docs for modder

docs/modders/HD_Graphics.md

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+# HD Graphics
+
+It's possible to provide alternative HD-Graphics within mods. They will be used if any upscaling filter is activated.
+
+## Preconditions
+
+It's still necessary to add 1x graphics as before. HD graphics are seperate from usual graphics. This allows to partitially use HD for a few graphics in mod. And avoid handling huge graphics if upscaling isn't enabled.
+
+Currently following scaling factors are possible to use: 2x, 3x, 4x. You can also provide multiple of them (increases size of mod, but improves loading performance for player). It's recommend to provide 2x and 3x images.
+
+If user for example selects 3x resolution and only 2x exists in mod then the 2x images are upscaled to 3x (same for other combinations > 1x).
+
+## Mod
+
+For upscaled images you have to use following folders (next to `sprites` and `data` folders):
+- `sprites2x`, `sprites3x`, `sprites4x` for sprites
+- `data2x`, `data3x`, `data4x` for images
+
+The sprites should have the same name and folder structure as in `sprites` and `data` folder. All images that are missing in the upscaled folders are scaled with the selected upscaling filter instead of using prescaled images.
+
+### Shadows / Overlays
+
+It's also possible (but not necessary) to add HD shadows: Just place a image next to the normal upscaled image with the suffix `-shadow`. E.g. `TestImage.png` and `TestImage-shadow.png`.
+
+Same for overlays with `-overlay`. But overlays are **necessary** for some animation graphics. They will be colorized by VCMI.
+
+Currently needed for:
+- flaggable adventure map objects (needs a transparent image with white flags on it)
+- creature battle animations (needs a transparent image with white outline of creature for highlighting on mouse hover)