fix viewportFlipY XOR condition

[CyCle1024]

Motivation

deko3d/source/dk_device.h

Lines 100 to 102 in 8cb7b4c

	// Rasterizer expects clip space +Y to point down. In UpperLeft mode with Y pointing up
	// (or LowerLeft mode with Y pointing down) we need to flip the incoming Y coordinate.
	bool viewportFlipY() const noexcept { return !isYAxisPointsDown() ^ !isOriginLowerLeft(); }

The implementation is not consistent with the comment. We intend to flip Y when UpperLeft mode with Y pointing up. But in this case, !isYAxisPointsDown() = true and !isOriginLowerLeft() = true, true ^ true = false. Same as LowerLeft mode with Y pointing down.

Key Change

The fix is simple, I think the comment is the correct logic, so just add a ! before return.

[fincs]

Thank you for identifying and providing a fix. I only have two concerns:

• Have you tested all combinations of DkDeviceFlags_Origin{Upper/Lower}Left/YAxisPoints{Up/Down} to make sure they work as intended?
• Can you simplify the expression using logic laws? For example, NOT (a XOR b) == a XOR NOT b == NOT a XOR b.

[CyCle1024]

• Have you tested all combinations of DkDeviceFlags_Origin{Upper/Lower}Left/YAxisPoints{Up/Down} to make sure they work as intended?

Sorry, I don't have much test that cover all the case, I only test on the case of OriginUpperLeft and YAxisPointsDown which is the default case, with a project called Melonds on switch. I fix the rendering problem with source code compiled deko3d.

• Can you simplify the expression using logic laws? For example, NOT (a XOR b) == a XOR NOT b == NOT a XOR b.

I can simplify it, but the reason I fix it in this way is for readability, since isYAxisPointsDown()=true and isOriginLowerLeft()=true is a pair of case that we should flip. Since it confuses you, I think I should change it.

[CyCle1024]

Here is logic table:

isYAxisPointsDown	isOriginLowerLeft	!isOriginLowerLeft	XOR Result	viewportFlipY()	Description
false (0)	true (1)	false (0)	0 ^ 0 = 0	false	Y-axis up, origin at lower-left
false (0)	false (0)	true (1)	0 ^ 1 = 1	true	Y-axis up, origin at upper-left
true (1)	true (1)	false (0)	1 ^ 0 = 1	true	Y-axis down, origin at lower-left
true (1)	false (0)	true (1)	1 ^ 1 = 0	false	Y-axis down, origin at upper-left