Legacy ImageMagick Discussions Archive

I have an sRGB-encoded JPEG image.
I would like to convert it to "RGB" format, i.e., pure image data in R-G-B triplets with no headers, etc.
In addition, I would like to convert it in two ways, one so that the R-G-B output is sRGB-encoded, and
one so that the R-G-B output is linear.

To convert to sRGB-encoded R-G-B, I did And to convert to linear R-G-B, I did but that produced output identical to the first case.

When I do then the output is different, but I'm unclear why or whether it did what I intended.

Looking at viewtopic.php?t=17207, it says

-colorspace : change the way actual pixel are represented
-set colorspace : tell image the data is represented this way, but does not touch the data

which matches my reading of the docs, but since .rgb format has no header I don't understand what
-set colorspace is "setting" in my case, or why it makes a difference.

Can someone please set me on the right path.
MV

First things first: What version IM are you using?

Sorry, meant to include in the question MV

There is no header with .rgb (it is just raw data). So if you want to change the data from sRGB (gamma 0.4545) to linear RGB (gamma 0.1), I think you can try doing that by using -gamma to do the conversion. I am not 100% sure about that. I would have thought -colorspace RGB might do the same. So try assuming your input jpg is sRGB.

convert input.jpg -gamma 1 output.rgb

If that fails then perhaps raw rgb is always linear?

Raw RGB is always assumed to be sRGB, I think.

Using "-gamma" or "-evaluate pow" will approximately change between RGB and sRGB. "-colorspace" is more accurate.


"-colorspace XXX" if image isn't already in colorspace XXX (according to the metadata), this changes pixel values and metadata.

"-set colorspace XXX" if image isn't already in colorspace XXX (according to the metadata), this changes metadata only.

A third fact you need to know: not all filetypes support all colorspaces. When the image is written to a file that doesn't support the colorspace, it is automatically converted a colorspace that is supported, as if "-colorspace XXX" was used.

I think the .RGB format supports only the sRGB colorspace.

If your image is in RGB colorspace and you want to write those values, unchanged, to a .RGB file, first use "-set colorspace sRGB". Then IM won't automatically convert it.