I'm experimenting with creating movies with QTKit. I'm creating the
movie from NSImages and experimenting with different codecs: Apple
Intermediate (icod), Photo-JPEG (jpeg) and Uncompressed YUV (2vuy).
Apple Intermediate and Uncompressed YUV look the same. Photo-JPEG
is very different.
None matches what I see when I open the Tiff files I use to create
the movies.
I see the resulting Apple Intermediate and Uncompressed YUV movies
are tagged as nclc 1:1:1, meaning Rec. 709 transfer function,
primaries and matrix - whereas the Photo-JPEG files have no color
information tag at all. That might explain why these look
different. But I do not understand why none looks like the original.
Does QTMovie's addImage:forDuration:withAttributes attempt to do
any color matching (including gamma correction) or does it not? Is
it perhaps just blindly tagging the movies but not actually
converting to the correct color space first?
Please file a bug report with your sample images (if possible) and a
clear method of reproducing your results. We have recently uncovered
at least one other color space issue with QTKit, which should be
fixed in a future QTKit release. I do not know of any workaround for
your particular issue. Sorry about that.
Here are the problems roots and workaround:
First: the tags in the final movie mean nothing (some codec's blindly
add nclc 1:1:1), it's up to YOU to set the correct tags. So to get the
colors looking right you have to convert the images to one of the
color space primaries supported by QuickTime, and set the nclc
accordingly. QTKit will do no conversion at all.
Second: QTKit is doing something with gamma. In my experiments, for
most codecs it is expecting the input images to be gamma 1.8 and then
converting it to gamma 2.2 (for some codecs it is leaving it as gamma
1.8 though, notably 2vuy and jpeg). So we need to convert the input
images to gamma 1.8 then pass it to QTKit; depending on which codec
was used, the results need to be tagged as either gamma 1.8 or 2.2.
By the way, the resulting gamma is a simple gamma encoding and is
quite different from the Rec. 709 transfer function, so using the nclc
for tagging will not look correct, instead set the nclc transfer
function to unknown and use the gamma extension to set gamma.
This is far from optimal as the image's gamma is possibly changed
twice for no reason... this can deteriorate the image's quality.
Third: up to here we have the primaries and gamma, now, most codecs
use YUV instead of RGB internally, and to get accurate colors QT needs
to know which matrix to use. This makes one wonder why QTMovie's
addImage:forDuration:withAttributes has no attribute for the YUV
matrix... anyway, each codec will do it's own YUV conversion! And even
worse, none will tag the Image Description correctly.
What this mean's is that (like with gamma) you need to test and come
up with a list of which codec uses which matrix and set the movie tags
accordingly. Here's the list for the codecs I tested (including the
gamma used):
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