Up­siz­ing your images

Hans We­ich­sel­baum ex­plores the no­tion of en­sur­ing the best-qual­ity im­age is main­tained when up-sam­pling and down-sam­pling your pho­to­graphs

The Shed - - Column - Hans We­ich­sel­baum

In the last edi­tion, we looked at the con­cept of im­age res­o­lu­tion. A fig­ure of 300 pix­els per inch (ppi) doesn’t tell you any­thing un­til your im­age gets onto pa­per (the same goes for ‘dpi’, which stands for ‘dots per inch’, or ‘drops per inch’). The size of a dig­i­tal im­age is de­ter­mined by the num­ber of pix­els, and noth­ing else. For ex­am­ple a 12-megapixel sen­sor will give you a 4000x3000-pixel im­age.

Only be­fore print­ing do we look at how many pix­els we get per cen­time­tre (or inch) of pa­per. Our ex­am­ple of a 4000-pixel-wide land­scape im­age will give you 571ppi for a 17.78cm (seven-inch)–wide print. This fig­ure drops down to 235ppi if you want a 43.18cm (17-inch) print, slightly larger than A3.

Op­ti­mum file res­o­lu­tion for print­ing

How many pix­els per inch do we need for best qual­ity? You’ll of­ten hear the num­ber 300ppi for print­ing and 72ppi for mon­i­tor dis­play. The 72ppi fig­ure is non­sen­si­cal, be­cause it will de­pend on the phys­i­cal size of your de­vice and the num­ber of pix­els it can dis­play. The res­o­lu­tion can be more than 500ppi for a typ­i­cal smart­phone dis­play.

Com­mer­cial prin­ters will rou­tinely ask for 300ppi files, and, in the pre­vi­ous ar­ti­cle, we looked at the his­tor­i­cal rea­sons for this fig­ure. It is still a good ball­park fig­ure to aim for when your images are head­ing for a printer. Ep­son inkjet prin­ters work with a na­tive res­o­lu­tion of 360ppi (or 720ppi if you tick the ‘Finest De­tail’ check­box). This means that your im­age is go­ing to be up- or down-sam­pled to 360 or 720ppi de­pend­ing on your print set­tings. The printer driver does the re­siz­ing au­to­mat­i­cally for you.

You do not need to panic if your file res­o­lu­tion works out to less than 360ppi. The 43.18cm print from our 12-megapixel cam­era (235ppi) will still look great. The trou­ble starts when you want to print your 12-megapixel cam­era file at A2 size (594mm/23.4 inches), which works out to 171ppi, or if you are in the habit of crop­ping your images. As a rule of thumb, you will get good re­sults from 240ppi prints viewed close up and from 180ppi photos hang­ing on the wall and viewed from a dis­tance. If the ppi gets too low, you’ll get alias­ing (the ‘jag­gies’), and the im­age will need to be up­sized.

Printer driv­ers in gen­eral do an ex­cel­lent job of up­sam­pling your images to 360ppi, or what­ever the na­tive res­o­lu­tion of your printer might be. You can also rest as­sured that the com­mer­cial printer you’re us­ing has a pro­fes­sional raster im­age pro­ces­sor (RIP) do­ing the best pos­si­ble job of up­siz­ing.

The dark art of up-sam­pling

De­spite all th­ese as­sur­ances, many peo­ple get ner­vous when they think of feed­ing a small file into a black box (the printer driver), yet out comes the per­fect 360ppi file. As we saw in the pre­vi­ous ar­ti­cle, Adobe Pho­to­shop gives you a num­ber of choices when you need to up-sam­ple your im­age files. Ob­vi­ously, there is more than just one op­tion.

‘Up-sam­pling’ means ask­ing the com­puter to ‘in­vent’ new pix­els be­tween ex­ist­ing ones. Need­less to say, there is no way that you can add de­tail that wasn’t in the file when it came out of the cam­era. Per­haps you re­mem­ber de­tails in the orig­i­nal scene, but they were not recorded be­cause the res­o­lu­tion wasn’t high enough. The soft­ware will in­ter­po­late be­tween ex­ist­ing pix­els, and add more based on the av­er­age value of those pix­els around them. This just gives the il­lu­sion of in­creased res­o­lu­tion, but it’s only more pix­els show­ing the same thing.

The prob­lem is that the im­age gets softer, los­ing sharp­ness and con­trast when it gets re­sized (the same ap­plies to down­siz­ing). Some in­ter­po­lat­ing al­go­rithms are bet­ter in keep­ing the im­age sharp, but are more likely to pro­duce re­siz­ing arte­facts. The chal­lenge is to find the best pro­gram, which can vary from im­age to im­age.

If your im­age comes from a scan, there is a sim­ple rule: never up-sam­ple. Es­pe­cially film scans: the film grain and the scan­ner’s sam­pling grid form in­ter­fer­ence pat­terns, which tend to make things worse when up­siz­ing. For the best qual­ity, scan at the max­i­mum op­ti­cal res­o­lu­tion, and then down-sam­ple to the re­quired size. Dig­i­tal cam­eras, on the other hand, are fairly tol­er­ant of re­siz­ing. If the orig­i­nal cap­ture was in fo­cus, us­ing the right soft­ware and up-sam­pling by 200–300 per cent can give you very good re­sults.

A few years back, be­fore dig­i­tal cam­eras took over the pho­to­graphic scene, there were many se­cret recipes that promised the user a su­pe­rior re­sam­pling qual­ity. One trick was to re­size the im­age not in one go but in a num­ber of smaller steps. Users went to great lengths writ­ing Pho­to­shop ac­tions, but to­day the avail­able al­go­rithms will be dif­fi­cult to beat.

Third-party soft­ware for up­siz­ing

There are sev­eral pro­grams that claim to do a bet­ter job than Pho­to­shop. One of the best-known soft­ware pro­grams for re­siz­ing images is Per­fect Re­size, which was orig­i­nally called ‘Gen­uine Frac­tals’ when it launched in 1996. Dis­trib­uted by ON1 soft­ware, it costs US$80, and you can down­load a 60-day trial.

It works as a stand-alone or as plug-in for Pho­to­shop, Adobe El­e­ments, and even Adobe Light­room. The in­ter­face is clean and in­tu­itive (see Im­age 1), and you sim­ply en­ter the new im­age di­men­sions (per cent, pix­els, cen­time­tres, or inches) and the pro­gram does the rest. The large pre­view shows you ex­actly what you will get, and there are also op­tions for sharp­en­ing, film grain, tiling, and gallery wrap. On the left side, you’ll find pre­sets for com­mon printer/pa­per com­bi­na­tions.

An­other pro­gram promis­ing you the best pos­si­ble re­sults is called ‘Pho­toZoom’ and comes from the BenVista group (see Im­age 2). There are two ver­sions, the Pro edi­tion, which weighs in at €169, and the more af­ford­able Clas­sic ver­sion for €69.

You can test-drive this pro­gram, but you’ll get wa­ter­marks all over your en­larged copy. The Pro ver­sion is a plea­sure to work with — it has a large pre­view, which can be set up as a split pre­view. The pro­gram of­fers 12 re­siz­ing al­go­rithms, and you get a live view of what the var­i­ous op­tions are do­ing to your im­age. There is the usual Un­sharp Mask fil­ter, plus con­trols for arte­fact re­duc­tion, crisp­ness, and vivid­ness. On top of that, you’ll get handy pre­sets, tailored for reg­u­lar/soft/de­tailed photos, re­duc­ing

Im­age 1 — Per­fect Re­size 10 from ON1

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