I ran the tests again. I am not sure what went wrong before, but now I do not get color results. Must have been some typo or copy/paste error.
Anyway, there are still problems.
1) -colorspace gray does not match -colorspace rec601luma, but instead matches -colorspace rec709luma
2) -colorspace gray seems to match all the 709 options, but not the rec601luma options, which it should be matching.
From the options page on colorspace at
http://www.imagemagick.org/script/comma ... colorspace
Gray
Gray = 0.298839*R+0.586811*G+0.114350*B
Rec601Luma
Gray = 0.298839*R+0.586811*G+0.114350*B
Rec709Luma
Gray=0.21260*R+0.71520*G+0.07220*B
So -colorspace gray should match -colorspace rec601luma and not -colorspace rec709luma
Here are the tests:
convert zelda3.jpg -colorspace gray 2zelda3_cgray.jpg
convert zelda3.jpg -colorspace rec601luma 2zelda3_c601.jpg
convert zelda3.jpg -colorspace rec709luma 2zelda3_c709.jpg
convert zelda3.jpg -grayscale rec601luma 2zelda3_g601.jpg
convert zelda3.jpg -grayscale rec709luma 2zelda3_g709.jpg
convert zelda3.jpg -intensity rec601luma -colorspace gray 2zelda3_i601.jpg
convert zelda3.jpg -intensity rec709luma -colorspace gray 2zelda3_i709.jpg
ccompare -metric rmse 2zelda3_cgray.jpg 2zelda3_c601.jpg null:
1457 (0.0222324)
compare -metric rmse 2zelda3_cgray.jpg 2zelda3_c709.jpg null:
0 (0)
compare -metric rmse 2zelda3_cgray.jpg 2zelda3_g601.jpg null:
1457 (0.0222324)
compare -metric rmse 2zelda3_cgray.jpg 2zelda3_g709.jpg null:
0 (0)
compare -metric rmse 2zelda3_cgray.jpg 2zelda3_i601.jpg null:
1457 (0.0222324)
compare -metric rmse 2zelda3_cgray.jpg 2zelda3_i709.jpg null:
0 (0)
Conclusion: -colorspace gray is using the wrong default mixing ratio.