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Archive for September, 2010

There are three common methods of capturing images digitally which are outlined below:

1. Scanning

A scanner is a device used to optically scan an image, handwriting, printed document or object and convert them into a digital image.

One of the most common pieces of hardware for image capture is a flatbed scanner; a device with a glass plate under a hinged lid, underneath the plate a moving light passes across and produces a digital image of the object that has been placed on the glass. It is attached to the computer either by a parallel port, USB port or SCSI port. Software called a TWAIN driver operates the scanner and is normally accessed via an image-editing program such as Photoshop. The TWAIN driver is specific to the model and make of scanner and is the user interface as well as the interface between the graphics program and the hardware. To use a flatbed scanner the item to be scanned is placed faced down on the glass plate, the lid is placed down on top of this. The scanner is normally accessed from within the image-editing program by the following commands: File – acquire – import – (scanner model) This will display a window showing the scanners controls and a preview of the item to be scanned. The controls specify the mode (colour, black and white, line art) and resolution as well as additional items such as tonal quality or colour balance, the scan can also be increased or decreased in size at this point. A portion of the item to be scanned can also be selected if the whole is not required. Once the scan is completed it is taken into the image-editing program to be further worked on and saved in an appropriate format. 

2. Screengrab

An screengrab is an image taken by the computer to record visible items displayed on the monitor. They are also commonly known as screenshots or screendumps.

Screengrabs are normally used to demonstrate a program, or can be used as a way of recording and archiving activity taking place on the monitor. PCs and Macs use different controls to create screengrabs. PC Microsoft Windows Press the print screen button on the keyboard. This captures a screengrab of the entire desktop and stores it in the PCs clipboard (a temporary storage space, where an item is placed when copied or cut). Alternatively, if the Alt key is held down when pressing the print screen button only the active window will be captured. The item can then be pasted into any applicable program. If a file of that image is required it will need to be pasted into an image-editing program and saved. The resolution of the screen grab will be 72 dpi.  Windows Vista Windows Vista has a snipping tool to make capturing screengrabs easier. It allows an area to be selected by clicking and dragging the cursor, when the mouse is released the ‘captured’ area appears in the snipping tool window from where it can be saved as an image file.  Mac Users can take a screengrab in various ways:

  1. Press command, shift, 3 to capture an image of the entire desktop 
  2. Press command, shift, 4 to capture a chosen area – crosshairs appear which are dragged across the screen, when the mouse is released a screengrab is made of the selected area. 
  3. Press command, shift, 4, spacebar to capture a specific application. The cursor changes to a camera and the application window will be highlighted. Once the mouse is clicked a screengrab is made. 

All screengrabs are automatically saved as PNG files on the desktop, the resolution of the files is 72dpi. By adding control to the above commands – the screengrabs are saved in the clipboard rather than saved as a PNG file. Another method of screengrab on the Mac is to use the utility Grab. This tool allows various methods of capture, the inclusion of cursors and files are saved as tiffs.  It is recommended screengrab files are saved as GIFs or PNGs. JPGs generally make screengrabs look blurry and blotchy especially if the files contain text.

3. Digital Camera

A digital camera contains no film but records images as digital objects, which can then be downloaded into a computer for further processing. The images are stored on a memory card situated in the camera.        


Digital cameras, like traditional film cameras range from compact point and shoot models with fixed retractable lenses to high-end single lens reflex cameras with interchangeable lenses. Digital cameras have electronic displays on the back, which are commonly used as a viewfinder in the average point and shoot model. It is known as live preview and allows for control of framing and exposure before taking the photo. There is also an array of options for controlling what kind of image is taken and video is normally included. Most cameras store images as JPEGS, though they can be stored, especially in more expensive high-end models as raw files, which are rather like digital negatives. However this option makes very large file sizes and is specific to the make of the camera, so the appropriate software is needed to read them once they have been transferred to a computer. There are various ways of transferring images to the computer. The most common method is to download straight from the camera, which is normally attached to the computer via a USB lead. Some cameras offer wireless connections via Bluetooth, or a card reader can be used, whereby the memory card is removed from the camera and inserted into a device that is capable of high speed transfer of data to the computer. Images can be downloaded and stored in digital photo collections such as iphoto. These programs allow the user to store and sort photos in the method most appropriate to them using categories such as date, event, person etc.. Photos can then be transferred from these collections by cutting and pasting or exporting so that that they can be worked on in image editing programs such as Photoshop.

Problems associated with image capture

Moiré patterns

Scanned images from printed materials can produce interference called a moiré pattern.

The interference is a conflict between two sets of fine patterned grids – the scanner samples and the halftone screen of the original image.  

 A printed halftone image is comprised of a pattern of dots – a black and white image is made up of a single screen of black while a colour image comprises four screens – black and the three primary colours (CMYK).

The scanned image is also composed of dots (pixels).

If these two sets of dots overlap at certain angles to each other they produce a moire pattern that often appear as wavy or rippled lines or a plaid like pattern.

