Re: [boost] [gil::io] Feedback for scanline_read_iterator

Subject: Re: [boost] [gil::io] Feedback for scanline_read_iterator
From: Michael Marcin (mike.marcin_at_[hidden])
Date: 2013-02-25 01:55:34

Christian Henning wrote:
 >
 > As far as I can tell tiff needs the row number to be read and also
 > tiff rows cannot be read in random order. Jpeg also needs to be read
 > in order which means there is no skipping.
 >
 > Now, because of that and to ensure a common interface among all
 > scanline_readers I need to pass the pos into the skip function.
 >

Seems like all the more reason to manage the line counter internally to 
the TIFF reader.

Nothing else afaik needs it as it's implicit in the other readers. And 
you need to have a detailed knowledge of the internal state of the tiff 
reader in order to pass in any correct value.

Mateusz Loskot wrote:
>
> It's not precise, because rows of some TIFFs can be accessed in random order.
> The spec confirms that too:
>
> "random access to individual scanlines can only be provided when data
> is not stored in a compressed
> format, or when the number of rows in a strip of image data is set to
> one (RowsPerStrip is one)."
>
> http://www.remotesensing.org/libtiff/libtiff.html
>

Even more of a reason if certain tiff formats get efficiency boosts by 
managing skips differently (noops for uncompressed, reads for compressed).