Nconvert - alpha channel transparency

Discussions on NConvert - the command line tool for image conversion and manipulation

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Nconvert - alpha channel transparency

Post by rladjadj »


I use NConvert command line tools to generate renditions in a DAM.

After image conversion from png to jpeg format, i waw some weird horizontal lines in the background of the result image.
According to the photographer of the original image, the issue comes from the alpha channel (alpha layer) : the transparency of the alpha channel is not conserved, resulting in the display of these strange lines.

While checking at the forum threads, i found a solution : i added the -merge_alpha parameter to my nconvert calls.
This works well : the results are ok now, for most of my generated renditions !
But, i still get these strange lines if i use the -grey parameter, which i need to generate black and white renditions.

Here are the nconvert command lines i use

- for a colored rendition (OK) :
nconvert -icc -q 100 -icc_in "ISO39 U320 K90.icm" -out jpeg -merge_alpha -resize 10% 10% -o %.Low_Resolution.jpg ${filename}

- for a black and white rendition : (i tested these 3 commands, but it's still KO)
nconvert -grey 256 -out jpeg -merge_alpha -resize 100% 100% -o %.Black_And_White.jpg ${filename}
nconvert -grey 256 -icc_in "ISO39 U320 K90.icm" -out jpeg -merge_alpha -resize 100% 100% -o %.Black_And_White.jpg ${filename}
nconvert -grey 256 -icc -q 100 -icc_in "ISO39 U320 K90.icm" -out jpeg -merge_alpha -resize 100% 100% -o %.Black_And_White.jpg ${filename}

Did anyone here has this issue before ?
Any solution to solve it for the black and white command ?

Thanks !
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Re: Nconvert - alpha channel transparency

Post by XnTriq »

Welcome to the forum!

Have you tried a different order of parameters (i.e. -merge_alpha before -grey 256)?
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Re: Nconvert - alpha channel transparency

Post by xnview »

please use -merge_alpha before -grey