Re: [Openexr-devel] C API calls

[Florian Kainz]

*   Make sure that the file you are trying to read does actually
    contain an image.  Start with a file that you know is good.
    You can download a number of images from www.openexr.com.

*   The ImfInputReadPixels call in your code should read

        ImfInputReadPixels (fp, y1, y2);

*   Your code snippets do not show how you compute width and
    height from the data window.  Assuming that

        width  = x2 - x1 + 1;
        height = y2 - y1 + 1;

    the ImfSetFrameBuffer call is incorrect; you will get a
    crash if x1 != 0 or y1 != 0.  The call should read

        ImfInputSetFrameBuffer (fp, rgba - x1 - width * y1, 1, width);

    Once your file reader works, I strongly encourage you to test
    it on all of the OpenEXR sample images that you can download
    from www.openexr.com, in particular the DisplayWindow/t??.exr
    files.

*   The C API is only for situations where for some reason you
    really cannot use C++.  You are probably better off using
    the C++ API, as you did in the first version of your code.

    The calls in the C API are straightforward wrappers around
    the corresponding C++ calls.  For example,

        ImfInputReadPixels (inputFile, s1, s2);

    is equivalent to

        inputFile.readPixels (s1, s2);

    The main difference is how errors are handled: the C++ call
    throws exceptions, while the C call returns 1 for success or
    0 for failure.  In C++ you handle errors like this:

        try
        {
            ...
            inputFile.readPixels (s1, s2);
        }
        catch (std::exception &e)
        {
            std::cerr << e.what() << std::endl;
        }

    In C you do this:

        if (!ImfInputReadPixels (inputFile, s1, s2))
        {
            fprintf (stderr, "%s\n", ImfErrorMessage());
            return 0;
        }


Thomas Lock wrote:
> I finally got it to work in debug mode, which exposed an error that wasn't
> coming up in release mode (not sure why). It was missing 1 library file to
> link to. I'm going to get reading and writing to work first in debug mode
> and then worry about release mode.
> At this point i have the writing working (verified with 2 other apps), but
> the reading seems to have a non-fatal error (image is black but correct
> dimensions). I don't think the buffer is getting filled properly. This
> function is simply the opposite of what the writing function does, so once
> the buffer is filled and converted to my array the rest is done outside this
> function. I added the lines of code i believe the problem is related to, but
> i can't seem to find any documentation on the C API so i am guessing.
>
> ImfRgba *rgba;
>
> rgba = (ImfRgba *)malloc(width * height * sizeof(ImfRgba));
>
> header = ImfInputHeader(fp);
>
> ImfHeaderDataWindow(header,&x1,&y1,&x2,&y2);
>
> ImfInputSetFrameBuffer(fp, rgba , 1, width);
>
> ImfInputReadPixels(fp, height, 0 );
>
>
>
> ----- Original Message ----- 
> From: "Florian Kainz" <address@hidden>
> To: "Thomas Lock" <address@hidden>
> Cc: <address@hidden>
> Sent: Thursday, July 20, 2006 12:05 AM
> Subject: Re: [Openexr-devel] C API calls
>
>
> > What does "fail" mean in this case?  Does the application crash?
> > If it does, have you tried using a debugger to get a stack trace?
> >
> > Does ImfOpenOutputFile return a null pointer?  If you do get a null
> > pointer, have you tried calling ImfErrorMessage() to find get some
> > diagnostic output?  Your example contains no error checking code.
> > Unlike the C++ interface, the C interface does not throw exceptions;
> > you must explicitly check the return values of the functions you call.
> >
> > Thomas Lock wrote:
> > > Hi Drew
> > >
> > >             I completely rewrote my code to meet the C API calls. This
> time
> > > i get zero warnings or errors but the app specifically fails on this
> call
> > > "file = ImfOpenOutputFile(filenames, header, IMF_WRITE_RGBA );". I tried
> a
> > > variety of filename options and nothing seems to work. I can't verify if
> my
> > > remaining code works, but i believe it is correct.
> > >
> > > extern "C" void WINAPI writeOpenEXR (char *filename, int width, int
> height,
> > > float* Bits128)
> > > {
> > >        int LineWidth=width*4;
> > >        float fwidth;
> > >        const char fileName[] = "c:\foo2.exr";
> > >        const char *filenames = "c:\foo2.exr";
> > >
> > >        ImfOutputFile *file;
> > >        ImfRgba *base;
> > >        base = (ImfRgba *)malloc(width * height * sizeof(ImfRgba));
> > >        fwidth = (float)width;
> > >        ImfHeader *header;
> > >        header = ImfNewHeader();
> > >        ImfHeaderSetDisplayWindow(header, 0, 0, width - 1, height - 1);
> > >        ImfHeaderSetDataWindow(header, 0, 0, width - 1, height - 1);
> > >        ImfHeaderSetScreenWindowWidth(header, fwidth);
> > >        file = ImfOpenOutputFile(filenames, header, IMF_WRITE_RGBA );
> > >
> > >        ImfDeleteHeader(header);
> > >
> > >        for(int h=0;h<height;h++)for(int b=0;b<width;b++)
> > >       {
> > >            ImfHalf half;
> > >
> > >            int i=h*LineWidth+(b<<2);
> > >            int j=h*LineWidth+b;
> > >
> > >            ImfFloatToHalf(Bits128[i], &half);
> > >            base[j].b = half;
> > >
> > >            ImfFloatToHalf(Bits128[i + 1], &half);
> > >            base[j].g = half;
> > >
> > >            ImfFloatToHalf(Bits128[i + 2], &half);
> > >            base[j].r = half;
> > >
> > >            ImfFloatToHalf(Bits128[i + 3], &half);
> > >            base[j].a = half;
> > >        }
> > >
> > >        ImfOutputSetFrameBuffer( file, base, 1, width );
> > >        ImfOutputWritePixels(file, height );
> > >        ImfCloseOutputFile(file);
> > >
> > >        free(base);
> > > }
> > >
> > >
> > >
> > >
> > > _______________________________________________
> > > Openexr-devel mailing list
> > > address@hidden
> > > http://lists.nongnu.org/mailman/listinfo/openexr-devel