Comparison Presets

Named sets of comparison settings (threshold and antialiasing) for common situations. Pass the name as preset to compare_file_odiff(), expect_snapshot_image() or snapshot_report(), or splice the values into other functions with do.call().

Usage

odiff_preset(name = c("strict", "default", "screenshot", "cross_platform"))

Arguments

name

One of "strict", "default", "screenshot" or "cross_platform".

Value

A named list with elements threshold and antialiasing.

Details

"strict"

threshold = 0, antialiasing = FALSE. Any change to any pixel fails.

"default"

threshold = 0.1, antialiasing = FALSE. The defaults of compare_images() and expect_snapshot_image().

"screenshot"

threshold = 0.1, antialiasing = TRUE. For browser screenshots (shinytest2, webshot2, chromote) and plots compared on the same platform. Browsers render the edges of rounded corners, circles and thin borders with slightly different anti-aliasing from run to run; odiff's anti-aliasing detection ignores those pixels. The colour threshold stays at 0.1 so that real colour changes are still caught.

"cross_platform"

threshold = 0.2, antialiasing = TRUE. For baselines shared across machines or operating systems. Also tolerates edges that move by up to about half a pixel, thicker anti-aliased borders and small colour or gamma shifts. The price: changes between colours of similar brightness (e.g. a blue element turning green) and very faint elements (light grey on white) can go unnoticed. Different fonts or text rendering are not tolerated; use snapshot variants or ignore_regions for those.

The values were calibrated with images rendered by ragg: shapes with rounded corners and 1px borders drawn at sub-pixel offsets (0.25 and 0.5 px), and with a different anti-aliasing rasteriser (cairo), should pass, while a new 10 x 10 pixel element, or a 10 x 10 pixel patch recoloured from blue to green, must fail. With the "default" settings the anti-aliasing-only differences fail (5 to 439 differing pixels); with "screenshot" they pass, except a 0.5 px shift of a bordered shape (2 pixels), which "cross_platform" passes too. A blue to green recolouring is detected up to a threshold of 0.14, which is why "screenshot" keeps 0.1. The calibration is part of the package's tests.

See also

compare_file_odiff(), expect_snapshot_image()

Examples

odiff_preset("screenshot")
#> $threshold
#> [1] 0.1
#> 
#> $antialiasing
#> [1] TRUE
#> 

if (FALSE) { # \dontrun{
# Use with any comparison function
do.call(compare_images, c(list("before.png", "after.png"),
                          odiff_preset("cross_platform")))
} # }