Digital image editing

Digital image editing is the process of altering digital images, whether they be digital photographs or other types of digitally represented images. Specialised software programs called vector graphics editors or bitmap graphics editors are the primary tools with which a user may manipulate, enhance, and transform images. These editors are capable of editing images in many diverse ways.

Digital imaging is closely associated with digital photography and is used extensively in the fields of science, medicine, and forensics.

Basics of image editing

Bitmap images are stored in a computer in the form of a grid of picture elements called pixels. These pixels contain the image's color and brightness information. Image editors can change the pixels to enhance the image in many ways. The pixels can be changed as a group, or individually, by the sophisticated algorithms within the image editors. The domain of this article primarily refers to bitmap graphics editors, which are often used to alter photographs and other raster graphics. However, vector-based software, such as Adobe Illustrator can be used to alter or construct other types of image, namely ones based upon vectors.

Image editing programs

Because of the popularity of digital cameras, image editing programs are readily available. Minimal programs, that perform such operations as rotating and cropping are often provided within the digital camera itself, while others are returned to the user on a compact disc (CD) when images are processed at a discount store. The more powerful programs contain functionality to perform a large variety of advanced image manipulations. Popular raster-based digital image editors include Paint Shop Pro, Adobe Photoshop, Visualizer Photo Studio, Pixel image editor, PixBuilder Photo Editor, Fo2Pix ArtMaster and the GIMP. For more, including free programs, see: List of bitmap graphics editors.

Digital data compression

Many image file formats use data compression to reduce file size and save storage space. Digital compression of images may take place in the camera, or can be done in the computer with the image editor. When images are stored in JPEG format, compression has already taken place. Both cameras and computer programs allow the user to set the level of compression.

Some compression algorithms are lossless, such as PNG, which means no image quality is lost when the file is saved. The JPEG compression algorithm uses a lossy format. The greater the compression, the lesser the quality. It utilizes the way the brain and eyes perceive colour to make loss of detail less noticeable.

Image editor features

Listed below are some of the most used capabilities of the better graphic manipulation programs. The list is by no means all inclusive. There are a myriad of choices associated with the application of most of these features.

Selection

One of the prerequisites for many of the applications mentioned below is a method of selecting part(s) of an image, thus applying a change selectively without affecting the entire picture. Most graphics programs have several means of accomplishing this, such as a marquee tool, lasso, vector-based pen tools as well as more advanced facilities such as edge detection, masking, alpha compositing, and color and channel-based extraction.

Layers

Another feature common to many graphics applications is that of 'Layers', which are analogous to sheets of transparent acetate (each containing separate elements that make up a combined picture), stacked on top of each other, each capable of being individually positioned, altered and blended with the layers below, without affecting any of the elements on the other layers. This is a fundamental workflow which has become the norm for the majority of programs on the market today, and enables maximum flexibility for the user whilst maintaining non-destructive editing principles and ease of use.

Image size alteration

Image editors can resize images in a process often called image scaling, making them larger, or smaller. High image resolution cameras can produce large images which are often reduced in size for Internet use. Image editor programs use a mathematical process called resampling to calculate new pixel values whose spacing is larger or smaller than the original pixel values. Images for Internet use are kept small, say 640 x 480 pixels which would equal 0.3 megapixels.

Cropping an image

Digital editors are used to crop images. Cropping creates a new image by removing a desired rectangular portion from the image being cropped. The unwanted part of the image is discarded. Image cropping does not reduce the resolution of the area cropped. Best results are obtained when the original image has a high resolution. A primary reason for cropping is to improve the image composition in the new image. LillyCroppedacp.jpg

Histogram

Image editors have provisions to create an image histogram of the image being edited. The histogram plots the number of pixels in the image (vertical axis) with a particular brightness value (horizontal axis). Algorithms in the digital editor allow the user to visually adjust the brightness value of each pixel and to dynamically display the results as adjustments are made. Improvements in picture brightness and contrast can thus be obtained.

Noise removal

Image editors may feature a number of algorithms which can add or remove noise in an image. JPEG artifacts can be removed; dust & scratches can be removed and an image can be despeckled. Noise tends to invade images when pictures are taken in low light settings. A new picture can be given an 'antiquated' effect by adding uniform monochrome noise. A more thorough discussion of camera noise can be found at:Explanation of noise

Removal of Unwanted elements

Most image editors can be used to remove unwanted branches, etc, using a "clone" tool. Removing these distracting elements draws focus to the subject, improving overall composition

Selective color change

Image editors have the ability to selectively change the color of specific items in an image, given that the selected items are within a specific color range.

Image gradient

A gradient tool provides gradual blend of color which can be considered as an even gradation from low to high values, as shown in the illustration with arrows added showing the directions along which the image fades from white to black.

Image orientation

Image editors are capable of altering an image to be rotated in any direction and to any degree. Mirror images can be created and images can be flipped (turned upside down). Images that are out of line can be straightened and perspective can be adjusted.

Merging of images

Many graphics applications are capable of merging one or more individual images into a single picture. The orientation and placement of each image can be controlled. The two images shown here were once individual studio portraits.

Special effects

Image editors usually have a list of special effects that can create unusual results. Images may be skewed and distorted in various ways. Scores of special effects can be applied to an image which include various forms of distortion, artistic effects, geometric and texture effects, and combinations thereof.

Change color depth

It is possible, using software, to change the color depth of images. Common color depths are 2, 4, 16, 256, 65.5 thousand and 16.7 million colors. The JPEG and PNG image formats are capable of storing 16.7 million colors (equal to 256 luminance values per color channel). In addition, grayscale images of 8 bits or less can be created, usually via conversion and downsampling from a full color image.

Lens correction

Photo manipulation packages have functions to correct images for various lens distortions including pincushion, fisheye and barrel distortions. The corrections are subtle, but can improve the appearance of some photographs.

Contrast change and brightening

Image editors have provisions to change the contrast of images and brighten or darken the image. Many an underexposed image can be improved by using this feature. Recent advances have allowed more intelligent exposure correction whereby only pixels below a particular luminosity threshold are brightened, thereby brightening underexposed shadows without affecting the rest of the image.

Sharpening and softening images

Graphics programs can be used to both sharpen and blur images in a number of ways. Portraits often appear more pleasing when softened (particularly the background, to make the subject stand out more — this is an example of shallow depth of field which can be achieved with a camera by using a large aperture, or artificially within software, by selecting the background portion of an image, and then blurring it). The red-eye effect, which occurs when flash photos are taken when the pupil is too widely open (thus reflecting back the colour of the blood-rich retina), can also be eliminated. Edge enhancement is an extremely common technique used to make images appear sharper, although many purists frown on the end result as less natural-looking.

Color adjustments

The color of images can be altered in a variety of ways. Colors and be faded in and out, and tones can be changed, and the color balance can be improved, which is important if the picture was shot indoors with daylight film, or shot on a camera that with an incorrectly adjusted white balance. Special effects, like sepia and grayscale can be added to an image. In addition, more complicated procedures such as the mixing of color channels are possible using more advanced graphics editors.

Conclusion

Image editors provide the means for altering and improving images in an almost endless number of ways. They accept images in a large variety of image formats, and after changes are made they allow the changed image to be saved in any number of image formats, at almost any size, and in varying degrees of compression. Most serious digital camera users will find the facilities of such a tool indispensable. In the hands of amateurs, they will obviously render amateurish results. However, for a beginner, the novelty of the power and features available to them, combined with their ease of use, image editing can be a lot of fun.

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