Scanner

An image scanner is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. A flatbed scanner is most commonly used for print and web.

The document or item is placed on a glass window for scanning. Under the glass is a bright light which illuminates the pane, and a moving optical array.

Images or objects to be scanned are placed face down on the glass, an opaque cover is lowered over it to exclude ambient light, and the sensor array and light source move across the pane, reading the entire area. An image is visible to the detector only because of the light it reflects. Transparent images don’t work this way, and need special accessories that illuminate them from the upper side.

Film can also be scanned, ‘slide’ (positive) or negative using equipment specially made for this. Usually, uncut film strips of up to six frames, or four mounted slides, are inserted in a carrier, which is moved by a stepper motor across a lens and CCD sensor inside the scanner.

Scanners usually read red-green-blue color (RGB) data from the array. This data is then processed  to correct for different exposure conditions, and sent to the computer via the  SCSI or USB ports. Color depth varies depending on the scanning array characteristics, but is usually at least 24 bits. High quality scanners have 48 bits or more color depth.

The color and brightness of each tiny area seen by a sensor is measured and recorded as a numeric value which represents the color there. This process is called digitizing the image. The data is organized into the same rows and columns to retain the location of each actual tiny picture area.

Each one of these sampled numeric color data values is called a pixel. Digital images consist of pixels, and are measured in pixels.

Scanners  have different resolutions depending on the model, measured in pixels per inch (ppi), or more accurately as Samples per inch (spi).
The choice of scan resolution is based on the needs of the output device that will process that image, normally a printer or a video monitor screen. Monitors and printers work very differently from each other and so different rules apply when making a scan for each of these.

When scanning for screen display you need only be concerned with pixel dimensions. A computer monitor displays approximately 96 pixels per inch, rounded off to 100 ppi. So, if an image is to appear approximately 1 1/2 by 2 inches on a web page, it would be scanned  at 150 by 200 pixels.

We should remember three main things

*  Use no more than 100 dpi/ppi when scanning for screen display
* Measure dimensions in pixels
* 100 pixels on screen is approximately one inch

Computer speed is not an issue, but disk space is – lots of large images will consume lots of disk space.

Screenshots

A screenshot, screen dump, screengrab, or print screen is an image taken by the computer to record the visible items displayed on the monitor.

Screenshots, screen dumps, or screengrabs can be used to demonstrate a program, a particular problem a user might be having or generally when display output needs to be shown to others or archived.

On a PC it is straight forward to make a screenshot. There is a ‘Print Screen’ key on the keyboard, probably labeled [PrtScn]. Press this when you have what you would like to copy on the screen.

Open an image editing programme, such as Photoshop, open up a new page, go to the Edit menu and choose Paste.

Use the crop tool to crop out unnecessary portions of the screen shot. This can be important if you are trying to make the screenshot small enough to load quickly onto a web page.

Go to the File menu and choose Save As.

Navigate to the folder where you want to save the image.

Type a file name for the image.

Select a file type.

To make a screen shot on a Mac, there are various options depending on your requirements.

To capture the entire desktop, press Command-Shift-3. The screen shot will be automatically saved as a PNG  file on your desktop.

To copy the entire desktop, press Command-Control-Shift-3. The screen shot will be placed on your clipboard for you to paste into another program.

To capture a portion of the desktop, press Command-Shift-4. A cross-hair cursor will appear and you can click and drag to select the area you wish to capture. When you release the mouse button, the screen shot will be automatically saved as a PNG file on your desktop.

To capture a specific application window, press Command-Shift-4, then press the Spacebar. The cursor will change to a camera, and you can move it around the screen. As you move the cursor over an application window, the window will be highlighted. The entire window does not need to be visible for you to capture it. When you have the cursor over a window you want to capture, just click the mouse button and the screen shot will be saved as a PNG file on your desktop.

Add Control to the two shortcuts above to place the screen shot on the clipboard instead of saving it to the desktop.

Screen shots almost always compress better and look better as GIFs. There is hardly ever a reason to save screen shots of application windows as JPEG files. The only exception is when the image contains continuous tone areas such as a photo or texture.

The first example was reduced to16 colors and saved as a GIF resulting in a file size of 3,177 bytes.  The second was saved as a JPEG with zero quality and resulted in a file size of 5,678 bytes.

Avoid reducing the pixel dimensions

If possible, it is best to avoid reducing the size of screen shots, particularly if they’ve already been saved to a paletted format like GIF. If the image is already in GIF format, convert the mode to RGB before resizing. Reducing the size of a screen shot also creates additional colors in the image which usually results in a larger GIF file. Usually you can get a smaller file size by using the full size image and reducing the color depth instead of the dimensions.

