don't conflate SetImageColorspace and TransformImageColorspace

[smparkes]

rmagick is conflating SetImageColorspace and TransformImageColorspace. This is the difference between "convert foo.png -set colorspace rgb ..." and "convert foo.png -colorspace rgb ..."

The former is like a reinterpret-cast in C++: it doesn't change the data, it just says the data is in the given colorspace. The latter is a transform that changes the data based on the original image colorspace and the parameter.

I need SetImageColorspace and, in the current code, it's not available if TransformImageColorspace is available.

This is a partial patch. I didn't wire in or test image.transform_colorspace(colorspace). If you're interested, I can ...

[bschwartz]

👍 to this. We've been bitten by this problem several times. Without this patch, if you do something like this:

image.colorspace = Magick::RGBColorspace

... it will trigger a colorspace transformation and mess up the colors of the image. Like the author of this patch, I'd expect that call to inform imagemagick that the image is RGB rather than transforming it to RGB.

This comment is related to this problem as well: rmagick/rmagick@b87199c

[freerobby]

👍 Please pull this in! Thank you @smparkes for the fix.

[jamesfzhang]

👍