Your comments to [Bug 408222]

[David Hurka]

Hi,

I knew Bresenham only from CNC controllers, and spotted it in QPainter 
documentation.

Could you maybe explain how it can be used for minification filtering, and how 
it would be different to nearest-neighbour? (Focusing on Cairo.) Terms 
Bresenham and Filtering don’t yield results in DuckDuckGo...

On Friday, 7 June 2019 17:47:36 GMT Tobias Deiminger wrote:
> Hi David,
> 
> Am 07.06.2019 15:37 schrieb David Hurka:
> > Hi Tobias,
> > 
> > On Friday, 7 June 2019 08:15:06 GMT Tobias Deiminger wrote:
> >> Am 05.06.2019 09:36 schrieb David Hurka:
> >> > Evince and Okular have to request pixmaps at some time, and have to
> >> > [...]
> >> > offset, hitting or missing pixels differently, so the input for
> >> > [...]
> >> 
> >> Do you mean it like that (arbitrary guessed examples):
> >> page.renderToImage( dpiX, dpiY, 0.1000, 0.1000, width, height, ...);
> >> //
> >> looks better
> >> page.renderToImage( dpiX, dpiY, 0.1001, 0.1001, width, height, ...);
> >> //
> >> looks worse
> > 
> > Yes, that’s what I mean.
> 
> Ok, but then changing these offsets happens all the time,
> during one single Okular session. Okular always requests
> a whole page or a crop region from it, determined by where
> the viewport is and what's the current zoom level.
> It never tries to extract a single object like an image or an
> annotation.
> So if a) is right, image quality would change frequently while
> you scroll/zoom around.
> 
> >> At a first glance I don't see subpixel stuff when looking
> >> at how Splash::drawImage fills Splash::bitmap, but I'm
> >> far from understanding it completely.
> > 
> > Hmm, what are you thinking about? RGB/VRGB/... subpixel rendering?
> > That’s not
> > what I mean.
> 
> No, just tried to guess your guess, maybe something
> about denormalization :-) But I see that's not the case (is it?).
> 
> > What I mean here would be a difference between Evince and Okular, not
> > between
> > Cairo and Splash.
> > 
> >> > So, Evince probably uses Cairo, which uses nearest neighbour in the
> >> > case of
> >> > the screenshot, and Okular probably uses Splash, which apparently does
> >> > not
> >> > only minification filtering.
> >> 
> >> Afaikt it's like this:
> >> 
> >> Qt5 frontend: In Splash::scaleImage, if an image is scaled down
> >> relative
> >> to its embedded size, or scaled up by more than 400% it uses
> >> bresenham.
> >> Else, if interpolation is explicitly wanted by the PDF image XObject,
> >> or if image is scaled in the range of 100% .. 400%, it uses bilinear
> >> interpolation.
> >> 
> >> Glib/cairo frontend: CairoOutputDev::getFilterForSurfac has same
> >> logic.
> >> It chooses between CAIRO_FILTER_NEAREST (= nearest neighbor)
> >> if scaled down, or scaled up by more than 400%. It use
> >> CAIRO_FILTER_GOOD (= box filter when < 75%, bilinear otherwise) for
> >> explicitly wanted interpolation or when scaled up in the range of 100%
> >> .. 400%.
> > 
> > Ok, if both work the same, the effect probably comes from somewhere
> > else.
> 
> They *do* differ in scaling algorithm (bresenham vs. nearest neighbor,
> bilinear vs. mostly bilinear). But they share the same logic to
> decide if a cheap or expensive algorithm shall be used. For the
> bug documents, we mostly see bresenham at work.
> 
> I think it could indeed make a visible difference.
> 
> >> > At least here I’m sure:
> >> > With nearest neighbour, the glyph edges are pixel edges. But with
> >> > magnification filtering, the pixel edges are widened to neighbouring
> >> > rendered
> >> > pixels, so the glyph edge becomes wider (smeared).
> >> 
> >> Guess glyphs are another story again. Both Qt5 and Glib frontend (via
> >> cairo)
> >> use freetype to render glyhps. But render settings may be different,
> >> don't know how they differ.
> > 
> > I didn’t mean freetype outlines or such. I said glyphs because I don’t
> > know
> > whether these signs may be called letters. I don’t know the language,
> > although
> > surrounding people think that when they see it on my screen. ;)
> 
> *gg*
> 
> >> > Cheers, David
> >> 
> >> After all, I see there's a subtle difference in the images in bug
> >> 408222, but
> >> honestly I couldn't even tell what looks better by only staring at it,
> > 
> > Yes, in the screenshot sections I have uploaded, the g-shaped glyph
> > looks
> > clearly better in Evince, but other glyphs look better in Okular.
> > 
> > Now it would be interesting whether the displayed zoom is close to the
> > native
> > resolution of the scan. If so, the effect would much certainly be
> > caused by
> > a).
> 
> pdfimage from poppler tools can give more insight here:
> 
> $ pdfimages -list test-1.pdf
> page   num  type   width height color comp bpc  enc interp  object ID
> x-ppi y-ppi size ratio
> ----------------------------------------------------------------------------
> ---------------- 1     0 image    1894  2798  gray    1   8  jpeg   no      
>   8  0 301   300  285K 5.5%
> 
> I.e., the image inside test-1.pdf has good resolution,
> and size is 1894 x 2798. We can also extract the image
> and view it to talk about the original quality:
> 
> $ pdfimages -all test-1.pdf ./ && gwenview ./-000.jpg
> 
> Because the rather big size, the images is actually downscaled
> when viewing it in Okular (at least on a non-HiDPI screen).
> For me, upscaling starts only when I view it
> at document zoom levels > ~390%.
> 
> >> and I would never have noticed a bug-worthy difference on my own.
> > 
> > Same for me, if a PDF is hard to read (like because it has serif
> > fonts), I
> > copy the text to Kate and reat it there. But I usually don’t read
> > scanned
> > PDFs.
> > 
> >> Shall we proceed there at all? Do we have means to quantify rasterized
> >> image quality?
> > 
> > I don’t know such means. A good workarround would possibly be to
> > provide a
> > zoom mode “Optimal for Scan”, which zooms to the native resolution of
> > the scan
> > (and makes sure that a) does not happen).
> 
> I think before taking measures we should figure out
> if there is actually a problem for > 1 users.
> But how can we figure it out?
> 
> Cheers Tobias