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Image Similarity

Source: vignettes/similarity.Rmd
similarity.Rmd

This article explores the use of {magick}’s image_compare function in the context of morphed images, since I couldn’t find any clear guides to what it was doing. I’ve implemented this for webmorph stimuli with the compare() function.

While this metric does capture something about similarity between morphed images, it is unlikely to be of any use as a metric for similarity between non-morphed images (e.g., it won’t be able to tell you if two face images are of the same person or not).

Create images to compare 

stimuli <- demo_stim()
s <- continuum(stimuli[1], stimuli[2]) |>
  mask("face") |>
  subset_tem(features("face")) |>
  crop_tem()

plot(s, nrow = 1)

Calculate metrics

Using all the available metrics, calculate similarity of each image to the first image.

mtypes <- magick::metric_types()
names(mtypes) <- mtypes
m <- lapply(mtypes, compare, 
            stimuli = s, 
            ref_stim = 1, 
            fuzz = 0, 
            scale = TRUE) # results scaled from 0 to 1

PSNR is Inf when the test and reference image are identical, so the highest non-infinite value scales as 1.0.

Check fuzz settings 

m2 <- lapply(mtypes, compare, 
             stimuli = s, 
             ref_stim = 1, 
             fuzz = 50,
             scale = TRUE)

Fuzz only does something for AE.

fuzzes <- seq(0, 20, 1)
names(fuzzes) <- fuzzes
m3 <- lapply(fuzzes, compare, 
             stimuli = s, 
             ref_stim = 1, 
             metric = "AE",
             scale = FALSE) # don't scale for this comparison

Conclusions

  • Increases with morph distance:
    • very strong negative exponential decay at 0 fuzz; more linear with higher fuzz: AE
    • strong negative exponential decay: PAE
    • slight negative exponential decay: Fuzz, RMSE
    • linear: MAE, MEPP, MSE
    • no idea: PHASH
  • Decreases with morph distance:
    • linear: NCC, Undefined
    • slight exponential decay: PSNR