This example is the original size.

The left hand example was reduced to 50% without converting the color mode to RGB. The resized image is not readable at all. This image is 2,567 bytes. The one on the right was reduced to 50% after converting the color mode to RGB. It’s a bit better than the example above, but the file size is 11,205 bytes. The resampling also caused the colors to increase to 256. It’s still not very readable, so was not worth resizing.

Avoid interpolation when increasing pixel dimensions.
When increasing the size of the screen shot, in most cases you should not use a resampling method that results in blending pixels or anti-aliased (smoothed) edges. Both Bilinear and Bicubic methods in Photoshop blend pixels, which results in blurry screen shots that don’t compress well. Instead, you should choose Nearest Neighbor in the resampling menu of the image size dialog to resize without blending.

The original image is 8 colors, 263 bytes. The second image was resized to 200% using “bicubic” setting, 256 colors, 3,238 bytes and the third was resized to 200% using “nearest neighbor” setting, 8 colors, 569 bytes.

Digital Cameras

A digital camera is a camera that stores images digitally rather than recording them on film. Once a picture has been taken, it can be downloaded  to a computer system, and then manipulated with a graphics program  and printed. Unlike film photographs, which have an almost infinite resolution, digital photos are limited by the amount of memory  in the camera, the optical resolution of the digitising  mechanism, and, finally, by the resolution of the final output device. Even the best digital cameras connected to the best printers cannot produce film-quality photos. The image must eventually be reduced to the resolution of the printer.

The big advantage of digital cameras is that taking photos is both inexpensive and fast because there is no film processing. This means you can experiment with many more pictures.

The memory card that holds all the images you take on your camera is placed into a slot in your printer or computer and the files are transferred as jpegs.

Digital cameras are made in a wide range of sizes, prices and capabilities. The majority are camera phones and compact cameras which are designed to be small and portable and are particularly suitable for casual and ‘snapshot’ use. Professional photographers and many amateurs use larger, more expensive digital single-lens reflex cameras (DSLR) for their greater versatility.

Moire patterns

Scanning images from printed material like magazines has a problem with a herringbone or crosshatched or dotted pattern in the scanned images, one example is shown below.

This interference is called a moiré pattern. In a scanned image, Moiré patterns are caused by interference between two sets of fine pattern grids, the scanner samples and the halftone screen in the original image.

Any image printed on a printing press (like a book, magazine, newspaper, postcard, calendar, etc.) is printed with halftone screen patterns. The printed image is composed of a pattern of dots. These fine dots cause optical problems in a scanned image because the scanned image is also composed of fine dots.

The two patterns of dots, the printed magazine’s 133 or 150 lpi screened pattern, and the 300 or 600 dpi scanner CCD cells, combine into maximums or minimums every several pixels in the image, depending on the spacing of the dots. It affects the overall light intensity in periodic patterns that become very visible.

One way to get around this problem is to use the Scanwizard Descreen filter. Then resample to 33% size (effectively 100 dpi size). Then use the unsharp mask filter.

Pixelation

Pixelation describes the blocky effect that occurs when an image is magnified to the point that the curves and overall picture of an image begins to give way to the pixels that make up the image. It is the display of a digitised image where the individual pixels are apparent to a viewer. This can happen unintentionally when a low-resolution image designed for an ordinary computer display is projected on a large screen and each pixel becomes separately viewable.

In the image above, the blocky image is revealed when zooming in.  This is because all of the original pixels were made larger.  The more an image is scaled, the poorer quality the image will have.

Pixelation is a problem unique to bitmaps. Alternatives such as vector graphics can scale to any level of detail. This is one reason vector graphics are popular for printing — most modern computer monitors have a resolution of about 100 dots per inch, and at 300 dots per inch printed documents have about 9 times as many pixels per unit of area as a screen.

In some cases, the resolution of an image or a portion of an image is lowered to introduce pixelation deliberately. This effect is commonly used on television news shows to obscure a person’s face or to censor something.

Colour casts

A colour cast is a tint of a particular colour, usually unwanted, which affects the whole of a photographic image evenly.

Certain types of light can cause film and digital cameras to have a colour cast. In general, the human eye does not notice the unnatural colour, because our eyes and brains adjust and compensate for different types of light in ways that cameras cannot.

In film, colour casts can also be caused by problems in development. Improper timing or imbalanced chemical mixtures can cause unwanted casts.

