efficient way to compare 2 images?

[TerryRitchie]

An example with 720p images at play:

Code: (Select All)

Randomize Timer

Screen _NewImage(1280, 720, 32)

'let's make this an unique and pretty image!
For i = 1 To 100
    Line (Rnd * _Width, Rnd * _Height)-(Rnd * width, Rnd * _Height), &HFF000000 + Rnd * &HFFFFFF, BF
Next

image2 = _CopyImage(0) 'identical copies for testing
image3 = _CopyImage(0) 'identical copy...  BUT
_Dest image3
PSet (Rnd * _Width, Rnd * _Height), &HFF000000 + Rnd * &HFFFFFF 'We've just tweaked it so that there's no way in hell it's the same as the other two now!
_Dest 0 'image3 is EXACTLY one pixel different from the other two.  Can we detect that?
image4 = _CopyImage(0) 'an identical copy once again, because 0 will change once we print the results

result1 = CompareImages(0, image2)
result2 = CompareImages(0, image3)
result3 = CompareImages(image2, image3)

Print "Current Screen and Image 1 Compare:  "; result1
Print "Current Screen and Image 2 Compare:  "; result2
Print "Image1 and Image 2 Compare        :  "; result3

Print
Print "Press <ANY KEY> for a speed test!"
Sleep

t# = Timer
Limit = 1000
For i = 1 To Limit
    result = CompareImages(image2, image3)
    result = CompareImages(image2, image4)
Next
Print
Print Using "####.####### seconds to do"; Timer - t#;
Print Limit * 2; "comparisons."


Function CompareImages (handle1 As Long, handle2 As Long)
    $Checking:Off
    Static m(1) As _MEM
    Dim s(1) As String
    m(0) = _MemImage(handle1): m(1) = _MemImage(handle2)
    If m(0).SIZE <> m(1).SIZE Then Exit Function 'not identical
    If m(0).ELEMENTSIZE <> m(1).ELEMENTSIZE Then Exit Function 'not identical
    s(0) = Space$(m(0).SIZE): s(1) = Space$(m(1).SIZE)
    _MemGet m(0), m(0).OFFSET, s(0): _MemGet m(1), m(1).OFFSET, s(1)
    If s(0) = s(1) Then CompareImages = -1
    $Checking:On
End Function

This function is going into my tool box! I ran across a need today to compare many images generated within code, not from files. I remembered this thread. Steve, this code works perfectly. Thank you for sharing.