Images of a Black People Sensual Art 600 Wide 600 Tall
Making fine prints in your digital darkroom
Pixels, images, and files
by Norman Koren
This pages introduces the fundamental concepts of digital images: what they are, how their size and resolution is specified, and how they are stored.
Pixels and images
| A digital image is a rectangular filigree of pixels , or "picture elements," illustrated past the image on the right, which consists of 52x35 pixels; 1820 total. Information technology has been enlarged fivex (to 260x175 pixels) to make the pixels visible. Pixels appear as squares when enlarged in this fashion. Digital images be as an array of bytes in a computer's RAM retentivity or as files in memory cards, hard drives, and CDs or DVDs. Several popular file formats are described below. |  |
Each pixel typically consists of viii bits (1 byte) for a Black and White (B&W) image or 24 bits (3 bytes) for a color image-- i byte each for Scarlet, Green, and Blueish. 8 bits represents 28 = 256 tonal levels (0-255). 16-bit B&W and 48-bit color image formats are also available; each pixel represents ii16 = 65,536 tonal levels. Editing images in 16/48-bits produces the highest quality results; you tin save images in 8/24 $.25 after editing is complete. An paradigm'due south resolution is the total number of pixels, e.k., 1600 x2000 = 3.two Megapixels, which corresponds to 3.ii Megabytes inside your reckoner for eight-chip B&W images or ix.vi Megabytes for 24-bit (3 bytes/pixel) color images. At that place are several other definitions of "resolution." See the explanation below.
Digital images are obtained from digital cameras or by scanning film or prints. Scanners are specified by their dpi or ppi resolution-- dots (actually, pixels) per inch they tin obtain from the source. Scanning the original source-- the negative or slide-- always produces better quality than scanning a print. Printers are specified by their dpi (dots per inch) "resolution," typically 720, 1440 or 2880 for Epson. This number is the stepper motor pitch, not the actual visual resolution. It typically requires several printer dots to stand for one image pixel. You don't demand to worry nearly the correspondence between image pixels and printer dots; this is handled by the epitome editor and printer driver software.
Image resolution and print size
 | Considerable confusion arises because image size is specified by the number of pixels, the "resolution" in dots or pixels per inch (dpi or ppi), and the physical size (width and height). Only the merely aspect that counts is the number of pixels. The prototype on the left was taken on the Canon EOS-10D, converted from RAW using the Canon File Viewer Utility (which I've since replaced with Capture 1 DSLR LE), adjusted for color (though far from finished), and so resized to 260x175 pixels wide in my favorite image editor, Picture Window Pro. |
. Picture Window Pro
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size = pixels/dpi (or pixels/ppi)It make no difference whether the image is 3437 dpi, 22.7xxv.i mm (the actual size of the EOS-10D digital sensor) or 17.07 dpi, fifteenxten feet (a billboard); each of the 3072102048 (half-dozen.iii one thousand thousand) pixels (6.three megapixels) is exactly the same. I didn't change the Resolution (dpi setting) when I resized the paradigm, hence the "size" of the new 20010133 pixel image size is tiny: 1.11ten0.74 inches. This "size" is completely unrelated to the size you see on your monitor.
To add together to the confusion, the discussion "Resolution" has several meanings. It can be the highest spatial frequency where a line pattern is visible: See the series on Image sharpness and MTF. It oft refers to the full pixel count of an image, e.chiliad., 3072x2048 pixels for the EOS-10D. I prefer either of these definitions to the dpi/ppi setting, which has nothing to practice with the total item present in an image, and which can be changed without changing a single pixel. Just we're stuck with it in prototype editing programs.
You lot can easily change Resolution (dpi) , hence Width and Meridian , without changing pixel count, i.e., you lot can rescale the prototype without resizing (i.e., resampling ) it. In Picture Window Pro, open the Resize dialog box (to a higher place), click on the pointer to the right of the Preserve, then select File Size and Proportions. When you can change either Width, Height, or Resolution (dpi), the other two follow. In Photoshop, open the Image Size dialog box, shown below, and leave Resample Image unchecked. Check Resample Image if y'all want t o change the pixel count-- to resize the image.
 | Flick Window Pro If you correct-click on the epitome in Picture Window Pro, then click on Display info, the Window Info box appears. The properties of the resized 200x133 pixel prototype are shown on the left. The Size numbers are the aforementioned every bit the New column in Resize dialog box, above. But the File Size, 8,305 pixels, is much smaller than the 79,800 byte image size (200x133 pixels x 3 bytes/pixel). This is the issue of JPEG pinch. GIF and PNG files are besides compressed, simply TIFF files are non: Image and file sizes are the same. I usually sharpen an image (using the simple Sharpen transformation,with Amount around lxx%) after resizing it down. |
In Picture Window Pro you select the print size when you impress. The Width and Height attributes are ignored. This doesn't exactly hold for Photoshop.
