> > On 2018-06-01 10:34 AM, D. Hugh Redelmeier via talk wrote: > * cjpeg - the IJG/libjpeg-turbo command offers easier access to JPEG > options than IM/GM's convert, such as greyscale > > * you can very often have a much lower JPEG quality than defaults, but > keep the originals somewhere safe 'cos JPEG is lossy > > * don't use arithmetic-coding compressed JPEGs: the browser support is nil > > * (almost all) JPEGs are made up of 8×8 px colour approximation cells. > If your image isn't an exact multiple of 8 px in each direction, you're > going to waste space in the file > > * consequently, cropping a JPEG can do unexpected bad things to the > output. jpegtran — another IJG/libjpeg-turbo tool — is one of the very > few programs that can do a true lossless crop of a JPEG file > > * JPEGs are for continuous-colour photographic images, and any sharp > change in colour — like you'd get in computer-generated or line graphics > — causes weird colour fringes / artifacts around the edges. The "modern" > way to deal with this is to rack the JPEG quality up to 95%+ and live > with huge files. The "right" way to deal with this is to add a *tiny* > bit of colour noise to the image, and it will kill the fringes off. > (JPEGs essentially store the result of signal processing the source > image rather than the image itself. If you've ever played with signals > (analogue or digital), sharp edges in any input waveform can cause > "filter ringing" or unwanted harmonics in the output. JPEG fringes are > unwanted filter harmonics, basically) > > * BUT … nobody really cares (or needs to) about JPEG size these days as > every webpage is megabytes of crap anyway. > > * almost nobody (outside specialist domains) uses the replacement for > the JPEG format, JPEG-2000. This is a shame, as it has some very nifty > features like progressive viewing (so an overview will only load a small > part of the file) and truly impressively tiny file sizes. JPEG-2000 > takes a fair amount of processing power to render (deemed too much/too > slow for the web circa 2001) and was until recently encumbered by some > annoying patents. Ubuntu deals with this very badly by patching a bunch > of tools (like ImageMagick) such that when you ask for JPEG-2000, it > gives you a JPEG. This is bad and wrong and they must get off my lawn > forthwith.
Hugh, you reminded of this presentation I watched a few days ago. < https://youtu.be/hQZ7Xg7q7zw>. Highly recommended. Regards, Clifford Ilkay +1 647-778-8696
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