are different then the aspect ratio changes adjusting to that
of the project (from 4:3 to 16:9). As a result the pixels enlarge
(similarly, adjusting to the design SAR: from 1:1 to 4:3), but without
crop/pad because they do not change the No. of pixels, but simply
those that are there extend horizontally.
source_2 (different WxH; different SAR; different DAR) ==> non-square
Pixels; flattened display; pad.
         The worst case because all aspect ratios are different
between source and project. Changing from smaller WxH_source to larger
WxH_project results in a pad i.e., an addition of side black bands.
Going from DAR 4:3 to 16:9 also results in a change in aspect ratio.
As in the case of source_1 the pixels stretch horizontally to change
from 16:15 to 4:3. The black sidebands arise precisely from the
extension of pixels that is not sufficent to fill the DAR_project; the
missing pixels are black and in such numbers as to fill the rest of
the project format.

source_3 (same WxH; same SAR; same DAR) ==> non-square Pixels; correct
display; no crop/pad.
         The best case since all aspect ratios are equal. The
rectangular pixels of the source are found to be the same in the
project, as are the DAR and the number of pixels of WxH. In fact, the
format of the project is the same as that of the source.

source_4 (differentWxH; different SAR; same DAR) ==> non-square
Pixels; flattened display; crop.
         Because of WxH_source > WxH_project we will have a crop, even
though the DAR is the same. This is because we go from DAR 1:1 to SAR
4:3 so the pixels are forced to extend horizontally. Since the DAR is
the same there must be a crop of the source to fit within the smaller
WxH of the project.

So:

same WxH ==> Correct display.
WxH_source < WxH_project ==> flattened display; pad.
WxH_source > WxH_project ==> flattened display; crop.

SAR ==> determines the horizontal size of the pixels (only the
horizontal base). It is the first of the factors that adapt the
WxH_source to the WxH_project.

DAR ==> determines the shape of the frame. It is the second of the
factors that adapt the WxH_source to the WxH_project.

>From Set Format you set the design (WxH and DAR) at this point the
sources will have to adapt to this format, either by changing DAR or
by changing the pixel shape via SAR.


Do the results and explanations sound right to you?

I leave the link if anyone wants to try the EDL and the four videos:

https://www.dropbox.com/scl/fi/hvltphw3yq76mkquok1wq/Archivio.tar.gz?rlkey=bxko4ee6z0awr4epusnpyj8p5&dl=0