Possible bug in FX hypot() function

[Matjaz.Kljun]

If I try to reproduce Anthony Thyssen example from page:

https://imagemagick.org/Usage/canvas/#gradient_complex

Code: Select all

magick -size 100x100 xc: +size xc:red xc:yellow -colorspace RGB ^
          -fx "ar=hypot( i/w-.8, j/h-.3 )*4; br=hypot( i/w-.3, j/h-.7 )*4; u[1]*br/(ar+br) + u[2]*ar/(ar+br)" ^
          -colorspace sRGB gradient_dist_ratio.gif

I get image with two extra vertical lines:



Command is run on Windows 7. IM version used is from release ImageMagick-7.0.8-51-portable-Q16-x64.zip.

On the other hand if the same command is run with the older version of IM ImageMagick-7.0.7-9-portable-Q16-x64.zip image produced is without extra lines:



The same command modified only with slightly bigger canvas size of “-size 101x101” also produce the image without extra lines. Something must be wrong in -fx expression calculation!

If the hypot() function is properly replaced with sqrt() function in -fx expresion, the produced image is without extra lines in old and also in new IM version.

After some debugging of hypot() function it looks like result is wrong when first argument is zero (0):

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$ magick -size 3x3 xc: -channel red -fx "aa=i; bb=j; Fhypot=hypot(aa,bb); Fsqrt=sqrt(aa*aa+bb*bb); debug(aa); debug(bb); debug(Fhypot); debug(Fsqrt)" null:
[0,0].red: aa=0
[0,0].red: bb=0
[0,0].red: Fhypot=0
[0,0].red: Fsqrt=0
[1,0].red: aa=1
[1,0].red: bb=0
[1,0].red: Fhypot=1
[1,0].red: Fsqrt=1
[2,0].red: aa=2
[2,0].red: bb=0
[2,0].red: Fhypot=2
[2,0].red: Fsqrt=2
[0,1].red: aa=0
[0,1].red: bb=1
[0,1].red: [color=#FF0000]Fhypot=0[/color]
[0,1].red: [color=#00BF00]Fsqrt=1[/color]
[1,1].red: aa=1
[1,1].red: bb=1
[1,1].red: Fhypot=1.41421
[1,1].red: Fsqrt=1.41421
[2,1].red: aa=2
[2,1].red: bb=1
[2,1].red: Fhypot=2.23607
[2,1].red: Fsqrt=2.23607
[0,2].red: aa=0
[0,2].red: bb=2
[0,2].red: [color=#FF0000]Fhypot=0[/color]
[0,2].red: [color=#00BF00]Fsqrt=2[/color]
[1,2].red: aa=1
[1,2].red: bb=2
[1,2].red: Fhypot=2.23607
[1,2].red: Fsqrt=2.23607
[2,2].red: aa=2

This is possible bug or I miss something obvious.

[snibgo]

Here is a simple test:

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magick xc: -fx "hypot(0.3,0.4)" txt:
# ImageMagick pixel enumeration: 1,1,65535,srgb
0,0: (32767.5,32767.5,32767.5)  #800080008000  srgb(50%,50%,50%)

magick xc: -fx "hypot(0,0.4)" txt:
# ImageMagick pixel enumeration: 1,1,65535,srgb
0,0: (26214,26214,26214)  #666666666666  grey40

magick xc: -fx "hypot(0.4,0)" txt:
# ImageMagick pixel enumeration: 1,1,65535,srgb
0,0: (26214,26214,26214)  #666666666666  grey40

In v7.0.7-28, the results are correct. There may be a more recent bug.

[magick]

Thanks for the problem report. We can reproduce it and will have a patch to fix it in GIT master branch @ https://github.com/ImageMagick/ImageMagick later today. The patch will be available in the beta releases of ImageMagick @ http://www.imagemagick.org/download/beta/ by sometime tomorrow.