There are two main causes of colour cast, sunlight (or cool light) and incandescent light (or warm light). High end digital cameras try to automatically detect and compensate colour cast and usually have a selection of manually set white balance settings to chose from. Otherwise, photo editing programs, such as Photoshop, often have built in colour correction facilities. For film, blue filters and amber filters are used to counter casts. Amber filters are used to reduce the blueish tint caused by daylight. Blue filters reduce the orange colour caused by incandescent light.

Image formats

The PNG, JPEG, and GIF formats are most often used to display images on the Internet.

JPEG

JPEG files are very small files for continuous tone photo images, but JPEG is poor for graphics. JPEG requires 24 bit color or 8 bit grayscale, and the JPG artifacts are most noticeable in the hard edges of graphics. This is the right format for those photo images which must be very small files, for example, for web sites or for email. JPEG is often used on digital camera memory cards.The JPG file is wonderfully small, often compressed to perhaps only 1/10 of the size of the original data, which is a good thing when modems are involved. However, this fantastic compression efficiency comes with a high price. JPEG uses lossy compression (lossy meaning “with losses to quality”). Lossy means that some image quality is lost when the JPEG data is compressed and saved, and this quality can never be recovered.

File compression methods for most other file formats are lossless, and lossless means “fully recoverable”. Lossless compression always returns the original data, bit-for-bit identical without any question about differences (losses).

Gif Graphic Interchange Format

GIF files (and other indexed color files) are good for graphics, but are poor for photos (too few colors possible). However, graphics are normally not 24 bit color anyway.

GIF uses indexed color, which is limited to a palette of only 256 colors (next page). GIF files do NOT store the image’s scaled resolution ppi number, so scaling is necessary every time one is printed. This is of no importance for screen or web images. GIF is an excellent format for graphics, and this is its purpose, especially on the web. Graphic images (like logos or dialog boxes) use few colors. Being limited to 256 colors is not important for a 3 color logo. A 16 color GIF is a very small file, much smaller, and more clear than any JPG, and ideal for graphics on the web.

TIFF – Tag Image File Format

TIFF is the format of choice for archiving important images. TIFF is THE leading commercial and professional image standard. TIFF is the most universal and most widely supported format across all platforms, Mac, Windows, Unix. Data up to 48 bits is supported.

TIFF supports most color spaces, RGB, CMYK, YCbCr, etc. TIFF is a flexible format with many options. The data contains tags to declare what type of data follows. TIFF can store data with bytes in either PC or Mac order. This choice improves efficiency (speed), but all major programs today can read TIFF either way, and TIFF files can be exchanged without problem.

TIFF image files optionally use LZW lossless compression. Lossless means there is no quality loss due to compression. Lossless guarantees that you can always read back exactly what you thought you saved, bit-for-bit identical, without data corruption. This is a critical factor for archiving master copies of important images.

Pict

PICT is a file format that was developed by Apple Computer in 1984 as the native format for Macintosh graphics. PICT files are encoded in QuickDraw commands. The PICT file format is a meta-format that can be used for both bitmap images and vector images.

The PICtor format is a device-independent raster images; the file header stores information about the display hardware (screen resolution, color depth and palette information, bit planes and so on) separately from the actual image information, allowing the image to be properly transferred and displayed on computer systems with different hardware. PIC files commonly stored palette-indexed images ranging from 2 or 4 colors to 16 and 256 colors, although the format has been extended to record true-color (24-bit) images as well.

BMP

The BMP file format, sometimes called bitmap or DIB file format (for device-independent bitmap). BMP is a graphics format used commonly as a simple graphics file format on Microsoft Windows platform.

BMP files are not used for transferring images on the Internet or other slow or capacity-limited media because they are too large and not typically compressed. For example an 800×600 image occupies nearly 1.5 megabytes. However this format it quite commen due to its simplicity and widespread familiarity in MS Windows.

PNG – Portable Network Graphics

PNG supports a large set of technical features, including superior lossless compression from LZ77. Compression in PNG is called the ZIP method, and is like the ‘deflate” method in PKZIP.

PNG incorporates special preprocessing filters that can greatly improve the lossless compression efficiency, especially for typical gradient data found in 24 bit photographic images. This filter preprocessing causes PNG to be a little slower than other formats when reading or writing the file (but all types of compression require processing time).

EPS – Encapsulated PostScript®

A file format used for both vector graphics and bitmaps. EPS files contain a PostScript description of the graphic data within them. EPS files are unique in that you can use them for vector graphics, bitmap images, type or even entire pages. Images can be saved in Illustrator and Photoshop and later amended. You can not use EPS on the web.