Legacy ImageMagick Discussions Archive

Ahh...i forgot about automatic chroma subsampling in IM.Thanks for the reminder and the help.


Happy new Year

The JPEG compression algorithm split the image into 8x8 blocks

http://fourier.eng.hmc.edu/e101/lectures/JPEG_diagram_1.gif

My thinking is that if we split an image into 8x8 blocks, feed each block to libjpeg and then merge the JPEG blocks into a lossless format(say PNG) , we should theoretically ...

P.S. If you, say, use -quality 80 to make the jpg images, then if you add -quality 100, you will get as close visual quality as you had before, but the file size will increase dramatically. So you probably want to make the new combined result have a similar or slightly larger quality than 80, but ...

@fmw42
Thanks for the great script snippets

I have a two part question

-For cropping and saving offsets parts
1.I could just count the number of images produced to know the value for the loop . Is there a cool one liner way to do it just after the convert
-For merge process
2.I need minimum ...

@snibgo
GNU parallel helps to make use of all cores.

Here is the situation.
Guetzli is very slow JPEG decoder

I am trying to split a PNG into small tiles and then convert them to JPEG with Guetzli.Then i want to join the JPEG tiles together to get the full JPEG compressed image.Small images are ok ...

JPG is what i am exactly trying to do.The filenames could be used to store the offset?

convert bench.png -crop 64x64 +adjoin bench.png_guetzli_%02d.png
parallel --will-cite 'guetzli --nomemlimit --quality 100 bench.png_guetzli_$(printf %02d {1}).png bench.png_guetzli_$(printf %02d {1}).jpg ...

Thanks a lot for your fast reply guys.
@snibgo
If i modify the image in some way , like a compression for example , then i will lose the offset.I want to be able to rejoin the tiles together later on irrespective of the manipulation i make with the image.

Thanks again for the help guys

I want to split any image into 64x64 tiles and then i need to join the tiles together to form the original image again.
The following from the docs work but how can i get the correct values for -tile parameter automatically.I intend to do this in a bash script later.

convert rose: -crop 20x20 ...

I don't get all the compare metric as stated in the documentation

https://www.imagemagick.org/script/command-line-options.php#metric

compare -list metric

AE
Fuzz
MAE
MEPP
MSE
NCC
PAE
PSNR
RMSE

Version: ImageMagick 6.7.8-9 2016-06-16 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999 ...

So far snibgo looks better and Fred's command and my attempts above make the image too sharp when compared to the original.

I have never used a custom script with IM thus far so forgive my ignorance.I run IM on a Unix environment and not windows.Do you have a unix version of the script.If possible could you upload the downsampled image from your script so that I can also visually compare.

Thanks a ton for the help

I am trying to get similar output in terms of quality that i get in photoshop with imagemagick while resizing an image.

Original : https://upload.wikimedia.org/wikipedia/commons/2/27/Mural_in_Northeast_Pavillion%2C_Thomas_Jefferson_Building_by_Elmer_E._Garnsey_11670u_edit.jpg
Photoshop settings ...