This forum is a good resource. There's a lot of other stuff, both good and bad, across the Internet. I'm not sure why PNG would be recommended as a default format to use in dye-sub printing. My guess is the people making the printers and print software are trying to simplify work flow details for the end users. Maybe the thinking is since PNG has certain limitations there are fewer things to mess up. Not everyone doing print work knows the various pros and cons of different file formats. Generally, the contents of the document should dictate the type of file used to output a print job. If the document is just a photograph or an entirely pixel-based image created in Photoshop then a PNG image might suffice. I'd still opt for a TIFF image rather than PNG because I know the image's resolution settings and color profile will be honored. If the document has a mix of pixel-based imagery and vector-based elements (logos, graphical items, etc) PDF will be a better choice since the vector-based elements will print at the maximum resolution the printer supports. I would compose layouts that have a mix of pixel-based imagery and vector-based elements using Adobe Illustrator. If it's a multi-page docment InDesign might be a better choice. Both applications would be exporting a PDF to send to the print application or RIP. The type of printer being used should dictate the color mode, be it CMYK or RGB. If the printer has extra ink colors, such as Light Cyan and Light Magenta, it can be better to compose documents in RGB color space. But gamut limits do still exist. No printer can output the entire RGB color range. Some caution must be used when working in that color space. Printers with extra ink colors can do a better job simulating spot colors, such as those from Pantone.
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