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 Author Topic: 1.50 or 1.67?  (Read 1967 times)
texshooter
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 « on: August 13, 2013, 07:46:47 PM » Reply

Jeff Schewe says to downsample for the web in increments of 50%, while Sean Bagshaw says 67%. Who is right?
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Schewe
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 « Reply #1 on: August 13, 2013, 10:25:03 PM » Reply

Actually, I said in increments not smaller than 50%....any flavor of BiCubic will toss pixels away indiscriminately...so if you downsize in a single step of more than 50% you loose more pixels than you keep...downsampling in increments can allow you do image processing while downsampling. YMMV

BTW, who is Sean Bagshaw? Written any books?
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texshooter
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 « Reply #2 on: August 13, 2013, 11:31:01 PM » Reply

He's Tony Kuyper's protege. In his video tutorial he recommends downsizing the original to 167% the size of the final (1st pass) then downsize that to the final size (2nd pass). But he's not the only one saying this. I'm reading this multiplier, two-thirds (1.67), all over the web as though it were a magic number, but I can't find any techy stuff to convince me it's true (or at least convince me the opiner knows what he's talking about).

If I understand your book Image Sharpening correctly, I should downsize a 5600 pixel wide image to a 1280 wide like this:

5600 x 50% = 2800 then
2800 x 50% = 1400 then
1400 down to 1280 [rounding off on the last pass instead of the first pass]

and most importantly, the multiplier above should never be lower than 50% because that would entail too many downsizing passes, which would throw too many pixels away.

Am I hot or cold?
 « Last Edit: August 13, 2013, 11:32:41 PM by texshooter » Logged
Schewe
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 « Reply #3 on: August 14, 2013, 12:23:53 AM » Reply

Correct...but the last pass using BiCubic Sharper while the previous steps are normal BiCubic...
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ablankertz
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 « Reply #4 on: August 15, 2013, 06:04:51 PM » Reply

>>Actually, I said in increments not smaller than 50%....any flavor of BiCubic will toss pixels away indiscriminately...so if you downsize in a single step of more than 50% you loose more pixels than you keep

You seem to be saying that downsizing in increments using bicubic preserves more information than downsizing in one step. Could you give us the math that explains it? Also, what about sinc based algorithms?
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Bryan Conner
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 « Reply #5 on: August 15, 2013, 11:14:23 PM » Reply

Correct...but the last pass using BiCubic Sharper while the previous steps are normal BiCubic...

Is there an advantage to doing the downsizing in this manner in Photoshop compared to allowing Lightroom 5 (or 5.2RC) to do this?  Thanks for sharing in this forum, Jeff.
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Schewe
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 « Reply #6 on: August 15, 2013, 11:37:18 PM » Reply

Since CC's Image Size is new, I can't tell you definitively that PS CC is "better" than LR's resampling (I think it might be). But it all depends on the original and the amount of resizing involved...upsample 50%? Prolly not much difference, upsample 100%, maybe a tilt to CC's Image Resize. 200-400%? Prolly CC's image resize with the correct reduce noise settings. Just understand that CC's Preserve Detail with reduce noise is new. So it will take a while for things to shake out.
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