Confusion about encoding WebP with sampling Factor

[Khawar Islam]

I have converted Kodak 24 images eith sampling factor 420.

convert test/images/kodim$i.png -quality $(($j*5)) -sampling-factor 4:2:0 test/jpeg/kodim$i/`printf "%02d" $j`.jpg

I need same procedure for WebP

convert test/images/kodim$i.png -quality $(($j*5)) -define webp:lossless=true test/webP/kodim$i/`printf "%02d" $j`.webp

Both convert lines are correct or not. I want to compare the results

[Jyrki Alakuijala]

WebP lossless stores data in RGB, which is similar (slightly better) than YUV444 and a lot better than YUV420.

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[Pascal Massimino]

On Tue, Oct 13, 2020 at 7:47 PM Khawar Islam <khaw...@gmail.com> wrote:

I have converted Kodak 24 images eith sampling factor 420.

convert test/images/kodim$i.png -quality $(($j*5)) -sampling-factor 4:2:0 test/jpeg/kodim$i/`printf "%02d" $j`.jpg

I need same procedure for WebP

convert test/images/kodim$i.png -quality $(($j*5)) -define webp:lossless=true test/webP/kodim$i/`printf "%02d" $j`.webp

If you want to compare *lossless* WebP, it should be against their PNG equivalent. They use the same colorspace (ARGB),

and same compression technique (lossless coding based on lz77 algorithm).

If you want to compare *lossy* WebP to JPEG, then you have no choice but using YUV420, which is the only available

colorspace available.

I'd advise using 'cwebp' instead of 'convert', to get better control over fine encoding options.

hope it helps,

skal/

Both convert lines are correct or not. I want to compare the results

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[Khawar Islam]

JPEG is lossy image compression and i want same thing for WebP. I am using imageMagick tool for conversion

Like for JPEG, below code generated  20 images with different quality factor of each kodak image for comparison

for i in {01..24..1}; do
echo JPEG Encoding test/images/kodim$i.png
mkdir -p test/jpeg/kodim$i
for j in {1..20..1}; do

convert test/images/kodim$i.png -quality $(($j*5)) -sampling-factor 4:2:0 test/jpeg/kodim$i/`printf "%02d" $j`.jpg

done

done

For WebP conversion

for i in {01..24..1}; do
echo WebP Encoding test/images/kodim$i.png
mkdir -p test/webP/kodim$i
for j in {0..20..1}; do
convert test/images/kodim$i.png -quality $(($j*5)) test/webP/kodim$i/`printf "%02d" $j`.webp
done
done

Am i correct?. the above code did not generates webP images with different quality factors. All images are same. Please help me

[James Zern]

Hi,

On Wed, Oct 14, 2020 at 1:48 AM Khawar Islam <khaw...@gmail.com> wrote:

JPEG is lossy image compression and i want same thing for WebP. I am using imageMagick tool for conversion

Like for JPEG, below code generated  20 images with different quality factor of each kodak image for comparison

for i in {01..24..1}; do
echo JPEG Encoding test/images/kodim$i.png
mkdir -p test/jpeg/kodim$i
for j in {1..20..1}; do

convert test/images/kodim$i.png -quality $(($j*5)) -sampling-factor 4:2:0 test/jpeg/kodim$i/`printf "%02d" $j`.jpg
done

done

For WebP conversion

for i in {01..24..1}; do
echo WebP Encoding test/images/kodim$i.png
mkdir -p test/webP/kodim$i
for j in {0..20..1}; do
convert test/images/kodim$i.png -quality $(($j*5)) test/webP/kodim$i/`printf "%02d" $j`.webp
done
done

Am i correct?. the above code did not generates webP images with different quality factors. All images are same. Please help me

This looks to be working correctly for me using:

$ convert -version

Version: ImageMagick 6.9.11-24 Q16 x86_64 20200718 https://imagemagick.org

As Pascal mentioned, you can also try cwebp to compare.

 

On Wednesday, 14 October 2020 at 17:00:50 UTC+9 pascal.m...@gmail.com wrote:

On Tue, Oct 13, 2020 at 7:47 PM Khawar Islam <khaw...@gmail.com> wrote:

I have converted Kodak 24 images eith sampling factor 420.

convert test/images/kodim$i.png -quality $(($j*5)) -sampling-factor 4:2:0 test/jpeg/kodim$i/`printf "%02d" $j`.jpg

I need same procedure for WebP

convert test/images/kodim$i.png -quality $(($j*5)) -define webp:lossless=true test/webP/kodim$i/`printf "%02d" $j`.webp

If you want to compare *lossless* WebP, it should be against their PNG equivalent. They use the same colorspace (ARGB),

and same compression technique (lossless coding based on lz77 algorithm).

If you want to compare *lossy* WebP to JPEG, then you have no choice but using YUV420, which is the only available

colorspace available.

I'd advise using 'cwebp' instead of 'convert', to get better control over fine encoding options.

hope it helps,

skal/

Both convert lines are correct or not. I want to compare the results

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[Khawar Islam]

Thank you Sir Pascal. Below command works fine

cwebp test/images/kodim$i.png -q $(($j*5)) -o test/webP/kodim$i/`printf "%02d" $j`.webp

What is the default chroma sampling factor of above command?

[Jyrki Alakuijala]

Your command creates a WebP lossy image at different qualities.

WebP lossy is always with YUV420 sampling, equivalent to imagemagick's -sampling-factor 2x2 (also -sampling-factor 4:2:0) for jpeg.

To view this discussion on the web visit https://groups.google.com/a/webmproject.org/d/msgid/webp-discuss/390f2eb2-f6e5-41ce-9ed2-752afc507d85n%40webmproject.org.

[Khawar Islam]

Thank you. Its mean that i am right.