Image Export Formats

[Midgardsormr]

Aliasing: Because a raster image is simply a grid of pixels, it is impossible to create a true slanted line or arc. The jaggedness seen at the edges of some images is called aliasing, and it is generally regarded as undesirable. However, if the number of colors available is limited, aliasing is unavoidable. See the image below for an example of aliasing.

Anti-aliasing: Anti-aliasing is a technique for reducing the jagged appearance of slanted lines and arcs. It blends the edges of such figures by adding intermediate colors, which fool the eye and make the edges appear smooth. This image shows how an aliased line is made to appear smoother through anti-aliasing:



Alpha Channel: There are two ways to achieve transparency in an image: Declare pixels of a certain color to be transparent, or add an additional channel of transparency information. This additional channel is called an alpha channel, and it can be used to make areas of an image partially or completely transparent. When separated from the rest of the image, an alpha channel appears to be a greyscale image. When this greyscale image is overlaid on the final image, areas that are black appear transparent, white areas are opaque, and grey areas are translucent, with the level of opacity determined by the value of the grey at that point. The image below illustrates an alpha channel. Notice how the background layer is invisible in the upper left and fully visible in the lower right and how that corresponds with the greyscale gradient. Alpha channels can also be used as selection tools in Photoshop, but that goes beyond the scope of this article. Here's a nice video tutorial for Photoshop: http://www.planetphotoshop.com/desig...a-channel.html



Bit Depth: In a computer, every piece of information is described by binary code. One bit is a single "switch" that is either off or on: 0 or 1. So a one-bit pixel has only two values: Black or white. By adding bits, the amount of information that can be stored increases exponentially. A two-bit image can have four values: 00, 01, 10, 11. Each bit added multiplies the number of possible colors by two. Therefore, the bit depth of an image is directly tied to the number of colors the image can display. It is also directly tied to the size of the image file. More bits used to describe each pixel means more space is required to store it. It is common to refer to the total bit-depth of an image as bits per pixel or bits per channel. So 8 bits might refer to a paletted image with a maximum of 256 colors or to an image with 8 bits per channel and millions of possible colors.

There is also a distinction between integer and floating point numbers. I won't go into great detail here, but the basics are that integer numbers, regardless of depth, have a finite luminance range, running from black at 0 to white at whatever number is at the top of the scale. Floating point numbers run from black at 0 to white at 1, but they also permit negative numbers and values beyond 1. Visually, this doesn't make sense, but it opens some options for High Dynamic Range imaging (HDRi).

Channel: Each pixel in an image is described by numbers. In a 32-bit RGBA image, each pixel is described by four 8-bit numbers whose values ( 0 - 255 ) determine the color and transparency of that pixel. Each of these numbers represents a channel: red, green, blue, and alpha (transparency). The channels are a way of organizing information about the pixels so that they can be more easily manipulated. Each channel, if viewed by itself, will appear to be a greyscale image, with the brightness of a given pixel indicating the amount of that color present in the full image.

Color Depth: Color depth is the same as bit depth, except that it is a term more specific to graphics. When dealing with images, the two terms are mostly interchangeable.

Compression: Compression is a process that reduces the size of a file. Compression can be either "lossy" or "lossless." Lossy compression discards information, so it can achieve very small file sizes at the expense of accuracy. Lossless compression retains all of the original information, so there are limits to how small a file can be made.

CMYK: A color model that uses four values: cyan, magenta, yellow, and black. CMYK images do not display properly on a computer monitor, but they are ideal for printing. CMYK color mixing is subtractive: adding inks to the page causes fewer frequencies of light to be reflected, resulting in a darker color. Theoretically, equal quantities of cyan, magenta and yellow should produce black. Pigments are never perfect, though, so the result is typically dark brown instead, which is why black ink is added to the system. So-called "rich black" is produced by mixing a small amount of the colored inks with the black ink. The CMY primary hues are the complementary colors of the RGB primary hues. If you use curves to reduce the green in your RGB image, that is equivalent to boosting the magenta.

Dithering: A process where noise is added to an image to counteract banding artifacts from reducing its color space. If you ever see banding in a very subtle gradient in your image, adding a little bit of noise or grain on that layer will often break it up and may be less obtrusive to the viewer.

High Dynamic Range imaging (HDRi): A relatively new term in imaging, HDR is all about capturing more detail in images by reclaiming portions of the luminance range that normally fall outside of the limits of an image format. At the moment, it is primarily a topic for photographers, but as displays with greater bit-depth emerge (I have a 10-bit display at my workplace, and 12-bit displays are currently on the market) the limits of 8-bit per channel images are going to start making them obsolete. There are HDR variants of several of the image formats detailed here, but OpenEXR looks like it will be the format of the future. For more information on this topic, I recommend consulting the blog of my friend Christian Bloch: HDR Labs. And if you're interested in even deeper coverage, he wrote the book on the topic: The HDRI Handbook 2.0, available on Amazon.

Paletted Image: An image for which the colors are preselected and stored in a table in the file rather than having color channels. Paletted images are frequently much smaller than images that use channels, but they are more difficult to work with. A paletted image is often temporarily converted to RGB when it is being edited, and the palette is reconstructed upon saving. Although most software does a good job of selecting a palette, some images might benefit from the user making a couple of decisions while saving to minimize file size.

Pixel: A single "dot" on a computer screen. A raster image is composed of a grid of pixels, each with its own color information.

Resolution: The number of pixels present in an image or computer display. Resolution is described with two numbers: the first is the number of horizontal dots (columns) and the second is the number of vertical dots (rows). 800 X 600 means 800 pixels wide by 600 pixels tall. This is distinct from the print resolution, which is described by the number of dots the printer places in a linear inch. The unit of print resolution is dots per inch (dpi), and in terms of image size, it is meaningless unless accompanied by the physical dimensions of the printed piece, measured in inches. So 300 dpi at 8"x10" is a useful description. The screen resolution of an image may be found by multiplying the horizontal and vertical measurements by the dpi. The screen resolution of the example is therefore 2400 x 3000 pixels (8 * 300 x 10 * 300).

RGB: A color model that uses three values: red, green and blue. RGB images must be converted to CMYK prior to being printed, but they display correctly on a computer monitor. RGB color mixing is additive: adding light to the spectrum causes more frequencies to be perceived by the eye, resulting in a brighter color. The RGB primary hues are complementary to the CMY primary hues. If you need less yellow in your RGB image, increase the blue.