Printing on dark garments can become a complicated process -- ease the headache with this advice.
If you are using a vector file that has been created or imported into CorelDraw or Illustrator, then creating the underbase file is usually simple. You will need to create an exact duplicate of the file and then change all of the pieces and outlines in the image that are to be underbased into black. the next step would be to adjust for any trapping by adding a small white outline to the appropriate parts of the image.
Creating an underbase for a bitmap or raster design is typically more complicated. the first step is to properly prep the file and make sure it is the correct size and resolution. next, duplicate the file and create a separation between the foreground and background so you can “float” all of the active elements in the design. this step will allow you to change the background to black by inserting a layer below the active pieces that are floating. once you have the design trapped on the outside by black, you can then convert it to L*a*b* color in the color mode menu (image/Mode/L*a*b* color). the final step to creating a working underbase is to select only the lightness channel from this file, and then copy and paste it into the original file as an extra channel. You can invert this new channel and then practice increasing the values in areas where you might need more solid color in the final print, depending on the color requirements of the original design.
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