SEM III CASE STUDY RETINA DISPLAY Retina Display is a brand name used by Apple for liquid crystal displays that, according to Apple, have a high enough pixel density that the human eye is unable to notice pixilation at a typical viewing distance. The term is used for several Apple products, including the iPhone, iPod Touch, iPad, and MacBook Pro. Because the typical viewing distance is different, depending on each device's use, the pixels per inch claimed to be of retina quality can differ, depending on the size of the display, with higher ppi for smaller displays and lower ppi for larger displays: 326 ppi for the smallest devices (iPhone and iPod Touch), 264 ppi for mid- sized devices (iPad), and 220 ppi for larger devices (MacBook Pro). When an Apple product has retina display, each user interface widget is doubled in width and height to compensate for the smaller pixels. Apple calls this mode HiDPI mode. Apple has applied to register the term "Retina" as a trademark in regard to computers and mobile devices with the United States Patent and Trademark Office, Canadian Intellectual Property Office, and in Jamaica. Retina Display models The displays are manufactured by different suppliers in the world. Currently, the iPad's display comes from Samsung, while the Macbook Pro, iPhone, and iPod touch displays are made by LG and Samsung. MULTIMEDIA Page 1 SEM III CASE STUDY Apple markets the following devices as having Retina Displays. Typical PPI (pixels ppcm (pixels Viewing Pixels per Model Resolution per inch) per cm) Distance Degree (PPD) (in/cm) iPhone 4/4S and iPod 960×640 Touch (4th generation) 10 inches 326 128 57 (25 cm) iPhone 5 and iPod 1136×640 Touch (5th generation) iPad (3rd/4th 15 inches 264 105 2048×1536 69 generation) (38 cm) MacBook Pro with 20 inches 220 87 2880×1800 77 Retina Display 15" (51 cm) MacBook Pro with 20 inches 227 89 2560×1600 79 Retina Display 13" (51 cm) MULTIMEDIA Page 2 SEM III CASE STUDY Technical definition When introducing the iPhone 4, Steve Jobs said the magic number for a Retina display is about 300 PPI for a device held 10 to 12 inches from the eye. One way of expressing this as a unit is Pixels per Degree (PPD) which takes into account both the screen resolution and the distance from which the device is viewed. Based on Jobs' magic number of 300, the threshold for a Retina Display starts at a PPD value of 53. 53 PPD means that a tall skinny triangle with a height equal to the viewing distance and a top angle of one degree will have a base on the device's screen that covers 53 pixels. Any display's viewing quality (from phone displays to huge projectors) can be described with this size-independent universal parameter. Note that the PPD parameter is not an intrinsic parameter of the display itself, unlike absolute pixel resolution (e.g. 1024 x 800 pixels) or relative pixel density (e.g. 72 PPI), but is dependent on the distance between the display and the eye of the person (or lens of the device) viewing the display; moving the eye closer to the display reduces the PPD, and moving away from it increases the PPD in proportion to the distance. It can be calculated by multiplying the distance to the screen times the resolution of the screen in pixels per unit length times twice the Tan of π divided by 360 (equal to half a degree in radians). Remember to use units, whether imperial or metric, consistently in applying this formula. If the distance to the screen is measured in inches, then the resolution of the screen must be in PPI. If the distance to the screen is measured in centimeters, the resolution of the screen must be in PPC. Two times the Tan of π divided by 360 can also be approximated with π divided by 180 (= 0.01745). In practice, thus far Apple has converted a device's display to Retina by doubling the number of pixels in each direction, quadrupling the total resolution. This increase, combined with the size-doubling of the user interface, creates a sharper interface at the same physical dimensions. MULTIMEDIA Page 3 SEM III CASE STUDY Some Stuff about PIXEL DENSITY Pixels per centimeter (ppcm), Pixels per inch (PPI) or pixel density is a measurement of the resolution of devices in various contexts: typically computer displays, image scanners, and digital camera image sensors. Ppcm can also describe the resolution, in pixels, of an image to be printed within a specified space. Note, the unit is not square centimeters. For instance, a 100×100 pixel image that is printed in a 1 cm square has a resolution of 100 pixels per centimeter (ppcm). Used in this way, the measurement is meaningful when printing an image. It has become commonplace to