I have 900 large, proprietary, multi-coloured PNG images of different sizes scanned @ 600 PPI ranging from 10MB to 50MB. The scans (ie photos) are of prints. The average size is 35MB. They all have a lot of white space (between 20 and 80%). I intend to reduce/convert these to WebP using Google's cwebp so they are close to 100KB when web-loaded.

For best results should I:

  1. Just convert PNG 600PPI to WebP (using the codes) or

  2. Reduce PNG 600PPI to, say, 300PPI THEN convert to WebP using the codes @ 75% OR

  3. Because of better (but lossy) JPG colour, should I first convert PNG600 to JPG (if so, to what size?) THEN convert JPG to WebP @ 75%? Is this idea of double conversion stupid?

(NB I will keep transparent images as PNG)! The web is full of "how to convert" PNG to WebP and how to batch convert. But none (that I have found) deals with the technical process of actual conversion/reduction in detail. The quality of final images is important because it is what I sell.

Advice from someone with greater knowledge of this process would be most helpful.

  • 1
    \$\begingroup\$ PPI when scanning determines resolution. PPI is meaningless in an intermediate file (assuming you don't also resize the number of pixels). It's practically meaningless in an image intended for web display. Please see: Is there any practical difference between saving at 300ppi or 72ppi? \$\endgroup\$
    – Michael C
    Sep 5 at 9:15
  • 1
    \$\begingroup\$ Scaling down (in pixels, not in PPI) will normally decrease the file size. Most conversion utilities will rescale as well as convert format in one step (better for final quality). WebP supports transparency so there is no need to keep images as PNG. WebP is well supported by browsers but not by all server software, which in that case won't identify the WebP files as images, which makes these difficult to insert into HTML pages.So first make sure that the server on which you put your images has a decent WebP support. \$\endgroup\$
    – xenoid
    Sep 5 at 10:35
  • \$\begingroup\$ Many thanks both of you (Michael C and xenoid). I am now studying what to do based on your replies. Thank you. \$\endgroup\$ Sep 5 at 15:14
  • 1
    \$\begingroup\$ The "white space" you mention - are you hoping to have that cropped off automatically? As has already been mentioned, you should forget about the ppi setting. Images have a width and a height - measured in pixels - that's what you care about. If you want to display the images online, then what dimensions do you need? That's for you to determine. Another consideration is whether or not you want to retain the originals (for example, in case larger images are needed at some point in the future). \$\endgroup\$
    – osullic
    Sep 5 at 18:50
  • \$\begingroup\$ Probably this should be moved to Graphic Design. \$\endgroup\$
    – Rafael
    Sep 5 at 21:18

1 Answer 1


Forget door number 3. There is no sense in converting to JPG and then to Webp.

But most importantly: You can not reduce from 50Mb to 100k and expect that image to be useful for print.

The logic is flawed in several places.

1. You need a small image for a website (100Kb), but when you actually want the file to use it, you download another (50Mb).

2. Let us assume you want prints of the same size as the original. Then you do not need 600PPI, so resampling the files is a good option. Note that the word I am using is resampling. This will reduce the file size by 1/4 and re-assign the metadata to 300PPI. Do not forget to save your originals.

3. If you want to enlarge them, then stop thinking about the PPI and define the pixel size.

4. If you want quality prints, you should not use any compression on WebP besides 100% quality. File size should be a secondary factor.

5. In reality, you should not consider WebP as a delivery file. WebP is a file format well used on webpages, but some applications meant to print will not recognize the file format. TIF is the most accepted, and generate a larger file. JPG with maximum quality is acceptable.

  • \$\begingroup\$ If you halve the linear dimensions in both directions, you reduce the file size by 3/4, to 1/4 of the original size. \$\endgroup\$
    – Michael C
    Sep 6 at 2:56
  • \$\begingroup\$ If 80% of that 50MB is white border, then you're actually only talking about 10MB of usable information. \$\endgroup\$
    – Michael C
    Sep 6 at 2:58
  • \$\begingroup\$ I am grateful for all the answers and have learned a lot! But I should say I don't want anyone to be able to print anything. For copyright reasons, I do not want people downloading perfect images, copying them and selling them on! I want people to SEE good images and want to buy them. I explain on my site the images they see are scans of original prints and are not perfect whilst the prints they may buy are! So a really good image that cannot print well is exactly what I am looking for! \$\endgroup\$ Sep 6 at 7:49
  • \$\begingroup\$ Then just make an image of a good size for a website, let's say 1000px height is good enough. \$\endgroup\$
    – Rafael
    Sep 6 at 8:07
  • \$\begingroup\$ Many thanks Rafael. osullic - No , it is not white border, it is background (of the item eg a battleship, a rose, a soldier in uniform etc. The item in each case sits alone in the centre of the image surrounded (and backed - through the rigging, the leaves etc) by white (or gray). I was wondering whether the fact of images having just the central image surrounded (and through-backed) by a single colour might affect the instruction given when converting to WebP? \$\endgroup\$ Sep 6 at 10:16

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