Yes, it's clear now, thanks for that detailed answer.
So my guess is that Photoshop writes data in a tag OIIO doesn't
recognize, to know this extra channel is alpha. (and hopefully Gimp can
read it, so he decides to use it as well.)
On 07/20/2015 09:33 PM, Larry Gritz wrote:
OK, so here's the deal:
TIFF (unlike OpenEXR) doesn't have any header field or other information that stores
channel names. So it is meaningless to talk about the channel names "in a TIFF
file." No such thing. All OIIO can do for TIFF is make up names that we think
correspond to what's there, based on other header fields.
For a file with >= 3 channels and whose PHOTOMETRIC tag is PHOTOMETRIC_RGB, we make the reasonable guess that
the channel names for the first three channels should be "R", "G", and "B".
But what if there's a fourth channel? What is it, and what is its name? TIFF
files contain a tag called EXTRASAMPLES describing the meaning of extra
samples. There are three choices:
1. ASSOCIATED -- it's an alpha channel, with the assumption that the color channels are
"premultiplied". We pass this along and call it "A".
2. UNASSOCIATED -- it's a mask, and the color channels are not premultiplied. We pass this on as channel
"A", and premultiply the color channels so we always present "associated alpha" (unless
the user has set the configuration to include "oiio:UnassociatedAlpha" set to 1, in which case we
do not premultiply and leave the alpha unassociated).
3. UNSPECIFIED -- this means that the extra channel is something else, not an alpha (associated or
unassociated). In that case, it seems wrong to call the channel "A", since the TIFF file
*** explicitly says that it's not alpha ***. So we call it "channelN", since the TIFF
file, as we said, specifies no name. (Examples of an extra channel that is not alpha would include
depth, unlit diffuse color, surface normal, etc.)
So, bottom line, the file you attached says in the TIFF header that the EXTRASAMPLES
value is UNSPECIFIED, meaning that it's not alpha. So we don't call it "A". I
don't really know what else to say.
You can rename channels with oiiotool:
oiiotool testalpha_olds.tif -chnames R,G,B,A -o out.tif
Although TIFF won't store the channel names you give it, the OIIO tiff output module will
recognize that since it's named "A", it's meant to be alpha, so the
EXTRASAMPLES value it writes should be ASSOCIATED. So this effectively just copies the
file and corrects the EXTRASAMPLES. I don't know how it got to have the UNSPECIFIED value
to being with.
Does this clarify?
-- lg
On Jul 6, 2015, at 8:39 AM, Michel Lerenard <[email protected]> wrote:
Hi,
I've stumbled on an issue I haven't been able to understand. I've attached a
file sent to our support team, which cannot be loaded properly. The file comes
from Photoshop.
While trying to understand what was happening, I noticed that in the ImageSpec channel names list, I get "R",
"G", "B" and "channel3". If I open the file in GIMP and resave it using another name, everything is
fine: the fourth channel is named "A".
I've looked at the TIFF plugin code, and I don't understand the part of the readspec
function that is renaming the fourth channel I have. From what I understand, at first the
function reads that there are four channels in the file, initializes the spec with
default names (R,G,B,A), then renames the channel "channel3" when it decides
that the extra channel is not alpha.
====>>>>>
// This extra channel is not alpha at all. Undo any
// assumptions we previously made about this channel.
if (m_spec.alpha_channel == c) {
m_spec.channelnames[c] = Strutil::format("channel%d", c);
m_spec.alpha_channel = -1;
}
====<<<<<
Could anyone explain to me how this works ? I'm completely lost with all the
EXTRASAMPLE stuff that is used to decide what to do with the last channel.
Thanks.
Michel
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Larry Gritz
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