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| 1 | +# Mathematical Explanation of Human-Like Mouse Movement Algorithm |
| 2 | + |
| 3 | +This document provides a **complete, step-by-step mathematical and |
| 4 | +visual explanation** of how the `move_naturally` function computes |
| 5 | +**Bezier curve control points** and moves the mouse in a human-like |
| 6 | +manner. |
| 7 | + |
| 8 | +Everything explained here corresponds directly to the code logic. |
| 9 | + |
| 10 | +------------------------------------------------------------------------ |
| 11 | + |
| 12 | +## 1. What Problem Are We Solving? |
| 13 | + |
| 14 | +Robotic mouse movement looks like this: |
| 15 | + |
| 16 | +- Constant speed |
| 17 | +- Straight lines |
| 18 | +- No curvature |
| 19 | + |
| 20 | +Humans move a mouse like this: |
| 21 | + |
| 22 | +- Accelerate → cruise → decelerate |
| 23 | +- Slight curves |
| 24 | +- Small randomness |
| 25 | + |
| 26 | +The solution used here is a **Cubic Bezier Curve**. |
| 27 | + |
| 28 | +------------------------------------------------------------------------ |
| 29 | + |
| 30 | +## 2. XY Coordinate System |
| 31 | + |
| 32 | +We use a standard XY graph: |
| 33 | + |
| 34 | +- X axis → horizontal |
| 35 | +- Y axis → vertical |
| 36 | + |
| 37 | +Example values used throughout this explanation: |
| 38 | + |
| 39 | + Point Meaning Coordinates |
| 40 | + ------- --------- ------------- |
| 41 | + S Start (100, 100) |
| 42 | + E End (400, 300) |
| 43 | + |
| 44 | +------------------------------------------------------------------------ |
| 45 | + |
| 46 | +## 3. Step 1 -- Direction Vector (Straight Line) |
| 47 | + |
| 48 | +From the code: |
| 49 | + |
| 50 | +``` python |
| 51 | +dx = x - start_x |
| 52 | +dy = y - start_y |
| 53 | +``` |
| 54 | + |
| 55 | +Calculation: |
| 56 | + |
| 57 | + dx = 400 - 100 = 300 |
| 58 | + dy = 300 - 100 = 200 |
| 59 | + |
| 60 | +So the straight-line direction vector is: |
| 61 | + |
| 62 | + SE = (300, 200) |
| 63 | + |
| 64 | +Distance between points: |
| 65 | + |
| 66 | + distance = sqrt(dx² + dy²) |
| 67 | + = sqrt(300² + 200²) |
| 68 | + = sqrt(130000) |
| 69 | + ≈ 360.56 |
| 70 | + |
| 71 | +------------------------------------------------------------------------ |
| 72 | + |
| 73 | +## 4. Step 2 -- Perpendicular Vector (Creates Curvature) |
| 74 | + |
| 75 | +To avoid straight movement, a perpendicular vector is created: |
| 76 | + |
| 77 | +``` python |
| 78 | +px = -dy |
| 79 | +py = dx |
| 80 | +``` |
| 81 | + |
| 82 | +Calculation: |
| 83 | + |
| 84 | + px = -200 |
| 85 | + py = 300 |
| 86 | + |
| 87 | +This vector points sideways relative to the direct path. |
| 88 | + |
| 89 | +------------------------------------------------------------------------ |
| 90 | + |
| 91 | +## 5. Step 3 -- Normalize the Perpendicular Vector |
| 92 | + |
| 93 | +Normalize to length 1: |
| 94 | + |
| 95 | + length = sqrt(px² + py²) |
| 96 | + = sqrt(200² + 300²) |
| 97 | + = sqrt(130000) |
| 98 | + ≈ 360.56 |
| 99 | + |
| 100 | +Normalized vector: |
| 101 | + |
| 102 | + px = -200 / 360.56 ≈ -0.555 |
| 103 | + py = 300 / 360.56 ≈ 0.832 |
| 104 | + |
| 105 | +------------------------------------------------------------------------ |
| 106 | + |
| 107 | +## 6. Step 4 -- Random Arc Offset |
| 108 | + |
| 109 | +From the code: |
| 110 | + |
| 111 | +``` python |
| 112 | +arc_scale = random.uniform(-0.2, 0.2) |
| 113 | +offset_x = px * distance * arc_scale |
| 114 | +offset_y = py * distance * arc_scale |
| 115 | +``` |
| 116 | + |
| 117 | +Assume: |
| 118 | + |
| 119 | + arc_scale = 0.1 |
| 120 | + |
| 121 | +Then: |
| 122 | + |
| 123 | + offset_x = -0.555 * 360.56 * 0.1 ≈ -20 |
| 124 | + offset_y = 0.832 * 360.56 * 0.1 ≈ 30 |
| 125 | + |
| 126 | +This offset bends the path. |
| 127 | + |
| 128 | +------------------------------------------------------------------------ |
| 129 | + |
| 130 | +## 7. Step 5 -- Compute Bezier Control Points |
| 131 | + |
| 132 | +Control points are placed **along the path** but shifted sideways. |
| 133 | + |
| 134 | +### Control Point C1 (25% of the path) |
| 135 | + |
| 136 | + C1_x = start_x + 0.25*dx + offset_x |
| 137 | + = 100 + 75 - 20 |
| 138 | + = 155 |
| 139 | + |
| 140 | + C1_y = start_y + 0.25*dy + offset_y |
