Chap27
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Chap27 Chap27 Presentation Transcript

  •  Explain image quality in terms of stroke, details, colour accuracy, dynamic range of the image.
  • In This Chapter, you’ll learn on:  Describe image quality.  Compare the quality of images acquired from various sources.  Improve image quality using appropriate software filters
  • Image Quality  Image quality is a characteristic of an image that measures the perceived image degradation (typically, compared to an ideal or perfect image). Imaging systems may introduce some amounts of distortion or artifacts in the signal, so the quality assessment is an important problem.
  • Factors that affect image quality  Sharpness determines the amount of detail an image can convey.  Noise is a random variation of image density, visible as grain in film and pixel level variations in digital images.  Dynamic range (or exposure range) is the range of light levels a camera can capture, usually measured in f-stops, EV (exposure value), or zones (all factors of two in exposure). It is closely related to noise: high noise implies low dynamic range.  Tone reproduction is the relationship between scene luminance and the reproduced image brightness.
  • Factors that affect image quality  Contrast, also known as gamma, is the slope of the tone reproduction curve in a log-log space. High contrast usually involves loss of dynamic range — loss of detail, or clipping, in highlights or shadows.  Color accuracy is an important but ambiguous image quality factor. Many viewers prefer enhanced color saturation; the most accurate color isn't necessarily the most pleasing.  Distortion is an aberration that causes straight lines to curve near the edges of images.
  • At full resolution, this image has clearly visible compression artifacts
  • Improve image quality using appropriate software filters  We can say that these effects or filters have a positive effect on image quality. But there is no definite way to improve an image quality 100% by just using software. It depends fully on the source of the capturing device.  For example, to scale up a 72 dpi resolution photo to 200%, despite the fact that you can achieve this by increasing the resolution in Photoshop or GIMP, you are not adding new detail into the photo. In effect, all you are doing is increasing the size of the pixels which rapidly decreases the quality as you scale higher.
  •  But Images can often be improved or transformed by using graphics editing software such as Photoshop. Such applications or programs usually have photo-enhancing tools and allows you to execute semi-automatic image enhancement with only the click of a button.  Some examples of such filters or tools are o blur/overlay o filter the background Blurring of the background to make the object more stand out
  •  Add a filter to the background to bring focus to the robot in the foreground.
  • Frequency Modulation Screen  Frequency Modulation Screening, also known as Stochastic screening, is a halftone process based on pseudo-random distribution of halftone dots to change the density of dots according to the desired gray level.  FM screening keeps the dots the same size and varies the frequency, or number, of dots and the location of those dots to simulate the original image. This illustration shows enlarged simulations of FM screening.
  • FM Screening Sample  You can see from these illustrations that, regardless of the type of screen employed, the eye perceives a shade of gray depending on the percentage of ink coverage.  In FM screening, the concepts of screen angle and frequency no longer apply. Because the dots are randomly placed, there is no direction (the screen angle used in AM screening) to the dots. The variable spacing of the dots means there is no fixed spacing, and therefore there is no screen frequency.
  • Who Can Benefit from FM Screening…  Applications for FM screening span the industry: newspapers, magazines, commercial printers. The key to successful use of any FM screening technology is summed up in one word: control. The user who successfully controls the production environment can realize the considerable benefits of FM screening.  Magazines Magazines with in-house image setting operations can use FM screening to produce high-quality color. Once the dot gain calibration has been performed in collaboration with the printer for the press, paper, and ink, the magazine can use these settings for every production run.
  • Newspapers  Newspapers can also use FM screening to good advantage. Newspaper industry tests with frequency modulated screening show higher quality color and image detail than with conventional screening, due to the relative coarseness of the conventional screen used with newsprint.  Newspapers can output pages more quickly at lower resolutions -- high enough to satisfy typographic quality requirements -- and the color images will be of much higher quality than if conventional halftone screening were used.
  • Define the term Image Resolution  Image resolution refers to the spacing of pixels in the images and is measured in pixels per inch(ppi). Resolution is the number of pixels or dots per inch (ppi/dpi). Typical resolution values vary between 72 ppi (pixels per inch) for a computer screen up to 2400 ppi for very high quality professional printing. The higher the resolution, the more detail is stored for the graphic. This means that a higher resolution image is higher quality, but it is also larger in file size.
