Re: [Openexr-devel] Y RY BY images

[Florian Kainz]

Jim Hourihan wrote:
> On Mar 14, 2007, at 3:51 PM, Florian Kainz wrote:
>
> > If you are using class RgbaInputFile to read the file, then the
> > RY and BY channel names are special; RgbaInputFile assumes that
> > they are sub-sampled with x and y sampling rates of 2.  When you
> > use class RgbaOutputFile to write a Y/RY/BY image, then the RY
> > and BY channels will automatically be sub-sampled.
>
> I should have included more info. I'm using Imf::OutputFile to write.
> I'm using Imf::InputFile to read (but this is clearly not an issue  because
> I can read Rec709_YC.exr and XYZ_YC.exr no problem).

When you are using Imf::OutputFile to write the file, you must specify
the sub-sampling rates in the header's channel list and in the frame
buffer slices.

> > A Y/RY/BY image with without sub-sampling can be read via the
> > general (arbitrary set of channels) interface, but RgbaInputFile
> > will reject it.  Maybe this should be fixed; the changes in class
> > RgbaInputFile would be manageable.  On the other hand, why would
> > you want to write Y/RY/BY images without sub-sampling?  You might
> > as well write RGB files.
>
> I want the data sub-sampled, but Imf was core dumping on me. However,  I 
> didn't
> realize it would do the sampling for me. Is there a way to prevent that?
> I already have the data sub-sampled in separate planes (as a result  of 
> using
> InputFile). My application caches the sub-sampled data for display.

Imf::RgbaOutputFile does the filtering and sampling for you.
With Imf::OutputFile you have to do it yourself.
You should be able to hand sub-sampled data to OutputFile;
after all that's what RgbaOutputFile does internally.

Function writeRead() near line 150 in file IlmImfTest/testCompression.cpp
contains code that writes and reads sub-sampled pixels using the
Imf::OutputFile and Imf::InputFile classes.

> I'll try handing it data that's not sub-sampled.
>
>     -Jim