/cc @nigeltao

The proposal is the following:

(1) In image/png:
Define Options as

type Options struct {
    PPI int
}

Add
EncodeOptions(w io.Writer, m image.Image, **o Options**) error
which would perform the same encoding as Encode(...), plus encode the PPI into the PNG file
Modify the Decode(...) function so that it also returns the PPI into the image.Config result

(2) In image:
Extend the image.Config struct with a new field:
PPI int

What would be left out of my patch is the corresponding PPI support in the image/jpeg library (my project only uses the image/png library). image/gif requires no patches as GIFs do not support PPI.

Specifically, the type should probably be called EncodeOptions, to allow us to to define a DecodeOptions at some point (e.g. to specify a destination buffer). That would mean renaming the function to something else, possibly EncodeImage.

Generally, though, PPI is just one example of image metadata, such as EXIF fields (e.g. #4341). There's a discussion of image metadata at #5050, but there's no concrete API proposal yet. I'd rather solve the general problem than specifically adding a PPI mechanism.

I agree this is only a subpart of the metadata issue.

How to solve？

hey @dmoklaf did your solution work/can i access it? I really need to generate some images with a higher value than the 72 default. I also wonder why this functionality still has not been added to the images package yet?

Hi, we finally didn't control the PPI, rather controlling instead the resolution of the image (in pixels) and its display width (with HTML/CSS). I may be wrong but it seems this library has only undergone mandatory Go maintenance (to support changes in other Go libs). This is unfortunate as the code is excellent and not much is needed to make it cutting edge.