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To add a 4K image to a presentation in BrightAuthor:connected or BrightAuthor, or to 4K HTML canvas, the presentation must be set to full resolution graphics. This is not required for 4K video.
Only the XC5, XT4, and XT5 support full resolution mode. Other Series 5 players do not support this mode.
You can only select Enable Full Resolution Graphics if Force Resolution is also selected in https://brightsign.atlassian.net/wiki/spaces/DOC/pages/404623943/Presentation+Settings#Display.
If your player or resolution doesn’t support full resolution graphics and you select 4K output and add an 1080p or larger image to your presentation or HTML page, the player will downscale the image to 1080p and then upscale it to 4K, reducing image quality. To get the best results, add the 1080p image to your presentation/HTML and let the player upscale it from 1080p to 4K.
Also see 4K Video Quality Overview