Yalantis/uCrop · error · CImgArgumentException

draw_spline(): Specified color is (null).

Error message

draw_spline(): Specified color is (null).

What it means

draw_spline() with endpoint tangents (x0,y0,u0,v0)-(x1,y1,u1,v1) validates that the color pointer is non-null before rasterizing the spline. A null color makes pixel output impossible, so CImgArgumentException is thrown.

Source

Thrown at ucrop/src/main/jni/CImg.h:50430

       The starting and ending velocities (\p u0,\p v0) and (\p u1,\p v1) can be deduced easily from
       the control points as
       \p u0 = (\p xa - \p x0), \p v0 = (\p ya - \p y0), \p u1 = (\p x1 - \p xb) and \p v1 = (\p y1 - \p yb).
       \par Example:
       \code
       CImg<unsigned char> img(100,100,1,3,0);
       const unsigned char color[] = { 255,255,255 };
       img.draw_spline(30,30,0,100,90,40,0,-100,color);
       \endcode
    **/
    template<typename tc>
    CImg<T>& draw_spline(const int x0, const int y0, const float u0, const float v0,
                         const int x1, const int y1, const float u1, const float v1,
                         const tc *const color, const float opacity=1,
                         const float precision=0.25, const unsigned int pattern=~0U,
                         const bool init_hatch=true) {
      if (is_empty()) return *this;
      if (!color)
        throw CImgArgumentException(_cimg_instance
                                    "draw_spline(): Specified color is (null).",
                                    cimg_instance);
      if (x0==x1 && y0==y1) return draw_point(x0,y0,color,opacity);
      bool ninit_hatch = init_hatch;
      const float
        ax = u0 + u1 + 2*(x0 - x1),
        bx = 3*(x1 - x0) - 2*u0 - u1,
        ay = v0 + v1 + 2*(y0 - y1),
        by = 3*(y1 - y0) - 2*v0 - v1,
        _precision = 1/(cimg::hypot((float)x0 - x1,(float)y0 - y1)*(precision>0?precision:1));
      int ox = x0, oy = y0;
      for (float t = 0; t<1; t+=_precision) {
        const float t2 = t*t, t3 = t2*t;
        const int
          nx = (int)(ax*t3 + bx*t2 + u0*t + x0),
          ny = (int)(ay*t3 + by*t2 + v0*t + y0);
        draw_line(ox,oy,nx,ny,color,opacity,pattern,ninit_hatch);
        ninit_hatch = false;

View on GitHub (pinned to f788b534b4)

Solutions

  1. Pass a valid color array with at least image.spectrum() elements.
  2. Guard the call with if (color) before draw_spline.
  3. Provide a default color constant in your rendering helper.
  4. Audit the color producer for null-returning error paths.

Example fix

// before
img.draw_spline(0,0,0,0,100,100,0,0,nullptr,1.0f);
// after
const unsigned char color[3] = {0,200,120};
img.draw_spline(0,0,0,0,100,100,0,0,color,1.0f);
Defensive patterns

Strategy: type-guard

Validate before calling

if (!color) {
  static const unsigned char blue[3] = {0,0,255};
  color = blue;
}
img.draw_spline(x0,y0,u0,v0,x1,y1,u1,v1, color, opacity, precision);

Type guard

template<typename T> bool hasColor(const T* color) { return color != nullptr; }

Try / catch

try {
  img.draw_spline(x0,y0,u0,v0,x1,y1,u1,v1, color, opacity);
} catch (const cimg_library::CImgArgumentException& e) {
  std::fprintf(stderr, "draw_spline: %s\n", e.what());
}

Prevention

When it happens

Trigger: Calling draw_spline(x0,y0,u0,v0,x1,y1,u1,v1,color,...) with color==nullptr; same root causes as any null color pointer: failed lookup, uninitialized buffer, optional parameter forwarded as null.

Common situations: Curved path rendering where color comes from a theme/config that failed to load; templated code where tc* is null on a fallback path.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/cfce2172ff36f72b. Report an issue: GitHub.