[Pfspd-users@sf.net] Cpfspd change proposal on aspect ratio behaviour

[Bram R. <bra...@ph...>]

Hi all,

Frank van Heesch triggered a disussion on the use of the display
aspect ratio in cpfspd and related tools.
This email outlines a CPFSPD change proposal.


Current situation

Cpfspd supports two display aspect ratios: 4:3 and 16:9.
These are both commonly used in current TV standards.
For several reasons, current tools and files do not use the 16:9
values properly - the 4:3 default is almost always used.
Consequently, almost every file is tagged as 4:3.
Up till now, we did not really pay attention to the aspect ratio.
Frank pointed out some good reasons to pay attention:
- Frank is working on an application that needs to use the information
- Wrong values are confusing


Requirements

- Appropriate defaults for aspect ration
  (so most tools will "automatically" set correct aspect ratio values).
- Support for "square pixel" aspect ratio
  (since many pfspd files contain a graphics image)


Boundary conditions

- Minimal or no change in API.
- Consistency with current API semantics.
- No change in pfspd file format.
  The file format describes only a __display__ aspect ratio (or image 
aspect ratio).
  Hence, a "square pixel" aspect ratio is only valid for a certain image 
width/height
  and cannot be preserved automatically over scaling operations.


Propsed solution

Both PV and PP will NOT be adapted. Main focus of these tools is 
visulisation
of the pixels. A deformation of the image aspect ratio is acceptable and 
clearly
favourable over scaling (requiering filtering with two main disadvantages: 
original
pixel values are lost and slowdown due to the extra calculations).

cpfspd.h
- Add P_AS_WH to pT_asp_rat.

cpfspd_get.c / p_get_aspect_ratio()
- Add P_AS_WH; use largest common divider from image width/height to check 
this value.

cpfspd_mod.c / p_mod_aspect_ratio()
- Add P_AS_WH; use largest common divider from image width/height to 
calculate this value.
Note that this is only "square pixel" for the current width/height 
setting.

cpfspd_hdr.c / p_set_header_values()                internal function
- Add P_AS_WH as parameter value; use largest common divider from image 
width/height
to calculate this value.
- Add P_UNKNOWN_ASPECT_RATIO to serve as "default" parameter value
to enable automatic setting depending on pT_size values:
P_4_3  for   P_SD, P_CIF,  P_QCIF
P_16_9 for P_HDp,  P_HDi

cpfspd_hdr.c / p_set_stream_header_values()                internal 
function
As p_set_header_values().

cpfspd_hdr.c / p_create_header()
- Pass P_UNKNOWN_ASPECT_RATIO (not P_4_3) to p_create_ext_header().

cpfspd_hdr.c / p_create_ext_header()
- No code change, funcionality changed by underlaying
p_set_header_values() / p_set_stream_header_values().


Non-cpfspd issues

Cpfspd application/tools that change image size (scale, border, crop, 
trim) need to handle the
aspect ratio explicitely. If input file has P_AS_WH then output must be 
explicitely set
at P_AS_WH to retain the "square pixel" property.


Your comments / suggestions / improvements are appreciated.

Regards, Bram.

PS: Frank, please subscribe to the relevant mailing list via 
https://sourceforge.net/mail/?group_id=137382

--
A.K. (Bram) Riemens
Principal Scientist, DSP group, Philips Research
Office: WO-p-94, Postbox WO02
High Tech Campus 36 (WO), 5656 AE Eindhoven, The Netherlands
Tel: +31 40 27 43833, Fax: +31 40 27 44675
E-mail: bra...@ph...