FindImg returns different maxLoc depending on DecodeImg or CaptureImg being used

[J7mbo]

Problem

• A screenshot is taken with robotgo.CaptureImg().
• When that screenshot is saved first, and then loaded using DecodeImg, then the X Y coordinates from FindImg() are correct. See code 1.
• When that screenshot is instead used directly in FindImg(), the results are different! See code 2.

This is a problem for those who want to use the images from memory, not having to save them to the filesystem first.

Code 1:

Note: please substitute for your own images.

robotgo.Save(robotgo.CaptureImg(), "template.png")

template, _, _ := robotgo.DecodeImg("template.png")
find, _, err := robotgo.DecodeImg("find.png")

a, b, c, maxLoc := gcv.FindImg(find, template)
fmt.Println(a, b, c, maxLoc)

This gives:

-0.26353332 0.99999934 (2802,1283) (1026,168)

Code 2:

template := robotgo.CaptureImg()
find, _, err := robotgo.DecodeImg("find.png")

a, b, c, maxLoc := gcv.FindImg(find, template)
fmt.Println(a, b, c, maxLoc)

-0.42405054 0.41302797 (664,132) (2869,116)

Note how the only difference between the two code snippets are that we're saving the image first and then loading it with DecodeImg.

Extra Info

---

• Note that this is a 14" MacOS with retina display. I have been noticing issues with scaling etc and having to do my own calculations to compensate. I'm not sure this is an issue here directly, though?
• gcv version (or commit ref): github.com/vcaesar/gcv v0.40.5
• Go version: go version go1.21.4 darwin/arm64

• Operating system and bit: MacOS M3 chip

Apple clang version 15.0.0 (clang-1500.0.40.1)
Target: arm64-apple-darwin23.1.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

[J7mbo]

PS - I know that this is just using MatchTemplate under the hood so if I need to open something in the gocv lib please let me know and I'll do that.