Source aspect ratio information missing in TouchFingerEvent generated by trackpads

[AlexGuo1998]

After enabling the hint SDL_HINT_TRACKPAD_IS_TOUCH_ONLY on macOS, interacting with the trackpad will generate SDL_TouchFingerEvent.

In the SDL_TouchFingerEvent struct, the XY coordinates are normalized to 0~1, which is acceptable for touchscreens, but not for trackpads:

• For touchscreens, the coordinates are mapped to the whole window, which we know the size/ratio.
• For trackpads, the coordinates are mapped to the trackpad, which the size/ratio is unknown.

This can cause problems.

For example, for a trackpad with a 2:1 aspect ratio, moving the finger one centimeter right may change the X coordinate by n amount, but moving one centimeter down will change the Y coordinate by 2 * n.

Without knowing the original trackpad's aspect ratio, the changes in X and Y coordinates are just not comparable.

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My suggestion is to store the device's aspect ratio as a floating number somewhere, for example in struct SDL_touch:

SDL/src/events/SDL_touch_c.h

Lines 26 to 34 in 6980325

	typedef struct SDL_Touch
	{
	SDL_TouchID id;
	SDL_TouchDeviceType type;
	int num_fingers;
	int max_fingers;
	SDL_Finger **fingers;
	char *name;
	} SDL_Touch;

And provide an API (e.g. SDL_GetTouchDeviceAspectRatio) to retrieve it.

For macOS, the ratio might be calculated with deviceSize.