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Old 02 November 2016, 11:37   #54
Jack Burton
It's all in the reflexes!
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Join Date: Nov 2009
Location: Wingkong warehouses
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Quote:
Originally Posted by robinsonb5 View Post
Not really, it's just that your particular test image doesn't cover the RGB range very well when you crop out just a small part of it.
If you're taking the top left 5x5 squares then you're only going to be testing with 25 blue levels out of 256, and some of those will be mapped to the same blue level after colour-reduction to 18-bit, so you'd probably only have 10 unique blue levels out of 64 represented.

Therefore your potential colour range of 262144 colours is reduced to 262144*10/64 = 40960, which is pretty close to the result you got.
If instead of cropping you do a nearest-neighbour scaling on your test image to reduce it to 512x512 pixels, then repeat your test I think you'll get a result much closer to 262144.
Yes, true, I didn't think about that.
I did some test again, I scaled the image to 512x512 as you advised me, and this time the color counting result was 63526 colors ! It's quite impressive because this time it's 4 times less colors than 262144 (from 24bit 512x512) whereas in my first test (1280x1280 -> 41082) it was 39 times less colors from the 24bit 1280x1280 source !
So you were right, in fact it really depends on the nature of the image displayed. For that matter landscapes give very good results, almost 24bit quality.
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