On 2018-06-01 10:34 AM, D. Hugh Redelmeier via talk wrote:
| Using convert is okay, but you won't end up with the smallest JPEGs that | way.
What do you recommend? (Lightning talk?)
oh I wish - the next couple of weeks I'm running more events than is sensible. anyway, of course I've overthunk this, so some notes in lieu: * 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. cheers, Stewart (Oh, and if you've made it this far: I see that local(ish) Linux-friend Bob Jonkman is standing as a Green candidate in Kitchener-Conestoga: https://gpo.ca/candidate/bob-jonkman/)