EWWW does not modify the DPI, as it is irrelevant for display on a screen. An image has a set number of pixels (sometimes called the resolution), regardless of the DPI, and changing the DPI has zero impact on how it displays on screen. However, EWWW can change the resolution. For example, if you upload images straight from your camera which are 4800×3600 and you know that they really only need to be 1800×1800 for your webpage, you can configure that on the Advanced tab.
But again, DPI means nothing on the web, so EWWW does not change it. In fact, if I’m not mistaken (which I could be), I think DPI is stored in the image metadata, which should be stripped most of the time anyway.
You can find more information about DPI/PPI here: https://99designs.com/blog/tips/ppi-vs-dpi-whats-the-difference/
Thanks for reply. I understand that DPI is irrelevant for display on a screen.
I also know that a 1800*1800 pic with 300dpi is few times larger file size than a 1800*1800 pic with 72dpi.
It’d be great if EWWW could detect images with large (and irrelevant DPI) and modify them to 72dpi to save space.
I think you still misunderstand what DPI is. If an image is 1800*1800, the dpi has no relation to the resulting file size. Only the full resolution itself has any bearing on the file size.
Hi @nosilver4u, thanks for your patience.
Here is an example: https://s28.postimg.org/5ht70djyl/dpi_compare.jpg
1. The original image (003.jpg) is 4032*3024 pixels with 314 DPI. The file size is 4.86MB.
2. I modify it to 72 DPI and save a new file. The new file size is only 1.46MB, with the same 4032*3024 pixels resolution.
3. By changing DPI from 314 to 72, the file size reduces from 4.86MB to 1.46MB.
Isn’t this show that the low DPI image has a smaller file size?
Thank you.
What are you using to modify that image? Sounds like it is decreasing the quality setting when you lower the dpi.
If you can post links to both images, it would be interesting to compare them and see what that software actually changed.
I use Paint.net
Sure.
I uploaded them at https://postimg.org/gallery/2br6rog0o/ (click the thumbnail then “Download original image”)
As I suspected, it is altering the quality (and/or recompressing) that is causing the decrease in file size. If it was merely DPI, the images should be identical pixel-for-pixel. But any time you re-save a JPG image, it reduces the quality (even if you used the same quality level every time). I ran a comparison with the DSSIM algorithm and 0.19% of the pixels were altered in the recompression. So it isn’t really the same image, Paint.net is just doing a nice job of compressing the image.
Just out of curiosity, I ran it through the TinyJPG algorithm we use on the API, and with only 0.5% pixels changed, it compressed it even farther, to 675kb.
Now I see… I can ignore the DPI when I edit an image for web, correct?
Right on! Now we’re on the same page.
