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OK - my manager hit me with this one today, and I need a reality check. She asked me to color-correct an image because, "images get darker in Photoshop when you make them smaller."

I've never seen anything that would lead me to that conclusion - and I've been with Photoshop since version 3 (still having difficulty fully trusting multiple undo's, by the way).

Is one of us crazy? And if it's me, in what way do you best compensate for the change? Levels?

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    If you resize using a good quality algorithm, the image should keep its same average color (including brightness). Can you show an example without offending your boss? – Kobi May 18 '11 at 4:22
  • Not really; I wouldn't feel comfortable sharing client images on a public forum. Thanks though. I'm pretty sure she's imagining it. – StormShadow May 18 '11 at 8:46
  • Depending on the sizes you often need to sharpen images too. I think this is a similar perceptial effect. – JamesRyan May 18 '11 at 16:44
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I've never noticed a color change from shrinking. Sometimes fonts will become pixelated, but not darker. I would compare the two side by side and adjust Gamma if necessary..

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    Yeah, neither have I. I have a feeling that she's resized a lot of images with some kind of vignetting around the edges (a lot of stock photos have this) and is perceiving the more closely encroaching dark corners as weighting the entire image towards the dark. I ended up bringing the RGB levels up and spiking the saturation a bit and she seems to feel validated. – StormShadow May 18 '11 at 8:45
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I've run into this. I don't know why it happens, but here's my workaround.

  1. I put all my layers in a folder.
  2. Duplicate the Folder
  3. Turn off the original folder.
  4. Merge the layers of the Duplicated folder.
  5. Now when you export, photoshop is resizing off of your merged layers and it seems to work fine vs. getting darker.
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In some cases, I agree with the manager.
Programmatically I tested one image using python(library: opencv).

First, I calculated the original image's average value of all RGB pixel values.
Second, I resized it 2 times bigger and shrink it to the original size.(I used cv2.INTER_AREA flag. There are many methods to shrink an image.)
I can inspect slightly decreased average of all pixels.
I don't know why this happen but maybe it's really true in some cases this weird phenomenon occurs.

Maybe this can be happen because of the color space, namely sRGB. This link handles about that. I think this can be a good reference.

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There's an obvious test here(that you haven't said you did): have you actually confirmed that your manager isn't nuts by directly comparing areas of color between an original and one of your downscaled images?

Even if there is, this is probably not directly a result of scaling, but rather something in the export options which come into play after the actual scaling within Photoshop, ie. the "Convert to sRGB" toggle, whether you're embedding a color profile, etc.

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The quality of the image is paramount and/or when you're using an already low-quality image within Photoshop (assuming it's already been rasterized) it'll pixelate and yes, interpolate the best it can but altogether degrade in quality.

But without an example to compare the two... even the a portion of the example to compare side-by-side it's gonna be like finding a needle in ... well you get the point.

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As posted by others, an image will not technically become darker if you shrink it (with a good algorithm).

However... our brain can play weird tricks on us with perception. A certain color may look "greenish" to us in a white surrounding, or totally different with another context. I can very well imagine that smaller images can -in certain cases- be perceived darker when made smaller. It seems very acceptable feedback to me if a user says:

"Hmmm, now that the image is smaller, it feels much 'darker'. I think we should make it a little brighter."

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  • Right. That sounds reasonable to me too. Much more reasonable however, than the blanket statement that resizing makes images darker in general. I decided that "Oh really?" will most probably be the most noncommittal and diplomatic response the next time I hear this theory expounded. – StormShadow May 18 '11 at 23:58

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