The Image Optimizer is a feature of The Forge which will optimize all your image assets so they load faster for all your players. It also allows on the fly image flipping or blurring.
Simply by using the webp image format, when the player’s browser supports it, allows to have huge reductions in size with no impact on the image quality. While some highly optimized JPG images can have similar sizes to webp (though with a much lower image quality level), we have seen in our tests, 3MB and 5MB images often reduced to 200KB or 400KB. Though in general, the improvement will be twice or three times smaller sizes.
If you have an image that has a resolution higher than 4096 pixels in either dimension, the optimizer will also automatically scale it down so its resolution is no higher than 4096x4096, ensuring that the image will load properly on all of your player’s machines, regardless of their graphics card’s capabilities.
You can read more about those features and detailed reasons for it in the The Assets Library FAQ item.
While the downscaling may not even be noticeable by most people, in some situations, the loss of image quality can be visible, especially at high zoom level, and you may want to keep a high resolution image for your background for example. In that case, we strongly recommend you to split the image into multiple tiles, rather than disabling the optimizer, to make sure that the image will load properly on everyone’s machines.
Disabling the optimizer however will force it to use the original image file you have uploaded, but you take the risk of having some of your players potentially complaining that they cannot see the art at all.
For your convenience, here’s the relevant part of the The Assets Library FAQ item that relates to high resolution images :
GPU Texture limit avoidance
Some Game Masters like to use very high resolution maps for their games, or simply use the maps they found or were given without thinking about the image’s resolution. This has the unfortunate effect that it may not work for all of their players. Images of a size above 16384x16384 pixels will simply not load in a browser. That’s a limitation of the WebGL technology, but there’s also a limitation imposed by each player’s individual GPU’s (Graphics card) capabilities.
While a map might load for you, it may appear entirely black to one or more of your players, because the limit imposed by their graphics card could be 8192x8192 or even 4096x4096. This is most often visible for people using integrated graphics cards, such as found in laptops.
To prevent this from happening, all images loaded within Foundry VTT will be automatically scaled down to a maximum of 4096x4096 resolution, making sure that your maps will load on everyone’s computer, regardless of their GPU capabilities.
The files will remain untouched however and can still be accessed in full resolution when accessed via the Assets Library on the forge.
Using high resolution maps
I know that some Game Masters may want to use high resolution maps and would not want to lose any of their hard earned pixels. In those cases, the best thing you can do is to actually split your map into multiple tiles that you would place on your scene. By using smaller tiles, you ensure that the map will be visible to all of your players at the high resolution you want it to be, regardless of their machine’s capabilities.
Do keep in mind however that this can have a detrimental effect on performance, as a 16384x16384 image alone would take over 2GB of your GPU’s limited RAM, even if it is split into sixteen tiles of 4096x4096 images. That can interfere with the proper functioning of the rest of the FVTT software, including the dynamic lighting layer which would also take an additional 2GB of RAM, and overall, the performance will be severely impacted.