Los Angeles InDesign User Group
Size Matters! Measurements in InDesign for Print, ePub and Web
Thursday, May 19, 2016
By Alvin Takamori
The May meeting of the Los Angeles InDesign User Group took place in the offices of InMotion Hosting, a local website hosting company. As our hosts, not only did they provide the space for the meeting, they provided pizza!
The presenter for the evening was Alan Gilbertson, a creative director and designer who creates visual branding, identity programs, and large format images in various media. The aim of his presentation, "Size Matter! Measurements in InDesign for Print, ePub and Web," was to broaden our understanding of resolution and output.
One of the first concepts he wanted to enlighten us with was the distinction between pixels and dots. Pixels are the units that represent the density of the visual information stored on an image. For instance, a photo of Mars can have an area 30 meters wide represented by 1 pixel. Dots are units generated by a printer. The number of dots in an area, like a square inch, indicates how detailed the output of a printer can be. The pixel information is a constant. You could compress the image and throw pixels away, but any attempt to add pixels would just be a guessing game by the computer as to what to put in the added pixels. Assuming that you don't want to do that, then the number of pixels in the image will not change. For instance, let's say you have an image that is 6000 pixels wide. If you tell a printer to use that image and create a 600 pixel per inch image, you will have an image 10 inches wide. If you create a 300 pixel per inch image, then the image will be 20 inches wide.
Next, Alan discussed lines per inch. A conventional offset press will use lines per inch as a unit of measuring output. This is a line of dots. The line can be printed at different angles, but the distance between the centers of the dots is the same. So to adjust the density of the ink laid on a page, the size of the dots vary and the distance between the lines can be adjusted. Knowing how many lines per inch that you are going to be printing, determines the density of the pixels that an image should have in order to optimize the quality of the print. The guideline is to multiply the number of lines per inch by 2 to calculate how many pixels per inch that an image needs. For instance, newspapers are printed at 75 lines per inch. So, an image for a newspaper needs to contain 150 pixels per inch to look good. (75 x 2 = 150) A fine art book will typically be printed at 200 lines per inch. Therefore, images for a fine art book need to contain 400 pixels per inch. (200 x 2 = 400) So much for the myth that all printed images should have 300 pixels per inch.
Following offset printing, Mr. Gilbertson discussed the features of stochastic printing. Instead of evenly spaced dots of various sizes used in offset printing, stochastic dots are randomly spaced with very fine dots that are all the same size. This type of printing produces finer detail and smoother transitions with a wider color gamut. If you employ this type of printing, you can use images with fewer pixels per inch than required for offset printing.
If you are printing using an inkjet printer, once again the density of the pixels that an image needs will vary based on the output. In this case, you need to know the number of dots per inch that the inkjet printer generates. Take that number and divide by 4 to determine the density of the pixels an image requires. For example, if you are using an inkjet printer that produces 1440 dots per inch you should be using images that contain 360 pixels per inch. (1440 ÷ 4 = 360)
Next, our presenter expressed his dissatisfaction with InDesign's export feature. Apparently, InDesign wants to export everything as a 72 ppi image. If you try to export images using a higher ppi, InDesign resamples the image without telling you. For instance, if you have a 600 pixel per inch image and you try to export it as a jpg file at 288 ppi, InDesign sees 288 as 4 times 72 ppi and takes the original 600 pixels and upsamples it 4 times to create a 2400 ppi jpg file. You don't want resampling because the computer is just guessing at what to put in the extra pixels that it's creating. So always export at 72 ppi, and if you need higher resolution let the overall dimensions increase.
If you are designing large images like a billboard, InDesign doesn't have a document size that big. In order to work on these large images in InDesign, you have to "work to scale." Essentially you work on a smaller sized image, but you pack it with a higher pixel per inch density. Images like a billboard are viewed at long distances, so less than 10 ppi can look okay. So you can fit these images on an InDesign document by working on a scaled down image size, for instance 1/12 scale. (1 inch = 1 foot) You could reduce the size even more. Let's say you need a 48' x 14' billboard that contains 9 ppi. In InDesign you could create an image a fraction of that size at 17.28" x 5.04", but at 300 ppi. If you increase the size of the image up to the billboard size you will get the 9 ppi image without any resampling of pixels.
Alan Gilbertson then touched on some aspects of producing images for digital billboards and screen displays in general. An interesting point he made was that jpg compression works on pixels in blocks of 8. So if you can create images using a pixel amount that is divisible by 8, you get a better result. For example, if you take a 160 x160 pixel image and convert it into a jpg file, it compresses into a noticeably smaller image (in terms of memory) than a 156 x 157 pixel image.
Our guest concluded his presentation with a question and answer session. In summary, some key points to remember are that dots are units of output by a printer,
while pixels are units of information in an image on a screen. Also, to detemine how many pixels an image should have, it's important to know exactly what you are outputting an image to, and there are far more choices than 72 ppi for web and 300 ppi for print.
Farthest Attendee Prize Winner
Mighty Deals—Nancy Fox
Raffle Prize Winners
eDocker CREATE! 6 month subscription. Value $774.00—Victoria Markle
Stock Layouts. Full access to Stock Layout template library. 3 month subscription. Value $299.00—David Morin
AcademyX. Credit toward any class. Value $250.00—Nancy Fox
Markzware. FlightCheck. 12 month subscription. Value $199.00—Param Sharma
Expo Creative Asset Manager for Mac from Insider Software. Value $149.00—David Morin
Font Agent Pro 6 from Insider Software. Value $99.95—Alan Gilbertson
InMotion Hosting. Web hosting and free domain. Value $90.00—David Morin
DTP Tools Cloud for InDesign. 6 month subscription. $77.40—Trish Hall
TypeDNA. Font management software. Value $49.00—Julie Worthington
Adobe Stock. 15 image licenses. $44.00—Susy Reuben
InDesign Magazine. 6 month subscription. Value $30.00—Marney Wilde
Pluralsight. 1 month subscription. Value $29.00—Alan Gilbertson, Julie Worthington
LA Web Professionals Group meeting tickets. Value $7.99—Jyndon Johnson, Greg Saunders, Param Sharma, Julie Worthington