[Mp4-tech] question about track header

[Dave Singer]

At 7:17 PM +0530 4/22/04, Swapna Matwankar wrote:
>Hello all,
>
>I have a question about track header in MP4 file.
>In the document, 'Information technology -- Coding of audio-visual 
>objects -- Part 12: ISO base media file format', following 
>information is given about width and height field in the tkhd box:
>width and height specify the track's visual presentation size as 
>fixed-point 16.16 values. These need not be the same as the pixel 
>dimensions of the images, which is documented in the sample 
>description(s); all images in the sequence are scaled to this size, 
>before any overall transformation of the track represented by the 
>matrix. The pixel dimensions of the images are the default values.
>
>Is it mandatory to scale the image to the height and width specified 
>in the tkhd atom? What if the LCD is smaller than these dimensions?

Two points:
a) in MPEG-4 (pure), the tkhd dimensions should be the same as the 
source dimensions.  Scaling etc., if needed, is done with BIFS.
b) obviously the entire visual composition and scaling implied by the 
sample description dimensions being scaled to the track dimensions, 
the track being affected by the matrix, and the overall movie matrix, 
give you the 'intended' notional picture.  How the terminal handles a 
picture that is over the display size is outside the scope of the 
standard.
OK?
>
>Thanks in advance.
>
>regards,
>swapna
>
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-- 
David Singer
Apple Computer/QuickTime
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