Image Resolution for Bi-tonal Scans
Resolution determines the quality of all scans. Resolution is the number of pixels per unit of measure (inches, in the United States, hence the abbreviation ppi) used to form an image. In general, bi-tonal scans intended for print usage should be at least 1200ppi. Resolution should be determined at the scanning stage based on the qualities of the originating image: (1) whether it is continuous or bitonal and (2) the size of the original. See the table below for further guidance.
Example of line art scanned at 1200 ppi. This setting is ideal for high-quality offset print production. Note the smooth curves.
Example of line art scanned at 72 ppi, which results in an image that appears blocky and sharp-edged. Pixelation will occur if line art is scanned at low resolutions. Low-resolution settings are not suitable for print.
Fig. 4.3—72ppi, with resolution artificially increased to 1200ppi.
Example of a low-resolution scan to which resolution has been artificially added. This is still inadequate for printing.
Determining Resolution Settings for Bi-tonal Resolution Scans
Save all scans in the TIFF or EPS format.
Continuous-tone or Bi-tonal?
To determine whether you should scan your original art in continuous-tone mode (8-bit) or in bi-tonal mode (1-bit), consider the following:
– If your original is a photograph or fine art drawing that contains multiple levels of gray tones, you should scan it in continuous-tone mode.
– If your original is a line drawing without gray tonalities and consists of only black-and-white lines, you should scan it in bi-tonal mode.
Line art: Images that contain only solid blacks and whites.
Pixelation: A Phenomenon in bitmap images that results from insufficient resolution or over-enlargement. Individual pixels become visible, especially on the edges of objects, creating a stair-stepped or jagged look.
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