Legacy ImageMagick Discussions Archive

What I want is exactly what's produced today by applying -grayscale rec709luminance to an srgb input image (no further formula clarification required), but identified as such as CieLuminance or something similar. I am not a colorspace expert, but LAB L is not the same as Y from YCbCr which is the s...

(I still think we should have a CieLuminance or CieY colorspace for linear grays.) Are these from CieLab or YCbYCr, namely linear versions of the first channel of CIELab or YCbCr? Please clarify. It is the Y in CIE 1931 XYZ, which is the same Y that L* in CIELab is defined nonlinearly in terms of, ...

I suppose IM could have -colorspace rec709luminance, but that is the same as -grayscale rec709luminance. At least that was the way it was supposed to be apart from a recent bug that changed its behavior and which in principle should be fixed in 6.8.7.1 when it is released. I'm not just looking for ...

Ah, I see what you are are saying. That changelog entry is old. I think IM shouldn't treat gray differently to colours. When it does (which it did from approx 6.7.9 to 6.8.5), we get into difficulties when a colour image becomes gray or nearly gray, or we use a gray image to modify a colour image, ...

I still think it might be cleaner to have a CieLuminance (linear grayscale) colorspace so that colorspace conversion for grayscale doesn't require a completely different internal identification mechanism (image->gamma) compared to all other colorspaces (where gamma has no effect). Somebody said we s...

I believe it is treating the grayscale colorspace as being nonlinear similar to sRGB (regardless of the gamma value), so -colorspace RGB does an sRGB-to-RGB gamma expansion, thus the data has been "doubly-expanded". This should be fixed. A "Gray" colorspace image with gama=1 shou...

I am not sure, but I believe that it should not matter if you do convert rose_luminance.png -colorspace RGB ....some processing... result.png or convert rose_luminance.png -set colorspace RGB ....some processing... result.png or just leave it out ...<snip/>... I would think that IM would know it is...

But my images were not properly linear since the IM gamma=0.4545 But what I'm asking is, once the fix is (back) in place, and I read into IM a 1-channel linear image properly identified as linear by gamma=1, can I explicitly expand it to a 3-channel linear RGB image via -colorspace so that I can do...

Yes, the PNG reader now sets image->gamma=1.0 when it reads a gAMA chunk with gamma==1.0. Okay, thanks! Now I'd like to read this image back into IM, which now knows it's linear, and make it a 3-channel linear image (all channels initially equal of course), so that I can do some further linear proc...

imbh convert rose: -colorspace sRGB -grayscale rec709luminance rose_luminance.png Also, when my 6.8.7-0(hdri) identify -verbose reads in your png from above, it seems to recognize gAMA=1 but nonetheless shows the image->gamma as 0.454545: png:gAMA: gamma=1 Gamma: 0.454545 Is this a part of the bug ...

Another interesting thing is how awful the rec709luma one looks (which is the same as the default -colorspace gray i believe!)

No, it should not. -colorspace gray is the same as -colorspace rec709luma and -grayscale rec709luma

Sorry, what should not what?

It does look like it produces a gamma=1 png, but it already did, before the fix, right? What I'm more interested in is this: imbh convert rose: -colorspace sRGB -grayscale rec709luminance -colorspace sRGB rose_luminance.sRGB.png (edited typo above) I want to see if that gives us a correct sRGB-gamma...

The -set colorspace RGB should no longer be needed once 6.8.7.1 is released... The gamma issue has been fixed. I don't see that setting the gamma property in itself solves the problem within IM processing. All I want to do is read in an sRGB photo, perform various operations on its linear luminance...

The -grayscale operator does not convert the input to a particular colorspace before performing its operation. See the last couple of posts in this topic. For example the -grayscale Rec709Luminance only in fact produces a Luminance (i.e. linear) if the initial colorspace of its input is sRGB (or sim...

I misspoke -- what I should have said is that the operation performed by -grayscale is independent of the initial colorspace, so that the result does depend on the initial colorspace as you point out. To see why this is confusing, just look at your example: %I%convert h8.png -grayscale Rec601Luma g1...