(iOS) Accelerometer Graph (convert g-force to +/- 128) granularity - ios

Question

I am using this Accelerometer graph from Apple and trying to convert their G-force code to calculate +/- 128.
The following image shows that the x, y, z values in the labels do not match the output on the graph: (Note that addX:y:z values are what is shown in the labels above the graph)
ViewController
The x, y, z values are received from a bluetooth peripheral, then converted using:
// Updates LABELS
- (void)didReceiveRawAcceleromaterDataWithX:(NSInteger)x Y:(NSInteger)y Z:(NSInteger)z
{
dispatch_async(dispatch_get_main_queue(), ^{
_labelAccel.text = [NSString stringWithFormat:#"x:%li y:%li z:%li", (long)x, (long)y, (long)z];
});
}
// Updates GRAPHS
- (void)didReceiveAcceleromaterDataWithX:(NSInteger)x Y:(NSInteger)y Z:(NSInteger)z
{
dispatch_async(dispatch_get_main_queue(), ^{
float xx = ((float)x) / 8192;
float yy = ((float)y) / 8192;
float zz = ((float)z) / 8192;
[_xGraph addX:xx y:0 z:0];
[_yGraph addX:0 y:yy z:0];
[_zGraph addX:0 y:0 z:zz];
});
}
GraphView
- (BOOL)addX:(UIAccelerationValue)x y:(UIAccelerationValue)y z:(UIAccelerationValue)z
{
// If this segment is not full, then we add a new acceleration value to the history.
if (index > 0)
{
// First decrement, both to get to a zero-based index and to flag one fewer position left
--index;
xhistory[index] = x;
yhistory[index] = y;
zhistory[index] = z;
// And inform Core Animation to redraw the layer.
[layer setNeedsDisplay];
}
// And return if we are now full or not (really just avoids needing to call isFull after adding a value).
return index == 0;
}
- (void)drawLayer:(CALayer*)l inContext:(CGContextRef)context
{
// Fill in the background
CGContextSetFillColorWithColor(context, kUIColorLightGray(1.f).CGColor);
CGContextFillRect(context, layer.bounds);
// Draw the grid lines
DrawGridlines(context, 0.0, 32.0);
// Draw the graph
CGPoint lines[64];
int i;
float _granularity = 16.f; // 16
NSInteger _granualCount = 32; // 32
// X
for (i = 0; i < _granualCount; ++i)
{
lines[i*2].x = i;
lines[i*2+1].x = i + 1;
lines[i*2].y = xhistory[i] * _granularity;
lines[i*2+1].y = xhistory[i+1] * _granularity;
}
CGContextSetStrokeColorWithColor(context, _xColor.CGColor);
CGContextStrokeLineSegments(context, lines, 64);
// Y
for (i = 0; i < _granualCount; ++i)
{
lines[i*2].y = yhistory[i] * _granularity;
lines[i*2+1].y = yhistory[i+1] * _granularity;
}
CGContextSetStrokeColorWithColor(context, _yColor.CGColor);
CGContextStrokeLineSegments(context, lines, 64);
// Z
for (i = 0; i < _granualCount; ++i)
{
lines[i*2].y = zhistory[i] * _granularity;
lines[i*2+1].y = zhistory[i+1] * _granularity;
}
CGContextSetStrokeColorWithColor(context, _zColor.CGColor);
CGContextStrokeLineSegments(context, lines, 64);
}
How can I calculate the above code to show the correct accelerometer values on the graph with precision?

Answer 1

I post this as an aswer not a comment, because I have not enough reputation, but what I'll write might be enough to send you in the right direction, that it even may count as an answer...
Your question still doesn't include what is really important. I assume the calculation of the xx/yy/zz is no problem. Although I have no idea what the 8192 is supposed to mean.
I guess the preblem is in the part where you map your values to pixel coordinates...
the lines[] contains your values in a range of 1/8192th of the values in the label. so your x value of -2 should be at a pixel position of -0.0000something, so slightly(far less than 1 Pixel) above the view... Because you see the line a lot further down there must be some translation in place (not shown in your code)
The second part that is important but not shown is DrawGridlines. Probably in there is a different approach to map the values to pixel-coordinates...
Use the debugger to check what pixel-coordinates you get when draw your +127-line and what you get if you insert the value of +127 in your history-array
And some Ideas for improvements when reading your code:
1.)Put the graph in it's own class that draws one graph(and has only one history. Somehow you seem to have that partially already (otherwise I cannot figure out your _xGraph/_yGraph/_zGraph) But on the other hand you draw all 3 values in one drawLayer??? Currently you seem to have 3*3 history buffers of which 3*2 are filled with zeros...
2.) use one place where you do the calculation of Y that you use both for drawing the grid and drawing the lines...
3.) use CGContextMoveToPoint(); + CGContextAddLineToPoint(); instead of copying into lines[] with these ugly 2*i+1 indecies...