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Photoshop 6.0
Photoshop'due south default Print... command tries to print images at the specified Width and Height. When you try to print the original 17.07x11.38 inch image on letter-sized paper, an annoying message appears, " The image is larger than the paper'southward printable area; some clipping will occur. " There are ii solutions.
- You tin can rescale the paradigm using the Image Size dialog box, according to the instructions above. You rarely need to resize information technology.
- Click on Print Options... instead of Print... in Photoshop 6 (Impress with Preview... in Photoshop CS). The first time you open up this box for an prototype, Calibration is set to 100% and Scale to Fit Media is unchecked. If you cheque Scale to Fit Media, Scale is adapted so the image fits the page. You may need to click Page Setup... to accommodate borders and orientation (Portrait or Landscape). Or you lot tin can exit Scale to Fit Media unchecked and manually prepare Scale. A small page preview in the Print Options box helps with the setting, which will be remembered every bit long as the paradigm remains open.
How many pixels to y'all need for a sharp print?
| | Perceived print quality* |
| | Outstanding. Every bit sharp every bit most printers can print; about every bit sharp as the heart can see at normal viewing distances. |
| | Fantabulous. Shut to 300 PPI for small prints, viii�x11 (or A4) and smaller. Outstanding quality in large prints, 11x17" (or A3) and larger, which tend to be viewed from greater distances. |
| | OK for big prints. Adequate, just not optimum, for small prints. |
| | Adequate, but non optimum, for large prints. Mediocre for small prints. |
*This table assumes that the image is precipitous. A more accurate tabular array, based the truthful image sharpness produced past the lens/photographic camera combination, is presented in the introduction to Sharpness in Imatest.com.
300 pixels per inch (ppi) is about every bit sharp as the eye can run across on an inkjet impress; it can be very impressive in a impress from a sharp image file. Remember, these numbers are actual pixels per inch on the impress, not the ppi setting of the paradigm file.
When an image is sent to the printer, the image editor or printer driver resizes it to the printer'south native resolution-- 720 dpi for Epson Photo printers; 600 dpi for HP and Catechism. No transmission resizing is required. There is some controversy about how good a job image editors do (particularly Photoshop). Read Qimage Print Quality Claiming to acquire more. I'yard pleased with the results I get from Picture show Window Pro.
Most digital images must be resized downwardly for the monitor display-- for web pages or email. Many people are careful to scale the resized images to 72 dpi. Absolutely unnecessary. I know of no spider web browser or viewing software that pays whatsoever attention to the dpi setting. Virtually monitors actually brandish fourscore-100 pixels per inch, anyway.
Image file formats
Several file formats are bachelor for image storage. The most of import are TIFF , JPEG , GIF , and PNG . The chief difference betwixt them is the type and amount of paradigm compression . Compression reduces the amount of storage space required past an image. For example, a 1600ten2000 pixel 24-bit colour epitome (3 bytes per pixel) requires 9.six megabytes to shop without compression; it requires considerably less with compression. There are two types of compression. - Lossless compression maintains all image particular, flake-past-bit. Typical compression ratios are thirty-50%, depending on the detail in the image. The effectively the detail, the less the pinch. Lossless pinch is used by the PNG format. It is available, though rarely used, with the TIFF format.
- Lossy compression sacrifices detail in order to achieve higher compression ratios. The amount of compression depends on the detail in the image and the quality level selected when the image is saved. JPEG and GIF use unlike types of lossy compression.
