On Sun Feb 08 2015 at 19:05:49 Larry Gritz <[email protected]> wrote:
> On Feb 7, 2015, at 6:21 PM, Justin Israel <[email protected]> wrote: > > Personally I would be happy with selecting the first 3 or 4 channels (or > the number supported by the output format) under that given circumstance, > but if we want to go the extra mile and do some checking, maybe that would > be a plus? > > > On one hand, it seems smarter to try to pull out the R, G, B among them, > regardless of the order. > > But that should be weighed against the distinct possibility that the > channels are not named simply, or are even mis-labeled. > > I'll think about this a bit. Maybe an even more convoluted approach is > best: If the channel names contain is an identifiable R, G, B, use those > (regardless of original order), otherwise use the first 3 channels and hope > for the best? > > I think most of the time these should be equivalent, because the OIIO > guidelines say that a format reader should reshuffle as necessary to make > R,G,B,A always be the first 4 channels, regardless of their appearance in > the file. > Actually, your mixture of the two approaches would probably be best. Funny thing... just this morning I got bit by assuming the first 3 channels would be ok... Turned out it was an exr with 4 channels: B matte G R Go figure. > > > Maybe this is a dumb question, but what do you think the average image > viewer does when displaying images with arbitrary channels? Do they look > for R,G,B labeled channels, or maybe load the first N channels? > > > Ha! I think the *average* viewer either crashes, refuses to load the image > at all, or loads it but totally botches the memory layout and thus > mis-draws the image, like this: > > image: R G B A X Y | R G B A X Y | > > display: | | | | > > (I hope that comes out right, it takes a mono font on those lines to make > the spacing right.) > > Hah. Well I meant viewers that can actually display the formats ;-) > > > > > On Sun Feb 08 2015 at 2:36:26 PM Larry Gritz <[email protected]> wrote: > >> OK, so how about this proposed heuristic for oiiotool: >> >> If the output format doesn't support as many channels as the input image, >> then it will perform the equivalent of "--ch R,G,B" (or "R,G,B,A", if the >> format does support an alpha channel) before saving. If the input doesn't >> contain (somewhere in it) channels named R, G, and B, then it's an error. >> >> Or should we just ignore the issue of specific channel names and write >> the first 3 (or 4) channels and call it a day? >> >> -- lg >> >> >> PS. WTF, Jono, don't you have someplace more important to be tonight? >> >> >> On Feb 7, 2015, at 5:20 PM, Justin Israel <[email protected]> wrote: >> >> Well in my case, one specific example of an input image is a non-subimage >> style stereo, with [RGB] and right.[RGB] labels. But oiiotool obviously >> should not have any knowledge of special labels, and the goal was to not >> have to pre-inspect the source images before converting to jpeg. >> >> Thanks Larry for breaking down the categories of when oiio wants to make >> best default choices, vs erroring out. It does sound like a really similar >> situation to the fact that it does automatically throw away the alpha >> channel. If I have a source image with extra channels, and no channels have >> specifically been selected, it would seem that choosing RGB by default >> would be what the user most likely wants. If I knew I wanted specialized >> channels, I would definitely select them. Otherwise, it should probably >> complain about the alpha channel just as it does complain about 6 channels >> passing through. >> >> >> On Sun Feb 08 2015 at 2:11:06 PM Jono Gibbs <[email protected]> wrote: >> >>> If the input file format had 3 obvious r g b channels and then 3 which >>> were not related to color (z, alpha, obj-id) then it seems like keeping the >>> r g b channels when going to JPEG is in the spirit of "do the obvious >>> thing". >>> >>> If it's a 6-channel color thing then it's unlikely any 3 channels make >>> sense. >>> >>> Does the input format have no information like channel names? >>> >>> --jono --mobile-- >>> >>> > On Feb 7, 2015, at 4:35 PM, Larry Gritz <[email protected]> wrote: >>> > >>> > On one hand, we don't want to do operations that are not supported in >>> the output format, thereby resulting in significant loss of data. On the >>> other hand, we can't be too trigger-happy with the errors, or it would be >>> impossible to get anything done. So when we encounter a request to do >>> something not possible with a given format, we try to ask "is there a >>> particular thing that the human almost certainly meant when they made this >>> impossible