Merging two single-colour photos for colour-changing artwork

[Transistor]

I'm making a magic mirror for a production of Snow White. It uses mirror tint film on acrylic for the mirror, but when stage lights are dim and internal LED lights are switched on it will show one of two images - Snow White or the Spirit of the Mirror.

Two faces.png

How can I merge these two faces so that I can make a print with both.

• The red face will disappear under red light but the blue will appear dark.

• The blue face will disappear under blue light but the red will appear dark.

Blends such as "Multiply" are not what is required as that will modify the lower image. I need something more like a reverse of the "Extract channel" feature so I can combine the red and blue images.

Any ideas? Thanks!

[jkm]

You'll need to ask on a Photoshop or ImageMagick forum to get a detailed answer.

This sort of illusion depends on having interleaved monochrome images, so that any particular pixel is either red OR blue. Not purple.

Think of the overall image as a series of lines: line 1 is from the blue image, line 2 is from the red image, line 3 is from the blue image, and so on. To avoid the final monochrome image appearing stretched, each source image must be decimated (some pixels removed) effectively halving the resolution.

It can be done with a horizontal interleave, a vertical interleave, or a grid interleave. It cannot be done using blend modes.

Ask on a Photoshop forum about how to interleave or interlace two images. It could also be done with ImageMagick; you could ask on a forum about that software as well. Here is an example: https://jqmagick.imagemagick.org/discou ... hp?t=29392

Good luck...

[Transistor]

It cannot be done using blend modes.

It can! You could think of the purple as microscopic particles of blue and red so it will reflect either and absorb the other. This is the same idea as your striped image but on a microscopic level.

I have an RGB LED strip as my light source with a switch on each colour. To avoid ambient light interference, these are installed in a large cardboard box with a viewing port. I got a paint colour card from the hardware store and used this to figure out what combination might work to give good enough contrast. The best I've got so far is as follows:

Faces both.jpg

Figure 1. The merged images.

To generate the combined image I used Inkscape, dropped the second image on top of the first and set the blend mode on the top image to 'Multiply'.

Faces blue.jpg

Figure 2. Photo of print under blue and green light. The blueish ink and paper reflect the G+B light and disappear while the red ink absorbs those colours and appears dark.

Faces red.jpg

Figure 3. Photo of print under red light. The red ink and paper reflect the red light so the face disappears and the mask is revealed.

For some reason that I haven't figured out yet it is very easy to get good contrast between the red ink and the G+B lighting. Finding a shade to absorb the R while not appearing dark in the G+B lighting is proving a bit tricky. I suspect this is due to the particular wavelengths of light emitted by the LEDs. I'll work on it some more to try and improve the contrast. The show is in twelve weeks or so!

Thank you for your interest in my project.

[jkm]

Blends such as "Multiply" are not what is required as that will modify the lower image. I need something more like a reverse of the "Extract channel" feature so I can combine the red and blue images.

First you say Multiply is not what you need. Now you say multiply works.

You’re moving the goalposts and switching your examples.

It can!

If that approach works on the images you actually asked the question about, then why did you ask the question?

You could also try taking your monochrome images and pasting them into the red and blue channels.

Or, since this is 2026 instead of 1926, you could forego the boxes, lighting rigs, and printed surfaces, and just install a panel display on stage as your “mirror” to display whatever you want, or project an image using a small projector.

Good luck….