spine-runtimes/todos/done/2025-01-21-10-06-07-cpp-clipping-broken-analysis.md

Now I'll perform a detailed comparison of the clipping implementations between the Java reference and C++ port, focusing on the clipTriangles methods and other critical functions.

Detailed Comparison: spine-libgdx vs spine-cpp SkeletonClipping

After analyzing both implementations line by line, I've identified several significant differences and bugs in the C++ port. Here's my detailed analysis:

CRITICAL BUG #1: Loop Index Increment Error

Location: C++ implementation, line 249 in clipTriangles with UVs

// C++ (line 249) - INCORRECT
i += 3;  // This increments i by an additional 3!
break;

Java reference (lines 268, 358):

// Java - CORRECT, no additional increment
index += 3;
break;

Impact: This bug causes the C++ implementation to skip triangles when clipping fails. The outer loop already increments i += 3, so this additional increment makes it skip every other triangle group, leading to incomplete clipping results.

BUG #2: Missing Continue Statement

Location: C++ implementation, lines 322-324, 332-336

// C++ - Missing 'continue' after adding vertices in valid intersection case
if (t >= 0 && t <= 1) {
    output->add(inputX + ix * t);
    output->add(inputY + iy * t);
} else {
    output->add(inputX2);
    output->add(inputY2);
    // Missing continue here!
}

Java reference (lines 414-418, 426-430):

// Java - Has continue statements
if (t >= 0 && t <= 1) {
    output.add(inputX + ix * t);
    output.add(inputY + iy * t);
} else {
    output.add(inputX2);
    output.add(inputY2);
    continue;  // Present in Java
}

Impact: Without the continue statements, the C++ code will always set clipped = true even when the intersection calculation fails, potentially affecting the algorithm's correctness.

DIFFERENCE #3: Return Value Logic

Java implementation (line 155):

return clipOutputItems != null;

C++ implementation (line 158):

return clipped;

Analysis: The Java version tracks whether any clipping output was generated (clipOutputItems != null), while C++ tracks whether any clipping occurred (clipped). These are subtly different conditions that could lead to different behavior.

DIFFERENCE #4: Additional Safety Check

C++ implementation has an additional safety check (lines 366-369):

if (originalOutput->size() < 6) {
    originalOutput->clear();
    return false;
}

This check ensures a minimum polygon size, which is absent in the Java version.

DIFFERENCE #5: Data Type Differences

• Indices: Java uses short vs C++ uses unsigned short
• Loop counters: Java uses int vs C++ uses size_t
• Array access: Java has bounds checking vs C++ direct array access

DIFFERENCE #6: Method Signature Variations

The C++ implementation has additional overloaded methods:

• clipTriangles(Array<float> &vertices, Array<unsigned short> &triangles, Array<float> &uvs, size_t stride)
• Different parameter types and organization

DIFFERENCE #7: UV Coordinate Handling

Java implementation: Has separate methods for different UV handling scenarios, with complex stride logic in the second clipTriangles method.

C++ implementation: Simplified UV handling with dedicated _clippedUVs array that's always populated when UVs are provided.

Minor Differences

1. Variable naming: Java uses camelCase (clipOutput) vs C++ uses underscore prefix (_clipOutput)
2. Memory management: Java relies on GC vs C++ manual array management
3. Array operations: Different API calls for array manipulation

Summary

The most critical issues in the C++ port are:

1. Triangle skipping bug (line 249) - causes incomplete processing
2. Missing continue statements - affects clipping logic correctness
3. Different return value semantics - may cause behavioral differences

The C++ implementation appears to be a reasonable port overall, but these bugs need to be fixed to ensure compatibility with the Java reference implementation. The triangle skipping bug is particularly serious as it would cause visible rendering artifacts.

Files analyzed:

• /Users/badlogic/workspaces/spine-runtimes/spine-libgdx/spine-libgdx/src/com/esotericsoftware/spine/utils/SkeletonClipping.java
• /Users/badlogic/workspaces/spine-runtimes/spine-cpp/src/spine/SkeletonClipping.cpp
• /Users/badlogic/workspaces/spine-runtimes/spine-cpp/include/spine/SkeletonClipping.h