| The image on the right is used as to illustrate compression. It is a 24-bit color 260x175 pixel prototype (3 bytes per pixel), and therefore contains 136,500 bytes (136.five kB) . It was stored as a 100% Quality JPEG, 46.3 kB in size. The compression ratio is 136.5/46.3 = two.95x . 100% JPEGs are virtually duplicate from uncompressed TIFF images-- they comprise a tiny loss that would only become visible subsequently the file was repeatedly opened and saved. The storage required for this paradigm in each format is shown below in (bold blueish) . Web browsers support JPEG, GIF, and PNG formats, but non TIFF. |  |
- TIFF files (identified by the .tif extension) are ordinarily uncompressed. Lossless compression is available, just not universally supported. TIFF is highly versatile; it can shop sixteen/48-bit images and metadata , such as Description, Artist's proper noun, and Copyright, in tagged fields. This makes TIFF files slightly larger than the images they comprise. TIFF is the format of selection for saving images intended for high quality printed output. (138.5 kB)
- PNG (.png) is another format worth considering. It uses lossless pinch: paradigm quality is equal to TIFF but file size tin can be considerably smaller, though generally larger than JPEG. PNG is supported by most web browsers and paradigm editors, just it isn't widely used-- it's undoubtedly the near underrated image file format. I occasionally use it for Web display of Black & White images, such as the gamma test pattern, because information technology compresses nearly likewise as JPEG, which doesn't support B&W. PNG was created to circumvent the LZW patent used in GIF and TIFF formats. (96.3 kB; 1.42x)
- GIF (.gif) is best for Spider web display of block graphics and charts. Its limited color palette makes it a poor choice for loftier-quality photographic images, although it's often used for thumbnails. GIF files tin can be very compact; they lack the wavy edge artifacts of JPEG files (38.2 kB; 3.6x)
- JPEG (.jpg) is the well-nigh popular format for Web display of photographic images. Its principal characteristic is lossy pinch, which can result in artifacts, most notably a wavy appearance most boundaries. The loss in quality and the stored prototype size depends on the Quality setting used to save the paradigm. College quality JPEGs are larger but have more detail and fewer artifacts. JPEGs typically range from 1/5 to 1/25 the size of TIFFs. This is illustrated beneath for an prototype saved past Film Window Pro with JPEG quality levels of 90%, seventy%, and xxx%.
| The images beneath illustrate the furnishings of the JPEG Quality setting on image compression with Picture show Window Pro. The dialog box is shown on the right. I employ quality levels between about 80 and 95% for most images in this site. |  |
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| ninety% quality; 15.iii kB; 8.ninex.Quality is close to 100%, but a piffling waviness-- the classic JPEG antiquity-- is visible in the sky near the copse. | 70% quality; 8.ix kB; 15.3ten. Image quality is notwithstanding pretty skilful, but wavy artifacts are significantly worse and detail in the rocks is reduced. | xxx% quality; half-dozen.1 kB; 22.4ten. Image quality is quite poor. Wavy artifacts are obvious; there is significant loss of detail, and foursquare zones are visible in the sky. |
Every programme has its ain JPEG Quality settings; there is no universal standard. Photoshop has settings between 0 and 12. IrfanView (a handy little editing utility) has settings between 0 and 100. The relationship between settings in unlike programs is anything but linear. Prisoner of war Pro Quality = seventy% (eight.nine kB) is roughly equivalent to Irfanview Quality = 50% (7.nine kB) and Photoshop Quality = 3 (33 kB in Photoshop CS; thirteen.8 kB in Photoshop 6). Photoshop files are considerably larger, even though have no embedded ICC profiles, perhaps because they have extra infinite reserved for Exif data (digital camera settings). This can be a disadvantage for small images used for Web display.
Many digital cameras store images in JPEG format. This tin lead to bug because image quality can deteriorate when images are saved and reloaded in JPEG format. The deterioration is tiny when files are saved at the highest possible JPEG quality, but it can go serious if less than maximum JPEG quality is selected. JPEGs should be used with circumspection for images intended for loftier quality quality printed output. If you plan to do much editing on a JPEG image, it'due south all-time to convert it to TIFF. No quality is lost when images are stored and reloaded in TIFF format.
Additional file formats You may occasionally meet these, but they're not recommended for general use.
Macintosh users should add the appropriate 3 leter suffix (JPG, TIF, PNG, etc.) to file names to go far easier for us Windows users to read.
- JPEG 2000 An update of JPEG that offers a college degree of compression at the same paradigm quality level every bit well as an option for lossless compression. Information technology also has options for fast display when full resolution isn't needed. It should be an splendid format one time it's widely adapted, just few web browsers back up it as of early on 2004. I volition avoid it until all major web browsers (Internet Explorer, Mozilla, and the Mac browsers) accept supported information technology for at least a year.
- PSD Adobe Photoshop Document. Designed past Adobe for apply within Photoshop; non supported by spider web browsers and nigh prototype editors. It sould not exist used for file interchange. To be used with programs other than Photoshop, PSD files mostly have to be transformed into a standard format such as TIFF. Irfanview is a bully footling program for translating them.
- BMP Windows bitmap format. A widely-supported uncompressed format that lacks the versatility of TIFF. TIFF should be used in its place whenever possible.
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| | Images and text copyright (C) 2000-2013 past Norman Koren. Norman Koren lives in Boulder, Colorado, where he worked in developing magnetic recording applied science for high capacity data storage systems until 2001. Since 2003 nigh of his fourth dimension has been devoted to the development of Imatest. He has been involved with photography since 1964. |  |
Source: http://www.normankoren.com/pixels_images.html
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