Draw lines and measure angles

MIT App Inventor Help
5

Thanks, this is basically what I was looking for.I have changed the app orientation to Landscape, and I have added the camera to capture background image, but the image doesnt keep its perspective (is stretched when applied as canvas background. Cannot get a setting to keep aspect ratio?
Secondly is there a way to edit(Move) the sprites once its placed if misaligned?

Thanks :smiley:

6

You will have to set the canvas to the aspect ratio of the image to avoid stretching/squeezing, there are extensions available to get the height/width of an image which you can use. Alternatively you could use an ImageSprite suitably placed and sized to load the image.

Sprites ? If you mean the circles (dots) and lines, then with this method, no to moving them or realigning. Once drawn, that is it, just reset and set again (it is admittedly easier with a mouse on the emulator than with a fat finger on a screen. With (a lot) more code you could use buttons to nudge a dot to the correct place, before confirming its position. Someone else may have a better approach....)

Or use the touch down / up events to position finger before setting the dot....

7

Thanks, and if for example have a preset of red dots and lines already placed (for example in the shape of a bicycle frame) the goal is to move the sprites to match the bicycle frame and indicate the angles? Can I for example use Balls, with lines connected?

8

Try this

drawLines.aia (3.9 KB)

The four balls are set to Origin at Centre
You can use the ball x and y positions for the calculations

Watch out for ball/sprite cannibalisn!
https://groups.google.com/d/msg/mitappinventortest/FuVKBNJwjfg/srrPgAmTCgAJ

image
image475×527 6 KB

9

Check out the pool cue code in this sample, for how to dynamically move and rotate a line with one end pinned to the Canvas ...

1 Like
10

This aia fixes the cannibalism

drawLines2.aia (4.8 KB)

1 Like
11

Awesome thanks, I've made good progress. Really appreciate the help.
Is there a way to zoom in on the canvas to make more precise adjustments to the ball positions?

12

See HERE
and
HERE

for some ideas....

13

A post was split to a new topic: I cannot get the Canvas to adjust to the aspect ratio of the Image Picker

14

Ok, I have got the canvas and balls to work, and set up a default layout.
Here is a diagram of lines and angles I require: Angles B-F

Lines
Lines781×751 64.5 KB

Here are the blocks I have, but doesnt work yet. Any ideas why?

angle Blocks
angle Blocks908×582 71.6 KB

Thanks,
Jakes

15

I haven't thought through the trigonometry on this, but at first glance it is necessary for your code to take into account the (x,y) values of three Balls:

  • shoulder
  • elbow
  • hand

That's 6 numbers.

I don't see you doing that.

1 Like
16

Correct @ ABG

Where is your "slope" calculation for BC ? (refer back to my first example)

17

Thanks for the feedback - I missed that. I have added the calculation for BC now, still no luck.
I see in your example you have used a list to get theX,Y points. In my example I tried to get them directly from the balls. Maybe its the issue here?

angle Blocks2
angle Blocks2869×591 85.8 KB

18

Assuming the cyclist doesn't scratch his nose or break his elbow,
the angle of his elbow B is going to be obtuse, the sum of 2 acute angles + 90 degrees.
This construction shows the two acute angles:

b21be5433b0487eee02bfe20f35d77f799aeac45
b21be5433b0487eee02bfe20f35d77f799aeac45781×751 81.1 KB

I added two necessary corner points to the diagram,
to give us 2 new X values and 2 new Y values for the arctan values you will need to add.

Heading clockwise around point B, here are the angles to add:

  • arctan((B.X-C.X)/(B.Y-C.Y))
  • 90 degrees
  • arctan((A.Y-B.Y)/(A.X-B.X))

based on the formula tan(acute angle) = opposite side length/adjacent side length

Please note that AI2 uses degrees, not radians.

19

Ok, Ive used formulas to calculate the angle of each segment, and then another formula to calculate the angle between the lines. Working fine, but the moment the angle go over 90 degrees, it shows wrong, For example it works fine from 180 degrees down to 90, but the moment I go less than 90, for example 60 deg, it shows 240.

angle Blocks4
angle Blocks41018×728 144 KB

I have tried something similar to this, as well:

angle Blocks3
angle Blocks31012×413 77.5 KB

Any idea what I can change?

20

Again, back in my first example, it output all three angles. Somehow, one has to figure out which angle is the correct one to apply, given it all depends on which way around you draw the lines, perhaps a user choice ?

21

The first section of your code computes some tangents, and drops them into variables with 2-letter names.

The special case you are complaining about might cause negative tangent values, which you can test for before deciding how to calculate your angles.

I am unfamiliar with the formula you use for calculating angles, with the "1 + tan()*tan()" denominator. Can you post a link to its derivation?

If your formula response is out of range but can be normalized, here are some normalization routines in this sample app:

22

The link is in post #2 of this topic

23

I see three Notes between the formula derivation and examples at

One of the notes warns against bad results if one of the lines is parallel to one of the axes.

For a really limber cyclist, I would fall back to a solution using the atan2 function, which is less susceptible to infinite slopes.

24

Thanks for all the usefull info. I will work through them and get to a solution :slight_smile:
Time to use more brain power