Pixelation

Pixelation is a problem unique to bitmapped images.

A bitmapped image is comprised of pixels in a grid each of which is assigned a location and colour value.

The image contains a fixed number of pixels and if enlarged or printed at a lower resolution than intended become jagged looking and lose definition. 

Resolution

Resolution is the dots of ink or electronic pixels that comprise a picture whether it is printed or displayed on screen.

It can be measured in several ways – the most common term in print being DPI (dots per inch). The more dots per inch , the more detail the image will contain. Low resolution images (e.g. 72dpi ) will not print out very well,  as they contain minimum information and can look pixelated. For printed images it is better to scan, output photos or create bitmapped images at a minimum resolution of 300dpi to produce good crisp results.

For web –based or multimedia images are displayed at the screen resolution of the monitor – which averages out at about 72ppi (pixels per inch). Therefore to use an image of higher resolution is only going to produce a larger file size and not a better image. As download times are a major consideration for web images and are dependent on the file size, the resolution does not need to be any more than 72dpi. Images will occupy larger or smaller areas of the screen depending on the size/resolution of the monitor. However it is best to plan ahead and output the image at the size required. This will avoid resizing and all the problems that this can bring such as pixelation.  

Colour casts

A colour cast occurs in photography when an unwanted tint of a particular colour affects the whole of an image evenly.

It is often a true reflection of the scene that has not been noticed by the photographer  as our eyes and brain adjust and compensate for different types of light in ways the camera cannot. Common colour casts are caused by incandescent or florescent lighting and sunlight. High end cameras can compensate for this by using settings such as white balance. Otherwise photos can be imported into  image editing programs such as Photoshop where there are tools for colour correction.

Image Formats

JPEG (Joint Photographic Expert Group)

JPEG images are the most commonly used file format. They are compressed images than retain the full range of colour in an image, but become compromised in clarity when significantly compressed where they tend to look blocky and fuzzy.

JPEG is a lossy format which means a file will lose or chuck out information in  order to compress and be able to produce a smaller file size.

There are different qualities of compression ranging from high to low – A JPEG saved as a high quality JPEG will retain more information than one saved at a low quality, and will therefore be a better image but also be a larger file size.

JPEGs are a good file format for photographic images, not so good for graphics with flat colour or text as compression can add artifacts and smear text, lines and edges.

GIF (Graphics Interchange Format)

GIF is a bitmap image format, which is in common use on the web because of its wide support and portability.

It is a lossless format , which means no loss of quality due to compression.

The format supports up to 8 bits per pixel, which allows a single image a palette of 256 colours that are chosen from the 24-bit RGB colour space. Because it is unable to display the full colour range of a JPEG (i.e. millions of colours) it is not a format suitable to photographs or images with a lot of detail. However it does make it ideal for flat graphics with text or lines and sharp edges as it does not produce artifacts caused by compression.

GIFs can be larger file sizes than JPEGs

GIFs can also be animated.

TIFF (Tag Image File Format)

Tiffs are very versatile and compatible across a range of projects from photos through to graphics. They support up to 48-bit colour space and can be compressed in various formats (which are not all supported across all systems, so care has to be taken in choosing the method of compression).

It is a lossless format, the files size can be large because of this, but  contain lots of detail. As they are lossless, the file is exactly the same as the original digital image and is therefore ideal for archiving or using as a master copy.

They are the standard universal format for high quality images.

PICT

PICT was developed by Apple in 1984 as the native format for Macintosh graphics. It is a meta-format, which means it can be used for both vector and bitmap images. PICTS were used to exchange graphics between various Macintosh applications but has since been superseded by PDFs. It is therefore not now in common usage.

BMP (Bitmap or DIB (Device-Independent Bitmap) 

BMP is the standard Windows image format. They can range from black and 1bit per pixel (black and white) up to 24 bit colour (16.7 million colours).

It is a lossless format and is suitable for photos, pictures or graphics, but creates large files sizes though there are rarely used methods of compression.

PNG (Portable Network Graphic)

This was a file format designed to replace the simpler GIF format , but does not support animation.

It has lossless compression , so there is no loss of data, so therefore no smudging or blurring.

It has a range of fifteen colour options – up to 24 bit (and for specialist requirements 48 bit true colour and 16 bit greyscale).

Like GIF,  PNG supports transparency.

PNG file sizes can be large (up to twice the size of JPEGS and three times the size of  GIFs).

They are good for both photos and graphics.

EPS (Encapsulated Post Script)

EPS is the standard file format for importing and exporting PostScript files.

It is primarily a vector file (images created by line and curves produced by mathematical objects are known as vectors) and can be rescaled to any size without loss of quality as they are not bitmap dependent. However EPS files can also contain bitmapped images.

EPS files are generated by drawing programs such as Illustrator and also layout programs such as In-Design for import into other relevant programmes. They are not a file format supported by the Web. 

 

 

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