IT Employment

Ask something
Discussion Locked

PNG image files, pHYs chunk

0 helpful
1 reply

by william_overington 2 views

Closed to new repliesThis discussion is closed. It stays readable, but it takes no new replies.

A need has arisen to save images in PNG format and to include information in a pHYs chunk within the PNG file about the aspect ratio of the pixels themselves. This is because of a requirement in the DVB-MHP (Digital Video Broadcasting - Multimedia Home Platform) specification.

quote

15.1.1 PNG Aspect ratios

PNG bitmaps shall carry a pHYs chunk indicating the pixel aspect ratio of the bitmap. This aspect ratio should be the same as that of the scene
containing the bitmap.

The PNG specification indicates that if the aspect ratio is absent square pixels should be assumed. To avoid overriding this specification the aspect ratio should be signalled explicitly.

end quote

The aspect ratio of the pixels is going to be somethinglike 16 wide to 15 high, that is, slightly on the landscape style rectangular side of square. This is so that the multimedia graphics handling facilities tie in as to
aspect ratio of pixels with the main picture of the television channel, so that graphics can be overlaid on top of the regular television picture in response to user input from a hand held infra-red control device.

A plug-in for Paint Shop Pro or a stand-alone program to just set the pHYs chunk would be suitable.

The DVB-MHP system is organized by a secretariat based at the European Broadcasting Union in Geneva. The http://www.mhp.org website provides
information about the DVB-MHP system. There is also a discussion forum at http://forum.mhp.org which is free to join and which has some recent
discussion about producing artwork to display using the DVB-MHP platform, including the issue of the pHYs chunk being set to indicate non-square
pixels. The requirement to set the pHYs chunk for PNG files is on-going for this area of broadcasting.

Does anyone know how the pHYs chunk can be set for non-square pixels please?

1 reply

by william_overington

Readers might like to know that some images of the general style that I am trying to save as PNG files with the pHYs chunk set for non-square pixels are in the Art Gallery in the http://www.users.globalnet.co.uk/~ngo webspace.

A direct link to the Art Gallery is as follows.

http://www.users.globalnet.co.uk/~ngo/art00000.htm

When saving images in the PNG format for broadcasting with the 720 pixel wide by 576 pixel high screen for the minimum system DVB-MHP broadcasting system, I hope to produce pictures where any circles appear as ellipses with the long axis vertical, so that when the file is displayed using the non-square pixels, shapes intended as circles will appear as circles on the screen. This is so that the television sets will not have to process the pictures for pixel shape, but can just place them pixel for pixel onto the screen.

Thus, for this application, the setting of the pHYs chunk is an entirely separate part of the process of producing the image file.

This discussion is closed to new replies.