request?" >>> > >>> > For example OpenEXR's least accurate pixel data type ('half') has more >>> precision and range than JPEG's most accurate type, so there will be data >>> loss, but we don't want to make every operation that starts with exr and >>> ends with jpg to be an error. So we just convert any input pixel data type >>> to UINT8 when outputting a JPEG. For most ordinary images, the data is LDR >>> and our eyes are mostly satisfied with 8 bits in an sRGB-mapped intensity >>> response, so this conversion will probably be fine. ("If you wanted to >>> preserve the HDR data, you should not have output to JPEG, dummy.") >>> > >>> > But what if we are asked to save more channels than a file format can >>> accommodate? If you just drop the channels, you're not just reducing >>> precision, you are losing whole sections of the original data. So at >>> present, we make it a hard error. >>> > >>> > As a special case (and maybe precedent?), we do just silently drop an >>> alpha channel when saving JPEG. JPEG cannot accommodate alpha, but it's so >>> common for an input to have alpha, it was painful for it to be an error >>> when saving to JPEG. It seemed that the obvious human interpretation was >>> "I'm using JPEG because this is final output for the web or for my mom to >>> view, the alpha that was valuable for intermediate computations won't be >>> needed for those purposes, so drop it." >>> > >>> > I think that at the low level of ImageInput, open() should fail if you >>> ask for more channels than can be supported in any obvious way. But for >>> oiiotool in particular, I think we can make more "best guess" heuristics. >>> > >>> > I'm certainly open to this being debated! >>> > >>> > When oiiotool is outputting to a format that doesn't support as many >>> channels as the image appears to have, do you think we should just silently >>> output the first 3 channels and drop the rest? Should this be the one and >>> only behavior? Or should there be a "strict/lax" option that determines >>> whether this (and potentially other conversions) are an error or silently >>> do whatever is necessary to complete the action somehow? If so, should the >>> default be strict or lax? >>> > >>> > -- lg >>> > >>> > >>> > (Note for the pedants in the crowd: most of the places I wrote "JPEG", >>> I'm not really talking about the JPEG compression, but rather the JFIF file >>> format.) >>> > >>> > >>> >> On Feb 5, 2015, at 3:56 PM, Justin Israel <[email protected]> >>> wrote: >>> >> >>> >> I was curious about some behavior in oiiotool v1.4.14, when dealing >>> with a source image that has 6 channels, and converting it to a jpeg. >>> >> >>> >> oiiotool source.exr -o out.jpg >>> >> # oiiotool ERROR: jpeg does not support 6-channel images >>> >> >>> >> I presume this bubbles up from libjpeg. Fair enough. But I know that >>> the mantra of OpenImageIO has usually been to try and do the right default >>> action, and attempt to avoid failures if possible. That being said, do you >>> think it would be more in line with that philosophy if the jpeg plugin >>> ensured it would only use 1, 3, or 4 of the first available channels, if >>> not using an explicit list already? That way you would still get a jpg >>> output, even if you passed it a 6 channel image, but could still explicitly >>> give is a channel list if you knew them up front. >>> >> >>> >> Currently I have to inspect the source image first before calling >>> oiiotool, and ensure that I pass it a reduced channel list if needed. >>> >> >>> >> Justin >>> > >>> > -- >>> > Larry Gritz >>> > [email protected] >>> > >>> > >>> > >>> > _______________________________________________ >>> > Oiio-dev mailing list >>> > [email protected] >>> > http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >>> _______________________________________________ >>> Oiio-dev mailing list >>> [email protected] >>> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >>> >> _______________________________________________ >> Oiio-dev mailing list >> [email protected] >> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >> >> >> -- >> Larry Gritz >> [email protected] >> >> >> >> _______________________________________________ >> Oiio-dev mailing list >> [email protected] >> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >> > _______________________________________________ > Oiio-dev mailing list > [email protected] > http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org > > > -- > Larry Gritz > [email protected] > > > > _______________________________________________ > Oiio-dev mailing list > [email protected] > http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org >
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