Black&White image - color component issue

Hi D_Fosse, Hi JG

Thanks for your help. Color management is an important tip, although colorspace may be more important than color profile, which is as i understand primarily a part of an output medium's calibration.

My conclusion: Originally, the image came as a colored sRGB out of the camera. While being converted to monochrome content(!) in postproduction, it remained in sRGB colorspace during postprocessing. When using ImageMagick for a batched final postprocessing step, it automatically converted the monochrome image  from sRGB colorspace to gray colorspace. ImageMagick does this automatic conversion when it detects that the  content(!) is monochrome. Then, Adobe Stock converted back to sRGB colorspace  when creating the thumbnail. It would seem that this conversion and back conversion is the root of the problem.

Note that ImageMagick offers parameters to deactivate the automatic grayscale conversion or to force the colorspace of the output file. Unfortunately, i was unaware of this issue when submitting the image.

Anyhow, i think we can settle this issue. I totally agree, don't use greyscale, use monochrome RGB instead. Be careful about the auto-grayscale option or the colorspace or type setting when using monochrome images in ImageMagick.

Once again, the conversion to greyscale colorspace was unintentional. ImageMagick did this behind my back. The only problem is that Adobe Stock won't accept the corrected image (in sRGB colorspace) while they accepted the image with the wrong grayscale colorspace. Is there any way to inform the reviewers about this issue so they will allow me to replace the image?

Thanks and regards

fb1701

Hi JG, Hi D_Fosse

Thanks for your input. After some research, experiments and thinking, here are some results:

* I agree that images may be displayed diffferently, depending on hardware (LCDs, OLEDs, Prints) and software (color profiles). But in this issue, it is not only about how a image is displayed. It is that the image is modified. You can see this in the following example. Here, i used ImageMagick to generate a gradient (greyscale from black to white). Then, i uploaded this image to Adobe Stock, downloaded the resulting thumbnail and appended the thumbnail to the original image:

You can see that the thumbnail image created by Adobe Stock (bottom) is brighter than the original image (top): The black end has become smaller, the white end has become bigger. The mid tones have become brighter. This can also be seen in the histograms:

Histogram of original image:

You can see that the tones are distributed equally. The spikes may be caused by quantization (rounding issues) and are thus irrelevant.

Histogram of the thumbnail generated by Adobe Stock:

You can see that the histogram is shifted to the right. The nonlinearity of the ramp-like shape may indicate a gamma value issue.

* I can confirm that this is a file format issue. This only happens for a 1 color components file. For a 3 color component RGB image (regardless if it has greyscale content i.e. all pixels have R=G=B values) there is no change of brightness.

* I used ImageMagick for a batch automated final postprocessing step. Unfortunately, i was unaware of the fact that ImageMagick will write a 1 color component file if it detcts that only grayscale tones are present in the image. This can be averted with the -type truecolor command line switch which will generate a 3 color components RGB file. Of course, this won't affect the content the of image. If it is grayscale it will remain grayscale, just in an other file format.

* As i see it, the tonal change in Adobe Stock has a signifcant impact. While the original file has a pencil drawing appearance, the modification by Adobe Stock makes the look faded or bleached out.

* I have already created a corrected version of the image. Using a copy of the image before the last postprocessing step and repeating the final postprocessing step with applying the -type truecolor command line switch, a file was created with the correct 3 color components RGB image file format - with the same content (i.e. it is still greyscale as intended). As there were no additional postprocessing steps,  jpg compression noise should be identical to the original file.

So regarding its contents, the corrected file should be a 1:1 identical replacement for the original file, just in another, more compatible file format. But, Adobe Stock won't accept it due to "technical issues".

So, this image was accepted:

And this image was rejected:

Regarding their content, they are exactly the same. The only difference is the file format.They were generated n the same (slightly modified by the -type truecolor 3 color channel file fomat switch) finalizing step. So, what makes AdobeStock accept the first one but reject the second one? Or do they prefer the artificially brighter version of the image?

Looking forward to your replies. Thank you.

Regards

fb1701

Hi JG

Thanks for the response.

Technically, it is a normal photo which came out of a normal camera. As the image was taken in a Contre-jour light situation (shooting against the sun), i decided to convert to B&W because of color saturation issues. It seems that for some reason, the photo processing software set the "Color Components" flag to 1 in the jpg file when it realized that there wasn't any color data. Normally the flag is 3 for colored  RGB data, obviously.

After doing some research about jpg data formats and metadata flags and doing some experiments, i'm quite sure that the image data (i.e. the actual pixel values) are fine. I think it is only the "Color Components" metadata flag which irritates the photo processing software of the Adobe Stock website while any other software seems to properly handle the "Color Components" flag. Or most of them just ignore it?

Some further explanation, as far i can understand this issue:

1. Data content of the image. If an image is converted to Black&White (or the image directly comes this way out of the camera if set to create B&W images), the RGB components are the same for all color channels. i.e. conly values between (0,0,0) and (255,255,255) will exist in the image. But for the software, it should be a normal image. Black and white and greyscale pixels also exist in normal, colored images, after all. So the only special thing about a B&W image is that only color tones between Black and White will occur. So, technically, a &W picture should be displayed just like a normal picture, as the only difference is that a B&W image lacks pixels where the RGB color components are not equal.

2. Data encoding. It seems that my processing software tried to be super smart to mark the image as a B&W image when it realized  the RGB values were identical for all pixels. This is done with the "Color Components" metadata flag. It is 3 for a colored image, it may be 1 for a B&W image.

So, the content of the image should be just fine. It is the only "Color Components" flag which seems to irritate the image processing functions of the Adobe Stock website. Note that i was able to change the "Color Components" flag from 1 to 3 (by loading and re-saving with an application which handles the flag correctly). However, Adobe Stock may possibly reject the image - although it is the same image! - when i re-upload it because you can never know for sure if an image be accepted or rejected, so i'm hesitant to try this (again).

Thank you and best regards

fb1701