bug#33226: chromium, firefox sharper than doc-view

[積丹尼 Dan Jacobson]

LI> I'm unable to reproduce this on the trunk -- PDFs (even after zooming)
LI> look fine to me in doc-view.  Are you still seeing this?

Try:
wget -O x.pdf 
https://www.ris.gov.tw/documents/data/2/4/4e050f2b-9778-4a7d-a764-241e43d659f1.pdf
chromium x.pdf # then zoom with the "+" button until 400%. (Yup 400
percent, or higher still.)

gv x.pdf # then zoom to "10.00", the maximum.

emacs x.pdf #and then use "+ runs the command doc-view-enlarge" many times.

We observe a /tmp/docview*/*.pdf-*/ is created and contains a png
conversion of the PDF, and a second factor file: page-1.png,
resolution.el. We note that the resolution.el has not changed. Inside is
"100" and that has not changed despite our zooming.

So that's probably why the file doesn't look as clear in docview vs. the
others! You must admit it is fuzzier at 400% zoom than the others.

(info "(emacs) DocView Navigation") says
   You can enlarge or shrink the document with ‘+’ (‘doc-view-enlarge’)
   and ‘-’ (‘doc-view-shrink’).  These commands work by reconverting the
   document at the new size.  To specify the default size for DocView,
   customize the variable ‘doc-view-resolution’.

doc-view-resolution is a variable defined in ‘doc-view.el’.
Its value is 100

    Documentation:
    Dots per inch resolution used to render the documents.
    Higher values result in larger images.

g runs the command doc-view-revert-buffer
but it doesn't make the enlarged document clearer.

One needs to set doc-view-resolution higher before opening the file.

But wait! *Not* on the INFO page is

(defcustom doc-view-scale-internally t
  "Whether we should try to rescale images ourselves.
If nil, the document is re-rendered every time the scaling factor is modified.
This only has an effect if the image libraries linked with Emacs support
scaling."
  :version "24.4"
  :type 'boolean)

And indeed, setting it to nil makes the bug go away, as proved by
resolution.el finnaly changing each time we hit "+".

So, this critical variable, if t, will cause scaling to silently
fail in half the cases ("if the image libraries linked with Emacs..."
fails. So should be nil by default, so that it never fails, I suppose.

Or, how about instead of failing, simply do the nil action if the t action 
fails!

By the way, when it is nil, one sees
DocView: process pdf/ps->png changed status to killed.
messages each time one hits "+".

Also I think there in /tmp/ it should save a copy of the previous few
scales each time we hit + or - so we can quickly zoom in and back out
etc.

emacs-version "26.1"