Our DTG (Digital Direct On Garment) printer is much like an inkjet printer that uses water-based inks suitable for fabric, much like the fabric paints used for fabric painting. Our prints are heat-cured for colour fastness.
When printing with a DTG printer the operator instructs the DTG printer whether it is printing on black, white, dark colours, or light colours. The DTG printer automatically prints a white under-base for black and dark colours, but for light colours this must additionally be specified by the operator. On white the DTG printer does not print a white under-base.
The DTG printer software has the ability to pick up whites and blacks. When printing on a white fabric it will pick up white and it will not print it, these areas are left blank and the background (t-shirt fabric) supplies the white. The same for printing on black, blacks will not be printed when printing on a black fabric. However, for any other colour a white under base is printed and the colour is printed on top of that to “simulate” white paper. Even when printing a red on a red fabric a white underbase is printed, the reason for this is that we cannot guarantee the shade of the red being exactly the same shade of the fabric. However, if the image has red and you want the red to be replaced by the red of the fabric, those areas must be left blank (transparent). For this we need to have a PNG or vector graphic. In order to get it transparent you have to cut it out in Photoshop or if it is a vector, in your vector editing tool, and save it as a PNG or SVG/EPS/etc., the PNG has the ability to save transparent areas.
The white under-base used when printing on black or coloured garments means that images being printed can't contain any fading into transparency e.g. outer glows or gradient fades because the white under-base has solid edges. For this reason we recommend any custom t shirts designed specifically for black or coloured garments have solid edging to the image.
For more information, you can read about Direct to garement printing on Wikipedia
See this design being printed start to finish: