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Bug 2892 - SDL_SYS_JoystickUpdate_XInput clamps the y axis wrongly
Summary: SDL_SYS_JoystickUpdate_XInput clamps the y axis wrongly
Status: RESOLVED WONTFIX
Alias: None
Product: SDL
Classification: Unclassified
Component: joystick (show other bugs)
Version: HG 2.1
Hardware: x86_64 Windows 7
: P2 normal
Assignee: Ryan C. Gordon
QA Contact: Sam Lantinga
URL:
Keywords: triage-2.0.4
Depends on:
Blocks:
 
Reported: 2015-02-26 10:50 UTC by gummikana
Modified: 2015-04-07 02:33 UTC (History)
1 user (show)

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Description gummikana 2015-02-26 10:50:20 UTC 
SDL_GameControllerGetAxis( joystick, SDL_CONTROLLER_AXIS_LEFTY )  was giving me a range of -32767 to 32767 instead of -32768 to 32767 (which the X axis me). I went investigating and I found the problem:

 SDL_PrivateJoystickAxis( joystick, 1, (Sint16)(-SDL_max(-32767, pXInputState->Gamepad.sThumbLY)) );

(and SDL_PrivateJoystickAxis( joystick, 3, (Sint16)(-SDL_max(-32767, pXInputState->Gamepad.sThumbRY)) );) in SDL_dxjoystick.c


The -32767 is correct. The problem is that pXInputState->Gamepad.sThumbLY is a SHORT meaning it's max positive is 32767, so that clamps the range from -32767 to 32767 and then it's flipped with the -sign. 

The way to fix this is to cast it to int:
SDL_PrivateJoystickAxis( joystick, 1, (Sint16)(-SDL_max(-32767, (int)pXInputState->Gamepad.sThumbLY)) );
Comment 1 Ryan C. Gordon 2015-03-25 00:03:09 UTC 
Are you sure this fixes it? If you pass a -32768 here:

SDL_PrivateJoystickAxis( joystick, 1, (Sint16)(-SDL_max(-32767, (int)pXInputState->Gamepad.sThumbLY)) );

That becomes:

SDL_PrivateJoystickAxis( joystick, 1, (Sint16)(-SDL_max(-32767, (int)-32768)) );

And that SDL_max() will return -32767, since that's greater than -32768...so you still end up clamped between -32767 and 32767, even with the explicit cast.

--ryan.
Comment 2 Ryan C. Gordon 2015-04-07 02:33:27 UTC 
Going to mark this as WONTFIX; we have to give up a value somewhere, and this one is as  good as any.

--ryan.