Defining images in PlanetPress Design requires that we first establish a few terms that will be used throughout this chapter.
Bitmap File Formats
A bitmap file format stores an image as a set of pixels. Common bitmap formats are BMP, TIFF, PNG and JPEG. Images such as photographs typically use a bitmap format.
The disadvantages of bitmap images are loss of image quality when scaling, and large file sizes for images with large pixel dimensions and color depths.
Vector File Formats: A vector file format encodes an image as a set of commands or mathematical equations. Encapsulated PostScript (EPS) is a common vector format. Images such as illustrations typically use a vector format.
In a vector format, the commands precisely describe how to draw the image. The advantage of images in vector format is that they scale without loss of color, detail, or sharpness. Depending on the complexity of the image they describe, they may also have a smaller file size than their bitmapped counterpart. The disadvantage is that they require the computer or printer to interpret the commands and render the image.
Metafile Format
A metafile image format can include both bitmapped and vector images. PlanetPress Design supports images in both Encapsulated PostScript (EPS) and Portable Document Format (PDF) metafile formats.
As for vector images, a metafile image file specifies a size for the image. In this case, if the metafile image contains bitmapped images, scaling it may affect the image quality of those bitmapped images.
Resolution
Resolution is a broad term that can refer to different things in different contexts. A detailed discussion of the different meanings and interrelationships is beyond the scope of this guide. Instead we provide This section provides a brief overview of the different possible meanings to clarify our use of resolution in PlanetPress Design.
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Term: |
Means: |
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Scan resolution |
The resolution at which you scan an image on a scanner. It refers to the number of samples per inch (spi), where each sample represents a single pixel in the resulting bitmapped image. |
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Display resolution |
The resolution of the monitor screen. For example 96 DPI means the screen displays 96 pixels per inch. Thus an image with pixel dimensions of 96 x 96 would measure 1 inch x 1 inch on this monitor screen. |
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Printer resolution |
The number of spots of ink the printer can produce per inch, commonly measured as dots per inch (DPI). The use of DPI as a measure can be confusing as it may be interpreted as meaning one dot of ink maps to one image pixel. In fact the number of spots of ink the printer uses to represent a single pixel can vary with the halftone screen used for a print job (the lines per inch (LPI)), the color depth of the image, and the algorithm the printer uses to represent individual colors. |
Color Depth
Color depth refers to the number of colors each individual pixel in a bitmapped image can display. Color depth is expressed in bits per pixel where each individual bit can take on one of two values (0 or 1).
Pixel Dimension
Pixel dimensions refer to the number of pixels in the width and height of a bitmapped image. Thus an image with pixel dimensions of 200 x 300 has a width of 200 pixels and a height of 300 pixels. Pixel dimensions do not define a fixed size for the bitmapped image. Rather the resolution determines the final physical dimensions of the image. See Resolution for help understanding resolution in PlanetPress Design.
You can adjust the pixel dimensions of a bitmapped image to adjust the size it occupies at a given resolution. This is called resampling the image, and it is particularly useful for large image files that contain more pixels than are necessary to produce the highest quality output.
Image Quality: Line Art
In line art quality, the edges of different elements of the image are clearly visible and sensitive to any increase or decrease in pixel dimensions that occurs through resampling. As an example of line art, think of a corporate logo. A logo typically uses flat areas of color with each area clearly distinguished from the other. PlanetPress Design uses a lossless compression for line art quality images. A lossless compression preserves all image information.
Image Quality: Photo
In photo quality, edges of elements are not as clearly delineated and are consequently less sensitive to any increase or decrease in pixel dimensions that occurs through resampling. As an example of photo quality, think of a photograph of a natural object or landscape; there are many subtle tones and gradations, even along the edges between elements. PlanetPress Design uses a lossy compression (JPEG compression) for photo quality images. A lossy compression does not preserve all image information. In the case of photo quality, you set a compression level that determines how much image information is lost. An appropriate compression level results in no visible deterioration of quality in the document output.