Coverting Color introduces JPEG compression artefacts.

[RefikTelhan]

Hi,

Color conversions made via Convert Color tool introduces "additional" JPEG compression artefacts. It seems that Convert Color re-compresses JPEG files at a low quality setting. If the original image contains some amount of JPEG artefacts due to prior low quality compression, the result gets worse.

Can we set this compression to "maximum quality" to reduce artefacts to a minimum?

Thanks and regards,

- Refik Telhan

[gary_sc]

That would also happen in PS as well: any time you save a JPG as a JPG, you'll increase the JPG degradation. If the original image was compressed to max, this will only make it worse.

I think the only way around this would be to save the image outside of Acrobat, open it in PS, convert it to 16-bit, save it as a TIF or PSD image, and convert the colors as you so wish, then save it as a JPG with limited (if any) compression. That should make it no worse than the original.

Alternatively, convert the image into a PNG format (which is what it should have been in the first place), and then place it into the original document.

Sorry, there are reasons for JPG images, but this is not one of them.

[RefikTelhan]

Hi Gary,

I know that any quality setting below the maximum setting causes progressive degredation and introduce additional artefacts. This is why only maximum quality compression is used in the prepress world.

The conversion in this case is a CMYK-to-CMYK conversion, US Web Coated (SWOP) v2 to ISO Coated v2 in relative colorimetric rendering intent with black point compensation. When I pull the image to Photoshop and do the same conversion there, the original visual appearance does not change.

Printers usually receive material as PDFs. They do have very limited control on how these PDFs are made. On many occasions, the images are as either untagged CMYK or tagged with US Web Coated (SWOP) v2 colorspace. This requires a conversion from US Web Coated (SWOP) v2 (the default CMYK colorspace of Photoshop) to the colorspace the printer uses for the specified paper.

The problem here is the JPEG compression setting used internally by Acrobat during the color conversion process. It should be set maximum. Apparently this is now set to some lower quality. We should be able to control the quality settings and set it to maximum.

Printers not have control on which image format is used in PDF file. But they should be able to maintain the quality level of the original image. I can see that the images in this particular case have been JPEG compressed not at the highest quality. Hence, the conversion process visibly degrades the appearance of the image by introducing additional JPEG artefacts.

As printers receive PDFs with several hundred pages, converting each image individually in Photoshop and saving back to Acdobat is not an option. Acrobat should be doing the conversion without changing the original visual appearance.

Best regards,

- Refik