There are several systems of digital image-retouching:
Manipulation for photo restoration or enhancement (adjusting
colors / contrast / white balance (i.e. gradational retouching), sharpness,
removing elements or visible flaws on skin or materials)
Creative retouching:
Used as an art form or for commercial use to create more
sleek and interesting creative images for advertisements. Creative retouching
could be manipulation for fashion, beauty or advertising photography such as
pack-shots (which could also be considered inherently technical retouching in
regards to package dimensions and wrap-around factors) One of the most
prominent disciplines in creative retouching is image-compositing. Here, the
digital artist uses multiple photos to create a single composited image. Today,
3D elements are used more and more to add extra elements or even locations and
backgrounds. This kind of image composition is widely used when conventional
photography would be technically too difficult or impossible to shoot on location
or in studio.
History:
Goebbels family
portrait photo in which the visage of the uniformed Harald, who was actually
away on military duties, was inserted and retouched
Before computers, photo manipulation was achieved by
retouching with ink, paint, double-exposure, piecing photos or negatives
together in the darkroom, or scratching Polaroids. Airbrushes were also used,
whence the term "airbrushing" for manipulation. Darkroom
manipulations are sometimes regarded as traditional art rather than job related
skill. In the early days of photography, the use of technology was not as
advanced and efficient as it is now. Results are similar to digital
manipulation but they are harder to create.
An early example of tampering was in the early 1860s, when a
photo of Abraham Lincoln was altered using the body from a portrait of John C.
Calhoun and the head of Lincoln from a famous seated portrait by Mathew Brady –
the same portrait which was the basis for the original Lincoln five-dollar
bill.
The 1980s saw the advent of digital retouching with Quantel
computers running Paintbox, and Scitex imaging workstations being used
professionally.[citation needed] Silicon Graphics computers running Barco
Creator became available in the late 1980s which, alongside other contemporary
packages, were effectively replaced in the market by Adobe Photoshop.
Raster images are
stored in a computer in the form of a grid of picture elements, or 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 graphics software, such as Adobe Illustrator, CorelDRAW, Xara
Designer Pro or Inkscape, are used to create and modify vector images, which
are stored as descriptions of lines, Bézier splines, and text instead of
pixels. It is easier to rasterize a vector image than to vectorize a raster
image; how to go about vectorizing a raster image is the focus of much research
in the field of computer vision. Vector images can be modified more easily,
because they contain descriptions of the shapes for easy rearrangement. They
are also scalable, being rasterizable at any resolution.
Automatic image
enhancement:
Camera or computer image editing programs often offer basic
automatic image enhancement features that correct color hue and brightness
imbalances as well as other image editing features, such as red eye removal,
sharpness adjustments, zoom features and automatic cropping. These are called
automatic because generally they happen without user interaction or are offered
with one click of a button or mouse button or by selecting an option from a
menu. Additionally, some automatic editing features offer a combination of
editing actions with little or no user interaction.
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, such as those used in PNG file
format, are lossless, which means no information is lost when the file is
saved. By contrast, the JPEG file format uses a lossy compression algorithm by
which the greater the compression, the more information is lost, ultimately
reducing image quality or detail that can not be restored. JPEG uses knowledge
of the way the human brain and eyes perceive color to make this 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 for
selecting rectangular or other regular polygon-shaped regions,
a lasso tool for
freehand selection of a region,
a magic wand tool
that selects objects or regions in the image defined by proximity of color or
luminance,
vector-based pen
tools,
as well as more advanced facilities such as edge detection,
masking, alpha compositing, and color and channel-based extraction. The border
of a selected area in an image is often animated with the marching ants effect
to help the user to distinguish the selection border from the image background.
Layers
Main article: Layers (digital image editing)
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 while 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
Main article: Cropping (image)
Digital editors are used to crop images. Cropping creates a
new image by selecting 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.
Uncropped image from
camera
Lily cropped from larger image
Histogram
Main article: Curve (tonality)
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.
Sunflower image
Histogram of Sunflower image
Noise reduction
Main article: Noise reduction
Image editors may feature a number of algorithms which can
add or remove noise in an image. Some JPEG artifacts can be removed; dust and
scratches can be removed and an image can be de-speckled. Noise reduction
merely estimates the state of the scene without the noise and is not a
substitute for obtaining a "cleaner" image. Excessive noise reduction
leads to a loss of detail, and its application is hence subject to a trade-off
between the undesirability of the noise itself and that of the reduction
artifacts.
