When you attempt to open a vector file—such as one created in Adobe Illustrator or Adobe Acrobat— in Photoshop, the program needs to first determine how to rasterize the file. Rasterizing refers to the process of translating vector-based file information into pixels. (For more information about these two file types—vector and bitmap—refer to the section "File Type" earlier in this chapter.)
To rasterize a file, Photoshop needs to give it absolute pixel dimensions and a file resolution because, although vector files are resolution- and size-independent, bitmap files require these sizes to display properly. Therefore, it's wise not to rasterize a file until you're sure of the size at which you intend to use it.
Along with being resolution-independent, vector files don't rely on anti-aliasing to create the appearance of smooth edges on curves. Anti-aliasing is a technique used by bitmap applications like Photoshop to add extra pixels of color around curved edges to give the appearance of smooth gradations from one color to another. Therefore, when you rasterize a vector file with lots of curves, I recommend you check Anti-aliased in the options window.
To use the Open command to open an existing vector file in Photoshop, first choose File I Open and locate the file in the directory structure before clicking OK. As long as the file is indeed a vector file, you'll be presented with an options window similar to the following:
At this point, you must specify the size at which Photoshop should build the file, both in physical height and width and in resolution. In addition, also specify the color mode. Finally, at the bottom of the options window, you can choose whether the file should be anti-aliased and whether Photoshop should constrain the proportions when sizing the image.
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