unexplained difference Photoshop-Gimp PNG filesize

[gympie]

I worked with Photoshop for several years and use the "Save for web" function daily.
I start to become somewhat familair with Gimp but I have one big issue.

As far as I know, there is no (standard) function "save for web" in Gimp.
So, I do it step by step.

As an example I exported a image from Lightroom in PSD format and open it in Gimp.
It is a image without transparency.
I use Gimp 2.10

First I resize / scale the image to acceptable size like 800 px wide, like I do in Photoshop, with cubic interpolation (I use bicubic in PS)
Then I use the export function, change the extension to .png (every checkbox is empty, like interlacing, save background color).
I have selected 8bpc RGB, compression level 9 and hit Export.
The .png file that is created is almost 3 times bigger then when I use the "save for web" function in PS.

- Flatten image is not the solution, there is no transparency.
- I assume I use already maximum compression for png.
- The number of pixels width-heigt are exactly the same in PS as in Gimp.

What steps do I forget to have a file size comparable to PS?

[rich2005]

You have to make sure you are comparing like-with-like. First try and I got a scaled down Gimp png slightly smaller file size than the corresponding PS png.

To get something a third of the file size are you using PS settings like this? screenshot: https://i.imgur.com/4lfNdKz.jpg

Check in Gimp the file properties. Are they exactly the same in both versions? Using PS PNG8 the image is indexed with 256 colours rather 64000-ish.
Use Image -> Image Properties screenshot: https://i.imgur.com/Y6UR1I9.jpg

edit:

Quote:...I have selected 8bpc RGB, compression level 9 and hit Export

8 bits-per-channel = PNG24 not PNG8

There is a web export plugin for Gimp, very old but still works more-or-less. see: https://www.gimp-forum.net/Thread-advice...8#pid13718 download the zip. Unpack the zip. Put the webexport64.exe in your Gimp 2.10 user profile plug-ins folder
C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\plug-ins

Find in the File menu -> Export for web . Looks like this: https://i.imgur.com/f9DIN0s.jpg

edit: If you need to process step-by-step
Scale the image. Do any other processing
Convert to indexed Image -> Mode -> Indexed
export the PNG (automatic pixel format)

[Ofnuts]

As an aside, if you want lossless images, you can now use the WebP format (built-in in 2.10). Supported by all major browsers these days.

[gympie]

"differences are now explained"   

@rich2005: boy, did I made a big mistake by thinking 8bpc (not knowing what bpc was, now I understand) is equal to 8bit png. That change (automatic mode) was the solution. Thank you very, very much to take the time to help me. I'm so happy...
I was aware of the plug-in but hesitated to use it because it is so old, and I have no problems doing it by hand.

@Ofnuts: It's a long way back that I read about webp but than there were no broad support, I sure will take a look, actually I already start experimenting after I read your post.


For the ones that searched for a solution for photoshop - gimp "save for web" 8bit png:

- scale down image: image, scale image
- convert it to image mode: image, mode, indexed (it that's not allowed, first change from 16 bits to 8 bits by "image, precision, 8 bits")
- export to png: file, export as, (change extension to png), export, automatic pixelformat, compression level 9 (that doesn't hurt the image quality)