Noise tends to invade images when pictures are taken in low
light settings. A new picture can be given an 'antiqued' effect by adding
uniform monochrome noise.
Removal of unwanted
elements
Main article: Inpainting
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.
Notice the branch in the original
The eye is drawn to the center of the globe
Selective color
change
Some image editors have color swapping abilities to
selectively change the color of specific items in an image, given that the
selected items are within a specific color range.
Selective color change
Image orientation
Image orientation (from left to right): original, −30° CCW
rotation, and flipped.
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 horizontally flipped or vertically flopped. A small rotation of several
degrees is often enough to level the horizon, correct verticality (of a
building, for example), or both. Rotated images usually require cropping
afterwards, in order to remove the resulting gaps at the image edges.
Perspective control
and distortion
Main article: Perspective control
Perspective control: original (left), perspective distortion
removed (right).
Some image editors allow the user to distort (or
"transform") the shape of an image. While this might also be useful
for special effects, it is the preferred method of correcting the typical
perspective distortion which results from photographs being taken at an oblique
angle to a rectilinear subject. Care is needed while performing this task, as
the image is reprocessed using interpolation of adjacent pixels, which may
reduce overall image definition. The effect mimics the use of a perspective
control lens, which achieves a similar correction in-camera without loss of
definition.
Lens correction
Photo manipulation packages have functions to correct images
for various lens distortions including pincushion, fisheye and barrel
distortions. The corrections are in most cases subtle, but can improve the
appearance of some photographs.
Enhancing images
In computer graphics, the process of improving the quality
of a digitally stored image by manipulating the image with software. It is
quite easy, for example, to make an image lighter or darker, or to increase or
decrease contrast. Advanced photo enhancement software also supports many
filters for altering images in various ways.[1] Programs specialized for image
enhancement are sometimes called image editors.
Sharpening and
softening images
Graphics programs can be used to both sharpen and blur
images in a number of ways, such as unsharp masking or deconvolution.[2]
Portraits often appear more pleasing when selectively softened (particularly
the skin and the background) to better make the subject stand out.[citation
needed] This can be achieved with a camera by using a large aperture, or in the
image editor by making a selection and then blurring it. Edge enhancement is an
extremely common technique used to make images appear sharper, although purists
frown on the result as appearing unnatural.
Selecting and merging
of images
Main article: Digital compositing
Photomontage of 16 photos which have been digitally
manipulated in Photoshop to give the impression that it is a real landscape
Many graphics applications are capable of merging one or
more individual images into a single file. The orientation and placement of
each image can be controlled.
When selecting a raster image that is not rectangular, it
requires separating the edges from the background, also known as silhouetting.
This is the digital analog of cutting out the image from a physical picture.
Clipping paths may be used to add silhouetted images to vector graphics or page
layout files that retain vector data. Alpha compositing, allows for soft
translucent edges when selecting images. There are a number of ways to
silhouette an image with soft edges, including selecting the image or its
background by sampling similar colors, selecting the edges by raster tracing,
or converting a clipping path to a raster selection. Once the image is
selected, it may be copied and pasted into another section of the same file, or
into a separate file. The selection may also be saved in what is known as an
alpha channel.
A popular way to create a composite image is to use
transparent layers. The background image is used as the bottom layer, and the
image with parts to be added are placed in a layer above that. Using an image
layer mask, all but the parts to be merged are hidden from the layer, giving
the impression that these parts have been added to the background layer. Performing
a merge in this manner preserves all of the pixel data on both layers to more
easily enable future changes in the new merged image.
Slicing of images
A more recent tool in digital image editing software is the
image slicer. Parts of images for graphical user interfaces or web pages are
easily sliced, labeled and saved separately from whole images so the parts can
be handled individually by the display medium. This is useful to allow dynamic
swapping via interactivity or animating parts of an image in the final
presentation.
See also: Slicing (interface design)
Special effects
An example of some special effects that can be added to a
picture.
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 transforms and texture
effects,[3] or combinations thereof.
