What causes "draw_rectangle(): Specified color is (null)." in Yalantis/uCrop?

Ensure the color buffer exists and points to at least spectrum() values before the call.

Yalantis/uCrop · error · CImgArgumentException

draw_rectangle(): Specified color is (null).

Error message

draw_rectangle(): Specified color is (null).

What it means

CImg's draw_rectangle() with a templated color pointer received a null color pointer. CImg throws CImgArgumentException because there are no pixel values to fill the rectangle with; drawing cannot proceed.

Solutions

Ensure the color buffer exists and points to at least spectrum() values before the call.
Use the array overload with a literal, e.g. unsigned char color[] = {255,0,0};
If color may be missing, early-return or draw with a default color instead of null.
Check the pointer for null before calling and handle the fallback path.

Example fix

// before
const unsigned char* color = vec.empty() ? nullptr : vec.data();
img.draw_rectangle(0,0,0,99,99,0, color, 1.0f); // throws when vec empty
// after
if (!vec.empty()) {
  img.draw_rectangle(0,0,0,99,99,0, vec.data(), 1.0f);
} else {
  const unsigned char black[3] = {0,0,0};
  img.draw_rectangle(0,0,0,99,99,0, black, 1.0f);
}

Defensive patterns

Strategy: type-guard

Validate before calling

if (!color || is_empty_color(color)) {
  static const unsigned char fallback[3] = {0, 0, 0};
  color = fallback;
}
img.draw_rectangle(x0,y0,z0,x1,y1,z1, color, opacity);

Type guard

template <typename tc>
bool color_is_valid(const tc* color, unsigned int spectrum) { return color != nullptr; }

Try / catch

try {
  img.draw_rectangle(x0,y0,z0,x1,y1,z1, color, opacity);
} catch (CImgArgumentException& e) {
  std::cerr << "Null color: " << e.what() << std::endl;
}

Prevention

Never pass .data() of an empty vector as a color
Use stack arrays for literal colors
Check pointers for null before any draw_* call

When it happens

Trigger: Calling img.draw_rectangle(x0,y0,z0,x1,y1,z1,color,opacity) where color==nullptr — typically an uninitialized pointer, a failed allocation, or an empty std::vector's data().

Common situations: Color array declared but never filled/allocated; .data() of an empty vector; refactoring changed a value overload to a pointer overload; conditionally-initialized color buffers.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/9f3f822cacf93837. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:52244

    //! Draw a filled 3D rectangle.
    /**
       \param x0 X-coordinate of the upper-left rectangle corner.
       \param y0 Y-coordinate of the upper-left rectangle corner.
       \param z0 Z-coordinate of the upper-left rectangle corner.
       \param x1 X-coordinate of the lower-right rectangle corner.
       \param y1 Y-coordinate of the lower-right rectangle corner.
       \param z1 Z-coordinate of the lower-right rectangle corner.
       \param color Pointer to \c spectrum() consecutive values of type \c T, defining the drawing color.
       \param opacity Drawing opacity.
    **/
    template<typename tc>
    CImg<T>& draw_rectangle(const int x0, const int y0, const int z0,
                            const int x1, const int y1, const int z1,
                            const tc *const color, const float opacity=1) {
      if (is_empty()) return *this;
      if (!color)
        throw CImgArgumentException(_cimg_instance
                                    "draw_rectangle(): Specified color is (null).",
                                    cimg_instance);
      cimg_forC(*this,c) draw_rectangle(x0,y0,z0,c,x1,y1,z1,c,(T)color[c],opacity);
      return *this;
    }

    //! Draw a filled 2D rectangle.
    /**
       \param x0 X-coordinate of the upper-left rectangle corner.
       \param y0 Y-coordinate of the upper-left rectangle corner.
       \param x1 X-coordinate of the lower-right rectangle corner.
       \param y1 Y-coordinate of the lower-right rectangle corner.
       \param color Pointer to \c spectrum() consecutive values of type \c T, defining the drawing color.
       \param opacity Drawing opacity.
    **/
    template<typename tc>
    CImg<T>& draw_rectangle(const int x0, const int y0,
                            const int x1, const int y1,

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