  • Advantages  The screening of four colors are no longer made with four different angles as with the traditional screen therefore it eliminates screening moiré.  FM screening does not create rosette patterns.  Halftone dot sizes can be as fine as 10 micrometres, which gives at the product a quality comparable to that of photographic prints.[citation needed]  The effects of misregistration is not completely eliminated, but the effect is certainly less apparent than in the traditional screening, this feature is very favorable for printing on rotary machines where the misregistration is very common due to effects such as web growth.  The use of FM screening allowed Archant, a UK regional publisher, to switch to fonts with "tiny holes"; such an "eco-font" permitted a reduction in ink without turning fine text grainy
  • Disadvantages:  The small dots used in FM screening require special care and cleanliness, especially when plates are made from films. Measure the Resolution of an image  The first thing you need to be aware of when dealing with digital images is the resolution of the image. Simply put, an images resolution is determined by how many pixels of information exist per inch within the image at 100 percent of the size it will be printed. Your digital images are very similar to a mosaic made with colored tiles. Pixels are the tiles with which the image is built. The more tiles you can use per inch, the more refined the detail can be within the image.
  •  Here's an illustration of the differences you might expect to see in image quality between a 72 pixel per inch image and a 300 pixel per inch image. The image on the left shows a detailed look at what 72 pixels per inch looks like up close.  72 pixels per inch is considered low resolution although it is perfectly fine for a website or other on-screen display, it is not really good enough for a high-resolution image setter which will reproduce your artwork to be printed on a printing press. The 300 pixel per inch image has over three times as much detail. 72 dpi close up 72 dpi imag 300 dpi image 300 dpi close up (Smaller in size as compared to an 300 dpi image)
  • When working with images destined for a printing press you should be using a general rule of thumb of 2 to 1.  The digital image should posses twice as many pixels per inch as the line screen in which your artwork will be printed at. So Cal Graphics likes to print images at 175 line screen or 150 line screen. So, using the 2 to 1 rule, your images should be scanned at 350 or 300 pixels per inch for maximum quality.  When in doubt, you should be safe saving your images at 100% of the size they will print, CMYK (or grayscale if you are printing 1 color), 300 pixels per inch for photos and 600 pixels per inch for hard-edged line art, and in TIF format.
  • Choosing the Resolution  Choosing the resolution of a new picture depends on several factors. If you are creating graphics for the web and most other non-printing applications, you can choose whatever you prefer because the image pixels are mapped 1:1 to the screen pixels. In these cases you will enter the image size in pixels and not a real world unit. I suggest you use the default resolution of 72 ppi so you get halfway meaningful numbers in information dialogs.  For printing, the most important factor is the hardware resolution of the target device. It makes no sense to choose a resolution that exceeds the capabilities of the device. While it is usually no problem for most printers to scale the image down, the larger size can significantly slow down processing and printing without any benefits.
  •  For black and white pictures you will usually match the image resolution to the printer resolution to get the best quality possible. Since no scaling is involved and the color information does not take a lot of space (1 bit per pixel), there are few reasons why you should go lower.  Grayscale or color pictures take some more consideration. You should keep in mind that almost all printing methods put a pattern of points of the four base printing colors (cyan, magenta, yellow and black) on the paper to fake a large range of colors. Especially with colorful scanned pictures, half or even a quarter of the hardware resolution will result in the best quality possible. Going higher will result in exactly the same picture and therefore waste precious resources. For example if you have one of the common ink jet printers with 300 dpi resolution in color mode, an image with 150 ppi will usually result in the best quality.
  • Identify the steps to modify image resolution based on specifications  The most common setting for computer monitors is 72 dpi; therefore, images intended for web-based or other on-screen projects should have a resolution of 72 dpi. This lower resolution will reduce file sizes and download times. The normal resolution for low-end printing is 150 dpi, and for high-end, professional, quality photographs the resolution should be 300 dpi or higher.
  • To change the resolution of a graphic, pull down the Image menu and select Image Size. The following dialog box will appear:
  • Change the resolution in the Resolution box (in blue above). Keep these facts in mind:  If you have a 72-dpi graphic and you change it to a higher resolution, you will not increase the quality of the graphic. The information was originally stored at 72 dpi, and the computer cannot “make up” more information to improve the graphic quality.  If you have a 300-dpi graphic and you change it to a lower resolution (say, 72 dpi), you will decrease the quality of the graphic. This is because you will lose the extra pixels of information. If you need to reduce the resolution of a graphic, save the original, high-resolution image in case you need it in the future.