FilledTriangle stretches too far horizontally

[timboldt]

I was using a FilledTriangle to draw the hands on a clock, and I noticed that the triangle for the minute hand goes outside the boundaries of the clock near the 3 o'clock and 9 o'clock positions.

Assuming there is nothing wrong with math.Sincos(), I can only assume that something is not quite right with FilledTriangle().

My logic looks something like this:

	const (
		hourRadius       = 55
		minuteRadius     = 95
		hourWidth        = 6
		minuteWidth      = 5
	)

	// Draw the clock hands.
	minuteAngle := float64(t.Minute()) / 60 * 2 * math.Pi
	mx, my := math.Sincos(minuteAngle)
	hourAngle := float64(t.Hour()*60+t.Minute()) / 60 / 12 * 2 * math.Pi
	hx, hy := math.Sincos(hourAngle)
	tinydraw.Triangle(
		display,
		x-int16(hourWidth*hy), y-int16(hourWidth*hx),
		x+int16(hourWidth*hy), y+int16(hourWidth*hx),
		x+int16(hourRadius*hx), y-int16(hourRadius*hy),
		black)
	tinydraw.FilledTriangle(
		display,
		x-int16(minuteWidth*my), y-int16(minuteWidth*mx),
		x+int16(minuteWidth*my), y+int16(minuteWidth*mx),
		x+int16(minuteRadius*mx), y-int16(minuteRadius*my),
		black)

[conejoninja]

At first glance the code seems ok, could you output the coordinates of the points to see if the error might be there? Which display are you using? I have drawn a few different clocks and gauges myself and don't remember any issue, but also don't remember setting it to 3-9 o'clock either. Code was based on Adafruit's GFX library https://github.com/adafruit/Adafruit-GFX-Library/blob/master/Adafruit_GFX.cpp#L619-L701 will check the implementation when possible.

[timboldt]

Okay, let me take a deeper look at it this weekend, and I'll see if I can find the exact inputs that reproduce the problem.

[spearson78]

I've also noticed some anomalies when filling triangles. I initally assumed this was due to the display driver I was implementing but I suspect something is wrong in tinydraw. I'll see if I can create a reproducible test.

[sago35]

No longer crashes due to #11.
However, the shape of the 3 o'clock and 9 o'clock locations still looks a little odd.
Also, the 6 o'clock spot seems a little thick.

And it seems that FilldTriangle() and Triangle() have different shapes.

souce code:
https://gist.github.com/sago35/46ff4e8380e06ea6151c359e33ddb751

[spearson78]

I created a pull request that re-uses the line drawing algorothm for filling the triangles unfortunately the changes were more significant than I anticipated when I started down this path. #12

[spearson78]

I created an additional pull request with some tests that reproduce the rendering issues. #13

[spearson78]

I finally spotted an additional error in the translation from c to go code. I should have noticed it with my fix for issue11 but somehow missed it. See #13

The rendering is in my opinion not as good as the changes I made in #12 but the change is much much simpler and easier to verify against the original code.

I'll leave it to the maintainers to decide which approach to take.

[sago35]

The FilledTriangle() is still a little misshaped, but it's much less of a problem now.
The following is a drawing using the latest version of tinydraw.