You definitely have to stage it -- copy your separate channels into the
interleaved form that the OIIO API wants.
But you don't need to do it for an entire image at a time via
ImageOutput::write_image(). For the sake of keeping memory under control, I
would recommend using write_scanlines() or write_tiles(). If you write several
scanlines at once, or a whole row of tiles/buckets at once, it can be more
efficient than writing just one scanline or tile per call (in particular,
OpenEXR is good at using threads to speed this up).
Also, imageio.h contains some helper routines for copying image-like data
around that will make this whole affair simple to program. In particular,
copy_image().
Conceptually, it looks like this. Let's say that you have separate buffers,
float *red, *green, *blue. You want something like this:
// Open the file and set up
int nchannels = 3;
ImageSpec spec (xres, yres, nchannels, TypeDesc::FLOAT);
if (you want a tiled file) {
spec.tile_width = spec.tile_height = tilesize;
}
ImageOutput *out = ImageOutput::create (filename);
out->open (filename, spec);
// Break it up into batches of at most 64 scanlines, or the tile size
int scanline_batch = tiled ? spec.tile_width : 64;
std::vector<float> tmp (xres*scanline_batch*nchannels);
for (int y = 0; y < yres; y += scanline_batch) {
int n = std::min (scanline_batch, yres-y);
// Copy the separate buffers into a contiguous staging area
OIIO::copy_image (1, xres, n, 1, red+y*xres, sizeof(float),
sizeof(float),
OIIO::AutoStride, OIIO::AutoStride,
&tmp[0]+0, nchannels*sizeeof(float),
OIIO::AutoStride, OIIO::AutoStride);
OIIO::copy_image (1, xres, n, 1, green+y*xres, sizeof(float),
sizeof(float),
OIIO::AutoStride, OIIO::AutoStride,
&tmp[0]+1, nchannels*sizeeof(float),
OIIO::AutoStride, OIIO::AutoStride);
OIIO::copy_image (1, xres, n, 1, blue+y*xres, sizeof(float),
sizeof(float),
OIIO::AutoStride, OIIO::AutoStride,
&tmp[0]+2, nchannels*sizeeof(float),
OIIO::AutoStride, OIIO::AutoStride);
if (spec.tile_height) {
out->write_tiles (0, xres, y, y+n, 0, 1, TypeDesc::FLOAT, &tmp[0]);
} else {
out->write_scanlines (y, y+n, 0, TypeDesc::FLOAT, &tmp[0]);
}
}
// clean up
out->close ();
delete out;
I'm just typing this off the top of my head (and glancing at imageio.h), I
haven't tried compiling it so I don't guarantee that I haven't made some
ghastly error. But that's the gist of what you'd need to do.
-- lg
On Jul 10, 2013, at 8:59 AM, Michel Lerenard wrote:
Hi,
I need to write images to the disk, and am facing a problem that seems so basic
i'm wondering if I'm not missing something obvious.
I need to write specific channels to an image. For example, write UVW values
next to RGBA 'standard' data.
Using directly OpenEXR, i would create the seven channels one after another and
fill each channel with matching data. Our application (Clarisse IFx) stores
data per channel so it would be pretty straigthforward.
I have seen no way using OIIO to do such a thing: there is no writeScanline
function that would take start / end channels: i can't write data channel per
channel.
The only thing I could do is build a huge interlaced buffer gathering all channels from
my image and call "writeimage", but i fear it would use an horrible amount of
memory and extra processing that seems unecessary.
Should I check the file format first and directly instanciate an EXROutput or
TiffOutput when needed ? How could I set channel names ?
Thanks in advance for your help,
Michel
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--
Larry Gritz
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