refer to PPI as DPI, which is incorrect because PPI always refers to input resolution. Good quality photographs usually require 300 pixels per inch, at 100% size, when printed onto coated paper stock, using a printing screen of 150 lines per inch (lpi). This delivers a quality factor of 2, which delivers optimum quality. The lowest acceptable quality factor is considered to be 1.5, which equates to printing a 225ppi image using a 150 lpi screen onto coated paper Screen frequency is determined by the type of paper that the image is to be printed on. An absorbent paper surface, uncoated recycled paper for instance, will allow the droplets of ink to spread (dot gain), and so requires a more open printing screen. Input resolution can therefore be reduced in order to minimize file size without any loss in quality, as long as the quality factor of 2 is maintained. This is easily determined by doubling the line frequency. For example, printing on an uncoated paper stock often limits the printing screen frequency to no more than 120 lpi, therefore, a quality factor of 2 is achieved with images of 240 ppi. MULTIMEDIA Page 4 SEM III CASE STUDY Calculation of monitor PPI Theoretically, PPI can be calculated from knowing the diagonal size of the screen in inches and the resolution in pixels (width and height). This can be done in two steps: 1. Calculate diagonal resolution in pixels using the Pythagorean Theorem: 2. Calculate PPI: where is diagonal resolution in pixels is width resolution in pixels is height resolution in pixels is diagonal size in inches (this is the number advertised as the size of the display). For example, for a 21.5 inch (54.61 cm) screen with a 1920×1080 resolution (in which = 1920, = 1080 and = 21.5), we get 102.46 PPI; for a typical 10.1 inch netbook screen with a 1024×600 resolution (in which = 1024, = 600 and = 10.1), we get 117.5 PPI. Note that these calculations may not be very precise. Frequently, screens advertised as “X inch screen” can have their real physical dimensions of viewable area differ, for example: Apple Inc.'s Mid-2011 iMac is advertised as a "21.5 inch (viewable) [...] display," but its actual viewable area is 545.22 mm or 21.465 inches. The more precise figure increases the calculated PPI from 102.46 (using 21.5) to 102.63. The HP LP2065 20 inch (50.8 cm) monitor has an actual viewable area of 20.1 inch (51 cm). MULTIMEDIA Page 5 SEM III CASE STUDY INSIGHTS Calculating PPI of camera screens Camera manufacturers often quote camera screens in 'number of dots'. This is not the same as the number of pixels, because there are 3 'dots' per pixel – red, green and blue. For example, the Canon 50d is quoted as having 920,000 dots. This translates as 307,200 pixels (x3 = 921,600 dots). Thus the screen is 640×480 pixels This must be taken into account when working out the PPI. Using the above calculations, you require the screen's dimensions, but other methods require you to have the total pixels, not total dots. 'Dots' and 'pixels' are often confused in reviews and specs when viewing information about digital cameras specifically. Scanners and cameras "PPI" or "pixel density" may also be used to describe the resolution of an image scanner. In this context, PPI is synonymous with samples per inch. In digital photography, pixel density is the number of pixels divided by the area of the sensor. A typical DSLR circa 2013 will have 1–6.2 MP/cm2; a typical compact will have 20–70 MP/cm2. For example Sony Alpha SLT-A58 has 20.1 megapixels on an APS-C sensor having 6.2 MP/cm2 since a compact camera like Sony Cyber-shot DSC-HX50V has 20.4 megapixels on an 1/2.3" sensor having 70 MP/cm2. Interestingly, as can be seen here, the professional camera has a lower PPI than a compact camera, because it has larger photodiodes due to having far larger sensors. Named pixel densities The Google Android developer documentation groups displays by their approximate pixel densities into the following categories: LDPI: Low density, ~120 dots per inch MDPI: Medium density, ~160 dots per inch HDPI: High density, ~240 dots per inch XHDPI: eXtra High density, ~320 dots per inch XXHDPI: eXtra eXtra High density, ~480 dots per inch XXXHDPI: eXtra eXtra eXtra High density, ~640 dots per inch Metrication The digital publishing industry often uses "pixels per centimeter" instead of "pixels per inch". MULTIMEDIA Page 6 SEM III CASE STUDY Resolution independence Resolution independence is a computing concept whereby elements on a computer screen are rendered at sizes independent from the pixel grid, resulting in a UI that is displayed at a consistent size, regardless of the size of the screen.