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June 3, 2011

What are Bit map, Raster and Vector Images in Graphics Design?

Bitmap images
Called raster images. They use rectangular grid of pixels to represent images. Each pixel is assigned a specific location and color value of RGB. Bitmap images used fore photographs and digital paintings as they can efficiently represent shades and color.

Bitmap images are resolution-dependent. They contain a fixed number of pixels, therfore lose details when scaled high.

Bitmap images require large storage space, hence compressed to keep file sizes down.

Vector graphics Vector graphics made of vector shapes that are made up of lines and curves defined by geometric characteristics. You can freely scale vector graphics without losing detail or clarity, as they are resolution-independent. They maintain crisp edges when resized or printed.

Vector graphics are the best choice for artwork, such as logos, that will be used at various sizes and in various output media.

Combining Vector and Raster Graphics
When combining vector graphics and bitmap images in a graphic.  The artwork on screen isn’t always will not look same in  print.

When your artwork contains transparency, Photoshop performs a process called flattening before printing or exporting. The default flattening process produces overlapping areas

Image Resolution The number of pixels per inch (ppi) in a bitmap image, if low will print smaller image with coarse-looking pixels. Using too high a resolution increases the file size without increasing the quality of the printed output, and also slows the printing of the artwork.

Printer resolution and screen frequency
The number of ink dots produced per inch (dpi) and the number of lines per inch (lpi) in a halftone screen the image resolution and printer resolution, and screen frequency determines the quality of detail of the print.

Color channels
Photoshop images have color channels.  Each channel stores information about color elements in the image. The number of default color channels depends on color mode. Grayscale, Duotone, and Indexed Color mode have one channel. RGB has three; and CMYK images have four. Channels in color images are actually grayscale images that represent each of the color components of an image. For example, an RGB image has separate channels for red, green, and blues color values.

In addition to color channels, alpha channels, can be added to an image for storing and editing selections as masks.

Bit depth
Bit depth specifies how much color information is available for each pixel in an image. The more bits of information per pixel, the more available colors and more accurate color representation. An image with a 1 bit depth of 1 has pixels with two possible values: black and white. An image with a bit depth of 8 has 256, possible values. Grayscale images with a bit depth of 8 have 256 possible gray values.

RGB images are made of three color channels. An 8‑bit per pixel RGB image has 256 possible values for each channel which means it has over 16 million possible color values. RGB images with 8‑bits per channel (bpc) are sometimes called 24‑bit images (8 bits x 3 channels = 24 bits of data for each pixel).

Monitor and Image Resolution
If your monitor resolution and your photo’s pixel dimensions are the same size, the photo will fill the screen when viewed at 100%.
When preparing images for viewing on‑screen, you should consider the lowest monitor resolution that your photo is likely to be viewed on.

About printer resolution
Screen frequencyScreen frequency is the number of printer dots or halftone cells per inch used to print grayscale images or color separations.Screen frequency is measured in lines per inch (lpi), or lines of cells per inch on screen. The higher the resolution of the output device, a higher screen should be used.

The relationship between image and printer resolutionPrinter resolution is different, but related to image resolution. To print a high quality photo on an inkjet printer, an image resolution of at least 220 ppi should be set.

ResamplingResampling is changes image data as you change in pixel dimensions or resolution. When you downsample, the number of pixels information is deleted from the image. When you upsample, it increases the number of pixels and their colors are interpolated. Downsampling can reduce image quality and loose sharpness. Applying the Unsharp Mask filter to a resampled image can help refocus the image details.

Interpolation methods
Nearest Neighbor - Fast but less precise. Replicates the pixels in an image using containing edges that are not anti-aliased, to preserve hard edges and produce a smaller file.

Bilinear - A method that adds pixels by averaging the color values of surrounding pixels.Produces medium-quality results.

Bicubic - A slower but more precise method based on an examination of the values of surrounding pixels. Produces smoother tonal gradations than Nearest Neighbor or Bilinear.

Bicubic Smoother - A good method to produce smoother results.

Bicubic Sharper  - Uses Bicubic interpolation with enhanced sharpening.
Changing an image’s pixel dimensions affects not only its on‑screen size but also its image quality and its printed characteristics.

Camera RawA camera raw file contains unprocessed, uncompressed grayscale picture data from a digital camera’s image sensor, along with information about how the image was captured. When you shoot JPEG files with your camera, the camera automatically processes the JPEG to enhance and compress the image. You have less control of processing here. 

To shoot camera raw images, you need to set your camera to save files in its own camera raw file format.

RGB Color model

RGB Color model assigns intensity value to each pixel. In 8‑bits-per-channel images, the intensity values range from 0 (black) to 255 (white) for each of the RGB (red, green, blue) components in a color image. For example, a bright red color might have a higher Red value, lower Green and Blue value. When all R, G, B values are equal the result is white and when they are all 0 produces pure black. Although RGB is a standard color model, the exact range of colors represented can vary, depending on the application or display device.

CMYK Color model CMYK model assigns a Cyan, Magenta, Yellow and Black percentages of ink colors. Over printing of these colors prints are produced.

The lightest colors are assigned small percentages of ink colors and the darker colors assigned higher percentage. A bright red might contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In CMYK images, pure white is generated when all four components have values of 0%. CMYK model is used for image to be printed using process colors.

Converting an RGB image into CMYK creates a color separation. It’s best to edit first in RGB and then convert to CMYK at the end of your process.

Lab Color mode
The CIE L*a*b* color model (Lab) is based on the human perception of color. The numeric values in Lab describe all the colors that a person with normal vision sees. Because Lab describes how a color looks rather than how much of a particular colorant is needed for a device (such as a monitor, desktop printer, or digital camera) to produce colors, Lab is considered to be a device-independent color model.

Duotone model Duotone mode creates monotone, duotone (two-color), tritone (three-color), and quadtone (four-color) grayscale images using one to four custom inks.

Bitmap model Bitmap mode uses one of two color values (black or white) to represent the pixels in an image. Images in Bitmap mode are called bitmapped 1‑bit images because they have a bit depth of 1.

Indexed Color modelIndexed Color mode produces 8‑bit image files with up to 256 colors.
Multichannel mode

Multichannel mode images contain 256 levels of gray in each channel and are useful for specialized printing.

You can convert between different color models which permanently change the color values in the image. As a result, some image data may be lost. Hence do as much editing as possible in the original image mode and save a backup copy with layers before converting.

Flatten the file before converting it. The interaction of colors between layer blending modes changes when the mode changes.

Web‑safe colors
The web‑safe colors are the 216 colors used by browsers regardless of the platform.

Printer resolution
Measured in ink dots per inch, also known as dpi. Generally, the more dots per inch, the finer the printed output you’ll get. Most inkjet printers have a resolution of approximately 720 to 2880 dpi. (Technically, inkjet printers produce a microscopic spray of ink, not actual dots like imagesetters or laser printers.)

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