Change color depth
An example of converting an image from color to grayscale
It is possible, using software, to change the color depth of
images. Common color depths are 2, 4, 16, 256, 65,536 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
down-sampling from a full-color image. Grayscale conversion is useful for
reducing file size dramatically when the original photographic print was
monochrome, but a color tint has been introduced due to aging effects.
Contrast change and
brightening
An example of contrast correction. Left side of the image is
untouched.
Image editors have provisions to simultaneously change the
contrast of images and brighten or darken the image. Underexposed images can
often 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. The exact transformation that is
applied to each color channel can vary from editor to editor. GIMP applies the
following formula:[4]
if (brightness < 0.0)
value = value * ( 1.0 + brightness);
else
value = value + ((1 - value) * brightness);
value = (value - 0.5) * (tan ((contrast + 1) * PI/4) ) +
0.5;
where value is the input color value in the 0..1 range and
brightness and contrast are in the −1..1 range.
Gamma correction
Main article: Gamma correction
In addition to the capability of changing the images'
brightness and/or contrast in a non-linear fashion, most current image editors
provide an opportunity to manipulate the images' gamma value.
Gamma correction is particularly useful for bringing details
that would be hard to see on most computer monitors out of shadows. In some
image editing software this is called "curves", usually a tool found
in the color menu, and no reference to "gamma" is used anywhere in
the program or the program documentation. Strictly speaking, the curves tool
usually does more than simple gamma correction, since one can construct complex
curves with multiple inflection points, but when no dedicated gamma correction
tool is provided, it can achieve the same effect.
Color adjustments
An example of color adjustment using raster graphics editor
The color of images can be altered in a variety of ways.
Colors can be faded in and out, and tones can be changed using curves or other
tools. The color balance can be improved, which is important if the picture was
shot indoors with daylight film, or shot on a camera with the white balance
incorrectly set. Special effects, like sepia tone 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.
The red-eye effect, which occurs when flash photos are taken
when the pupil is too widely open (so that light from the flash that passes
into the eye through the pupil reflects off the fundus at the back of the
eyeball), can also be eliminated at this stage.
Printing
Control printed image by changing pixels-per-inch.
Controlling the print size and quality of digital images
requires an understanding of the pixels-per-inch (ppi) variable that is stored
in the image file and sometimes used to control the size of the printed image.
Within Adobe Photoshop's Image Size dialog, the image editor allows the user to
manipulate both pixel dimensions and the size of the image on the printed
document. These parameters work together to produce a printed image of the
desired size and quality. Pixels per inch of the image, pixel per inch of the
computer monitor, and dots per inch on the printed document are related, but in
use are very different. The Image Size dialog can be used as an image
calculator of sorts. For example, a 1600 × 1200 image with a resolution of 200
ppi will produce a printed image of 8 × 6 inches. The same image with 400 ppi
will produce a printed image of 4 × 3 inches. Change the resolution to 800 ppi,
and the same image now prints out at 2 × 1.5 inches. All three printed images
contain the same data (1600 × 1200 pixels), but the pixels are closer together
on the smaller prints, so the smaller images will potentially look sharp when
the larger ones do not. The quality of the image will also depend on the
capability of the printer.
Clipping path:
Clipping path services are professional offerings provided
by companies for extracting objects or people from still imagery, and typically
includes other photo editing and manipulation services. Addressees of such
services are primarily photography and graphic design studios, advertising
agencies, web designers, as well as lithographers and printing companies.
Clipping path service companies commonly reside in developing countries such as
Bangladesh, Philippine, India, and Pakistan, which can provide their services
at comparatively low cost to developed countries, fostering outsourcing of such
activities.
3d clipping path:
Clipping planes are used in 3D computer graphics in order to
prevent the renderer from calculating surfaces at an extreme distance from the
viewer. The plane is perpendicular to the camera, a set distance away (the
threshold), and occupies the entire viewport. Used in real-time rendering,
clipping planes can help preserve processing for objects within clear sight.
The use of clipping planes can result in a detraction from
the realism of a scene, as the viewer may notice that everything at the
threshold is not rendered correctly or seems to (dis)appear spontaneously. The
addition of fog—a variably transparent region of color or texture just before
the clipping plane—can help soften the transition between what should be in
plain sight and opaque, and what should be beyond notice and fully transparent,
and therefore does not need to be rendered.
