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.

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,

View on GitHub (pinned to f788b534b4)

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.