[v1.104] JPEG decoding with dithering

[XnTriq]

1. Go to Settings → Load format settings… → Read.
2. Set DCT to Floating.
3. Set Dithering to None.
4. Convert a JPEG to PNG.
5. Repeat the conversion with Dithering = Ordered.
6. Repeat the conversion with Dithering = Floyd-Steinberg.

The resulting PNGs are identical

[Xn User]

Which method compression in JPEG file was be used ? This very need for report , I mean what 1x1 1x1 1x1 or 1x2 1x1 1x1 or 2x2 1x1 1x1 )
And test was be do with classic JPEG compression methods or JPEG LI compression methods ?

[XnTriq]

testorig.jpg

Encoder: libjpeg
Chroma subsampling: 4:2:0 (2×2,1×1,1×1)

[xnview]

testorig.jpg
Encoder: libjpeg
Chroma subsampling: 4:2:0 (2×2,1×1,1×1)

if i check the code of libjpeg, it seems that dithering is only used when quantize with colormap

[Xn User]

Right , Dithering must be do use only for 1x2 1x1 1x1 or 2x1 1x1 1x1 or 2x2 1x1 1x1 , for 1x1 1x1 1x1 dithering not need use )
I also asked in JPEGLi https://github.com/libjxl/libjxl/issues/4146 , you can see what XnView MP and ACDSee Pro 5.0 with IDE_ACDstd 5.0.56.0000 reading file with differ , in result XnView MP with Floyd-Steinberg colors a little rude if compare with ACDSee Pro , I see rude gradients

[Xn User]

Please , add all this algorithms https://surma.dev/things/ditherpunk/ and will be learn which is better
In the end will better if this algoritms will be apply optionaly to all lossy or images with less colors
this for testing methods of dithering https://surma.dev/lab/ditherpunk/lab.html

[XnTriq]

Related: Ximagic Quantizer

[user0]

if i check the code of libjpeg, it seems that dithering is only used when quantize with colormap

yep,

• Color quantization [legacy feature]
  is an optional post-decoding step that maps full-color pixel data to a limited palette
  
  • Dithering [legacy feature]
    is an optional component of color quantization that introduces structured noise to mask quantization errors, reduce visible artifacts such as banding and improve perceived color transitions

since these steps are not applicable when rendering 24/32/48-bit images, the following should be removed:
- Dithering combobox in Settings>Formats>Read - JPEG
- -dither fs switch when lib is used internally


Some details

Doc
it is clearly written in DJPEG DETAILS section of usage.txt
"Note that these switches (-dither) have no effect unless color quantization is being done."

• libjpeg
  https://www.ijg.org/files/jpegsr10.zip

  Code: Select all

  	-colors N	Reduce image to at most N colors.  This reduces the
  	or -quantize N	number of colors used in the output image, so that it
  			can be displayed on a colormapped display or stored in
  			a colormapped file format.  For example, if you have
  			an 8-bit display, you'd need to reduce to 256 or fewer
  			colors.  (-colors is the recommended name, -quantize
  			is provided only for backwards compatibility.)
  

  Code: Select all

  	-dither fs	Use Floyd-Steinberg dithering in color quantization.
  	-dither ordered	Use ordered dithering in color quantization.
  	-dither none	Do not use dithering in color quantization.
  			By default, Floyd-Steinberg dithering is applied when
  			quantizing colors; this is slow but usually produces
  			the best results.  Ordered dither is a compromise
  			between speed and quality; no dithering is fast but
  			usually looks awful.  Note that these switches have
  			no effect unless color quantization is being done.
  			Ordered dither is only available in -onepass mode.
  

• libjpeg-turbo
  https://github.com/libjpeg-turbo/libjpe ... /usage.txt

  Code: Select all

          -colors N       Reduce image to at most N colors [legacy feature].
          or -quantize N  This reduces the number of colors used in the output
                          image so that it can be stored in a colormapped file
                          format.  This feature cannot be used when decompressing
                          lossless JPEG images.  (-colors is the recommended
                          name.  -quantize is provided only for backward
                          compatibility.)
  

  Code: Select all

          -dither fs      Use Floyd-Steinberg dithering when quantizing colors
                          [legacy feature].
          -dither ordered Use ordered dithering when quantizing colors [legacy
                          feature].
          -dither none    Do not use dithering when quantizing colors [legacy
                          feature].  By default, Floyd-Steinberg dithering is
                          applied when quantizing colors.  This is slower but
                          usually produces the best results.  Ordered dithering
                          is a compromise between speed and quality.  No
                          dithering is faster but usually looks awful.  Note that
                          these switches have no effect unless color quantization
                          is being done.  Ordered dithering is only available in
                          -onepass mode.
  

Code
dither_mode is ignored if quantize_colors is false

Step	Code File	Action
Parsing	djpeg.c	captures -dither and -colors flags
Master Control	jdmaster.c	sees quantize_colors == TRUE; enables the quantizer
Processing	jquant*.c	checks cinfo->dither_mode to decide which math loop to run (1or2 pass)
Output	wr*.c	writes the dithered, low-color pixels to a BMP/GIF/PPM file

Test

1. get libjpeg-turbo binary
2. decode jpeg
   • default output

     Code: Select all

     djpeg.exe -bmp -outfile test_0.bmp test.jpg

     Note: by default the following applied as well:
     -dct int
     chroma fancy upsampling
   • add dithering

     Code: Select all

     djpeg.exe -bmp -dither fs -outfile test_1_dither_fs.bmp test.jpg

3. compare
   default vs dithering - images are the same

ps: hope this is enough to discourage this ignorant behavior