| 141 | + = 100 + 50 + 30 |
| 142 | + = 180 |
| 143 | + |
| 144 | + C1 = (155, 180) |
| 145 | + |
| 146 | +------------------------------------------------------------------------ |
| 147 | + |
| 148 | +### Control Point C2 (75% of the path) |
| 149 | + |
| 150 | + C2_x = start_x + 0.75*dx + offset_x |
| 151 | + = 100 + 225 - 20 |
| 152 | + = 305 |
| 153 | + |
| 154 | + C2_y = start_y + 0.75*dy + offset_y |
| 155 | + = 100 + 150 + 30 |
| 156 | + = 280 |
| 157 | + |
| 158 | + C2 = (305, 280) |
| 159 | + |
| 160 | +------------------------------------------------------------------------ |
| 161 | + |
| 162 | +## 8. XY Graph Diagram |
| 163 | + |
| 164 | + |
| 165 | + |
| 166 | +* **Dashed line**: Robotic straight path |
| 167 | +* **Curved line**: Human-like Bezier movement |
| 168 | +* **Start/End**: The mouse journey |
| 169 | +* **C1/C2**: Invisible control points pulling the curve |
| 170 | + |
| 171 | +------------------------------------------------------------------------ |
| 172 | + |
| 173 | +## 9. Bezier Curve Mathematical Formula |
| 174 | + |
| 175 | +Cubic Bezier equation: |
| 176 | + |
| 177 | + B(t) = (1−t)³S |
| 178 | + + 3(1−t)²tC1 |
| 179 | + + 3(1−t)t²C2 |
| 180 | + + t³E |
| 181 | + |
| 182 | +Where: - `t ∈ [0, 1]` - `B(0) = Start` - `B(1) = End` |
| 183 | + |
| 184 | +------------------------------------------------------------------------ |
| 185 | + |
| 186 | +## 10. Example: Bezier Point at t = 0.5 |
| 187 | + |
| 188 | +### X coordinate: |
| 189 | + |
| 190 | + B_x(0.5) = |
| 191 | + (0.5³ * 100) |
| 192 | + + 3*(0.5²*0.5*155) |
| 193 | + + 3*(0.5*0.5²*305) |
| 194 | + + (0.5³ * 400) |
| 195 | + |
| 196 | + ≈ 235 |
| 197 | + |
| 198 | +### Y coordinate: |
| 199 | + |
| 200 | + B_y(0.5) = |
| 201 | + (0.5³ * 100) |
| 202 | + + 3*(0.5²*0.5*180) |
| 203 | + + 3*(0.5*0.5²*280) |
| 204 | + + (0.5³ * 300) |
| 205 | + |
| 206 | + ≈ 223 |
| 207 | + |
| 208 | +So the midpoint lies at: |
| 209 | + |
| 210 | + (235, 223) |
| 211 | + |
| 212 | +------------------------------------------------------------------------ |
| 213 | + |
| 214 | +## 11. Acceleration (Ease-In-Out) |
| 215 | + |
| 216 | +The function applies an **Ease-In-Out Quadratic** easing to the time variable `t`. This transforms linear time (constant speed) into "perceived" time `alpha` (variable speed). |
| 217 | + |
| 218 | + |
| 219 | + |
| 220 | +### The Math |
| 221 | + |
| 222 | +We use a piecewise function based on the progress `t` (from 0 to 1): |
| 223 | + |
| 224 | +1. **Acceleration Phase** (Start to Halfway, `t < 0.5`): |
| 225 | + ```math |
| 226 | + alpha = 2 * t^2 |
| 227 | + ``` |
| 228 | + * At `t=0`, speed is 0. |
| 229 | + * Velocity increases linearly. |
| 230 | + |
| 231 | +2. **Deceleration Phase** (Halfway to End, `t >= 0.5`): |
| 232 | + ```math |
| 233 | + alpha = -1 + (4 - 2 * t) * t |
| 234 | + ``` |
| 235 | + * This is an inverted parabola. |
| 236 | + * Velocity decreases linearly to 0 at the end. |
| 237 | + |
| 238 | +### Why this specific formula? |
| 239 | + |
| 240 | +At the midpoint `t = 0.5`: |
| 241 | +* **Formula 1:** $2 * (0.5)^2 = 0.5$ |
| 242 | +* **Formula 2:** $-1 + (4 - 1) * 0.5 = 0.5$ |
| 243 | + |
| 244 | +The values match perfectly, and more importantly, the **derivative (speed)** matches, ensuring a perfectly smooth transition with no "jerk" at the halfway point. This mimics the physics of a hand starting a movement, reaching peak speed, and slowing down to stop. |
| 245 | + |
| 246 | +## 12. Why This Looks Human |
| 247 | + |
| 248 | + Feature Effect |
| 249 | + ---------------------- --------------- |
| 250 | + Perpendicular offset Curved motion |
| 251 | + Random arc No repetition |
| 252 | + Bezier curve Smooth path |
| 253 | + Ease-in-out Natural speed |
| 254 | + |
| 255 | +------------------------------------------------------------------------ |
| 256 | + |
| 257 | +## 13. Final Result |
| 258 | + |
| 259 | +✔ Natural-looking mouse movement\ |
| 260 | +✔ Hard to distinguish from real user\ |
| 261 | +✔ Smooth curves and timing\ |
| 262 | +✔ No sharp jumps |
| 263 | + |
| 264 | +------------------------------------------------------------------------ |
| 265 | + |
| 266 | +### End of Detailed Explanation |
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