### 1. unit axis intercepts [solved]

- Posted by petelomax Sep 20, 2023
- 857 views
- Last edited Sep 23, 2023

I've written a natty little routine which bisects a bezier curve into sub-pixel fragments and that

part is working fine, and once I've got {x1,y1} and {x2,y2} within a 1x1 pixel square, I want to

know whether they intersect the unit axis, for which "ceil(min(x1,x2))!=ceil(max(x1,x2))" and the

same for the unit y axis is fine. However, I also want to know **where** it intercepts the unit axis,

which btw is floor(max(x1,x2)), and also incidentally I'm happy with the edge cases of 0.1..1 not

being an intercept but its partner 1..1.9 being one, and wouldn't want both deemed one anyway.

Once it is that small, I think it is safe to treat the curve fragment as a straight line.

I'm sure that's all clear as mud.

So I've got minx, maxx, unitx, ditto y, but in want of a hitux and hituy. I think the problem is

(flat|steep) * (\|/) * (up|down or left|right) and I'm currently staring at (a roughly estimated)

40 or so glitches in a test set of 625 cases and a brain clogged with min/max/ceil/floor/abs when

maybe I don't really need or want any of them?? I somehow believe there is a dirt-simple solution

itching to get out, but I just can't see it.

To recap/summarise, given some {x1,y1}, {x2,y2}: where does it hit a straddled whole x, ditto y?

You cannot make any assumptions about the first point being left or right or above or below the

other one, but you can assume they *are* strictly less than 1 apart, in both directions.

### 2. Re: unit axis intercepts

- Posted by ChrisB (moderator) Sep 20, 2023
- 852 views

I asked ChatGPT, because my brain exploded.

It sounds like you want to determine whether a line segment defined by two points {x1, y1} and {x2, y2} intersects the unit x and y axes, and if it does, where it intersects them. Additionally, you want to handle the special case where a segment from 0.1 to 1 on the axis is not considered an intercept, but 1 to 1.9 is. Here's a possible approach to solve this problem:

1. Calculate the intersection points of the line segment with the unit x and y axes.

To find the intersection points with the unit x-axis, you can use the equation of the line:

y = m * x + b

Where m is the slope of the line and b is the y-intercept. In your case, you can calculate the slope as:

m = (y2 - y1) / (x2 - x1)

Then, calculate the y-coordinate of the intersection point with the unit x-axis (where x = 0):

y_intersection_x = m * 0 + b = b

So, you have found the intersection point with the unit x-axis, which is {0, y_intersection_x}.

To find the intersection point with the unit y-axis, you can use the same equation with x = 1:

y_intersection_y = m * 1 + b

2. Check if the intersection points are within the desired range.

You mentioned that you want to consider an intercept only if it's within certain ranges. For the unit x-axis, you can check if the y-coordinate of the intersection point is within the range [0.1, 1.9]. For the unit y-axis, you can check if the y-coordinate of the intersection point is within the range [0.1, 1.9].

Here's some Python-like pseudocode to implement this:

```python # Calculate the slope of the line m = (y2 - y1) / (x2 - x1) # Calculate the y-intercept b = y1 - m * x1 # Calculate the intersection point with the unit x-axis y_intersection_x = b # Calculate the intersection point with the unit y-axis y_intersection_y = m * 1 + b # Check if the intersections are within the desired range if 0.1 <= y_intersection_x <= 1.9: # Intersection with unit x-axis is within the desired range hitux = True else: hitux = False if 0.1 <= y_intersection_y <= 1.9: # Intersection with unit y-axis is within the desired range hituy = True else: hituy = False ```

This code should help you determine whether the line segment intersects the unit x and y axes within the specified ranges. If it does, the `hitux` and `hituy` variables will be set to True, indicating an intersection, and you can also retrieve the intersection points if needed.

### 3. Re: unit axis intercepts

- Posted by ChrisB (moderator) Sep 20, 2023
- 848 views

I also asked it towrite the code in Euphoria

To determine where a Bezier curve intersects the unit axis and whether it straddles a whole pixel, you can follow these steps in Euphoria:

-- Function to find the intersection points with unit axis function findIntersectionPoints(x1, y1, x2, y2) -- Determine whether the curve is flat or steep flat = abs(y2 - y1) < abs(x2 - x1) if flat then -- Swap x and y coordinates for a flat curve x1, y1 = y1, x1 x2, y2 = y2, x2 end -- Determine whether the curve is going up or down up = y1 > y2 -- Calculate the intersection point on the unit y axis if up then hituy = floor(y2) else hituy = ceil(y2) end -- Calculate the intersection point on the unit x axis hitux = ((hituy - y1) / (y2 - y1)) * (x2 - x1) + x1 if flat then -- Swap x and y coordinates back if it's a flat curve hitux, hituy = hituy, hitux end -- Check if the intersection point straddles a whole pixel straddles_x = ceil(x1) != ceil(x2) straddles_y = ceil(y1) != ceil(y2) return hitux, hituy, straddles_x, straddles_y end -- Example usage x1 = 0.5 y1 = 0.5 x2 = 1.5 y2 = 1.9 hitux, hituy, straddles_x, straddles_y = findIntersectionPoints(x1, y1, x2, y2) if straddles_x then ? "X-axis intersection at ", hitux else ? "No X-axis intersection" end if straddles_y then ? "Y-axis intersection at ", hituy else ? "No Y-axis intersection" end

This code defines a `findIntersectionPoints` function that calculates where the Bezier curve intersects the unit axis (both X and Y) and whether it straddles a whole pixel. The function accounts for whether the curve is flat or steep and whether it's going up or down. It also provides example usage and prints the results.

### 4. Re: unit axis intercepts

- Posted by ChrisB (moderator) Sep 20, 2023
- 855 views

Oh, i have no idea whether any of this makes sense, I just wanted to copy and paste your question into chatgpt and see what happened.

It also didn't want to play global thermonuclear with me, which was a bit disappointing.

Cheers

Chris

### 5. Re: unit axis intercepts

- Posted by petelomax Sep 20, 2023
- 815 views

I give it 11/10 for "brazenly pretending to be useable"... It actually hit the nail on the head,

in that I was (stupidly) trying to use minx + *something*, when I should have been using

x1 +[/-] *let the sign of the slope dictate which way*. Anyway, so far, it's certainly progress.

When I tried, didn't get anything near as good, no idea why not (and boy was it slow).

Next time you use ChatGPT, you should try pasting errors from trying to run the code,

since that should (hopefully quite quickly) teach ChatGPT to emit better Phix code.

Thanks, Pete

### 6. Re: unit axis intercepts

- Posted by katsmeow Sep 21, 2023
- 771 views

It also didn't want to play global thermonuclear with me, which was a bit disappointing.

Wrong hardware. GTW was played on a massively configurable parallel Thinking Machine CM-5 (named WOPR in the movie, and was a prop, not an actual computer), of the sort NASA once used to descramble satellite images. The confusion may be because the company that made the Connection Machine was Thinking Machine Corporation. I don't know what they were thinking. The movie's game was ported to 8-bit computers of the early 1980's, and just this year Russia threatened to use nukes again so , still playable, i guess.

Kat

### 7. Re: unit axis intercepts

- Posted by ChrisB (moderator) Sep 22, 2023
- 768 views

Heh heh.

Well spotted! (Miss Falcon?)

Cheers

Chris