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If you put this image on a black background you see something different than on a white background. Does anyone have a idea how it works?

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It's not a transparent PNG and placing the image over different colored backgrounds does nothing here. There was a trick to alter an image when the cursor highlighted it - only worked in IE. But I can't find info on that now. –  Scott Aug 22 '12 at 9:59
    
I might've seen image like this before cause it seems a bit familiar, but I can't remember. One thing I noticed is that the image seems to render differently in different browsers ( Safari showed the other image for me ). Plus saving it to my desktop, I can immediately see the other image and when I open it up in photoshop and adjust the curves I can bring out the other one as well. i.stack.imgur.com/rhev7.jpg –  Joonas Aug 22 '12 at 11:26
    
it has to do with how browsers change the colors when the image is selected. –  DA01 Aug 22 '12 at 14:58
    
So the answer is the browser and (possibly) your gamma settings? My Safari showed a worse version of the img above from Joonas where the interlacing was much more apparent. The FF version shows the Batman but more blurred than Joonas's and darker. I'd post a screenshot but I'm lazier than Joonas ;) –  RKS Aug 23 '12 at 2:02
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I remember getting insta banned from a forum in my youth for making my avatar have a hidden nekkid lady in it when highlighted. Ah to be young again. –  Ryan Aug 24 '12 at 2:51

4 Answers 4

OK, I finally got around to looking at the example image, and the trick is pretty simple: gamma correction.

As others have noted, the image is composed of two interleaved pictures: out of every 2 × 2 pixel block, three pixels have RGB values in the range 0 to 210, and show the "red tabby kitten on bed" image, while one pixel has RGB values in the range 214 to 255 and shows a very heavily lightened version of the "batman cat" image.

Here's a small section of the image, scaled up by a factor of 8, with no gamma correction applied. This is what you'll see if you open the image in a program that doesn't understand PNG gamma correction and zoom in:
A small section of the image, zoomed in, no gamma adjustment
As you can see, the light pixels (which contain the "batman cat" image) just pretty much look white. At normal magnification, they blend in with the other image, which has much higher contrast, and just make it look somewhat lighter.

However, the PNG image also contains a gAMA chunk, which specifies a file gamma value of 0.023. This is an extremely small gamma value; more typical values would be between 1.0 and 0.45. When opened in a program that supports PNG gamma correction, this causes the image to be darkened so much that the "kitten on bed" image literally becomes invisible — all its colors are mapped to black — while the colors of the "batman cat" image are mapped to more ordinary values.

For example, here's the same zoomed-in section of the image after gamma correction:
Same image section, zoomed in, with adjusted colors

So, to conclude, the appearance of this image does not depend on the color of the background. Rather, it depends on whether the program you use to view it supports PNG gamma correction (and is willing to apply such an extreme gamma value) or not.

By the way, the gamma correction value used in the image seems a little too extreme: at least on my screen, the "batman cat" image shows up a lot more nicely if you double the gamma.

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The image is two images interlaced. Interlacing is, very basically, where two images are displayed simultaneously by showing a single line (or pixel) of each one in an alternating pattern.

Usually one finds this in TV and video broadcasting since the frame rates mask the interlacing effect.

If one were to take this image and use a deinterlace filter ("even fields"), the batman image would be gone. Also, the batman cat image becomes "primary" when zoomed to 50% or when using the scale feature of photoshop, probably becasue of interpolation/resampling. Curiously, when you commit the scaling change and photoshop re-renders at full quality (it uses a fast render method when manipulating transforms), the other cat becomes "primary" again. The same scaling to 25% eliminates the batman cat as well.

As far as "why" the display changes depending on software, that is most probably a function of the methods used by various software for rendering images. I checked the header for the file and it looks to be properly formed, but there may be some trick or hack implemented in one of the optional sections (it has 3 optional chunks IIRC) which exploits a rendering bug. Personally, I think it is just the method used interpolate based on the rescaling stuff I mentioned above. Also, there is a gamma section in the header and on my display the image is REALLY dark, so it may just exploit the fact that some pixels are lighter and the rest blend in with a dark background.

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The image appears dark here to me as well, however, saving the file I can only see the other cat and a faint "haze" of the Batman cat as if it's a semi-transparent overlay. Zooming in you can see the interlaced pixels. –  RKS Aug 23 '12 at 1:57
    
try zooming out. The software used has an impact on the effect as well. –  horatio Aug 28 '12 at 14:47

I ran a test on the PNG you linked to and got the same picture both times so I can't see what you are asking about.

PNGs have an alpha channel which allows the colors to have varying degrees of opacity. This might be the cause of the change with the different colored backgrounds.

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I'm going to ignore the actual picture in your question, and just answer the question implied in the title: How to create a PNG image the looks different on a black background than on a white one?

Specifically, the method I'll describe will allow you to combine any two grayscale images A and B into one PNG file C, so that C looks like A on a black background and like B on a white background, with one condition: every pixel of A must be equal to or darker than the corresponding pixel of B. (If the original images don't quite satisfy that condition, the method I'll describe will force them to, leading to some "ghost image" leakage from one to the other. Depending on the images, this may or may not be visually apparent.)

The trick, of course, is to make use of the PNG alpha (= opacity) channel. Specifically, given a pixel with color c and opacity α, the pixel will have the apparent color a = αc on a black background and b = αc + (1−α) on a white background. (Here, color 0.0 represents black and 1.0 represents white.) Turning this around, given target pixel colors a and b, we can solve these equations for c and α:

  • 1 − α = ba
    ⇒   α = 1 − (ba) = 1 − b + a,   and
  • c = a / α

(This, of course, assumes than ab; otherwise the transparency 1 − α would be negative, which the PNG format, alas, does not support.)

OK, so how can we do these calculations in a graphics editor like the GIMP or Photoshop? Well, we really just need two operations — subtraction and division — and, as it happens, GIMP (and, if I'm not completely mistaken, Photoshop too) has both of these implemented as layer composition modes.

In fact, the process is essentially the same as the one I described in this answer for reconstructing the alpha channel of an image from versions superimposed on black and white backgrounds; the only difference is that we start from two arbitrary images — for example, these two cat photos from Wikimedia Commons:

Cat---Black---Moertel---(Gentry).jpg White_kitten.jpg

I've cropped these images, converted them to grayscale and adjusted their color levels so that they almost — but not quite, just to show what happens — satisfy the relative lightness condition ab. (Tip: the "lighten only" and/or "darken only" layer modes are handy for checking this.)

Now, I'll do the following steps:

  • Open both images as layers of a single picture.
  • Make a copy of the black cat layer, change its layer mode to "subtract" and merge it down into the white cat layer. The result will be the transparency of the final image.
  • Make a copy of the transparency layer we just created, invert it, change its layer mode to "divide" and merge it down to the remaining copy of the black cat layer. This will be the luminance channel of the final image.
  • Move the transparency layer to top and make it visible, and use the channels dialog to transfer it to the selection. Then select the luminance layer and cut the selection out of it. Finally, delete the transparency layer.
  • Save the remaining layer as PNG. It should look like this:

Black and white cats in one image

This page has a white background, so you should see the white kitten image (but with some faint "ghosts" of the other image where the ab condition wasn't satisfied), but you can download the image above and confirm that it looks quite different on a black background. Or just compare these flattened versions on black, white and purple backgrounds respectively:

Flattened version with black background Flattened version with white background Flattened version with purple background

The last version shows what's going on: the purple areas are where the PNG above is transparent, allowing the background color to show through. Conversely, the black and white areas are where the PNG image is opaque, making the color independent of the background.

Ps. Here's the full size version of the PNG image.

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