How do I create a dynamic array of arrays (of arrays)? - c++

Question

I'm trying to create a dynamic array of arrays (of arrays). But for some reason the data gets corrupted. I'm using the data to generate a texture in a OpenGL application.
The following code works fine:
unsigned char imageData[64][64][3];
for (int i = 0; i < 64; i++)
{
for (int j = 0; j < 64; j++)
{
unsigned char r = 0, g = 0, b = 0;
if (i < 32)
{
if (j < 32)
r = 255;
else
b = 255;
}
else
{
if (j < 32)
g = 255;
}
imageData[i][j][0] = r;
imageData[i][j][1] = g;
imageData[i][j][2] = b;
}
std::cout << std::endl;
}
glTexImage2D(target, 0, GL_RGB, 64, 64, 0, GL_RGB, GL_UNSIGNED_BYTE, imageData);
Problem is, I want to be able to create a texture of any size (not just 64*64). So I'm trying this:
unsigned char*** imageData = new unsigned char**[64]();
for (int i = 0; i < 64; i++)
{
imageData[i] = new unsigned char*[64]();
for (int j = 0; j < 64; j++)
{
imageData[i][j] = new unsigned char[3]();
unsigned char r = 0, g = 0, b = 0;
if (i < 32)
{
if (j < 32)
r = 255;
else
b = 255;
}
else
{
if (j < 32)
g = 255;
}
imageData[i][j][0] = r;
imageData[i][j][1] = g;
imageData[i][j][2] = b;
}
std::cout << std::endl;
}
glTexImage2D(target, 0, GL_RGB, 64, 64, 0, GL_RGB, GL_UNSIGNED_BYTE, imageData);
But that doesn't work, the image gets all messed up so I assume I'm creating the array of arrays (of arrays) incorrectly? What am I doing wrong?
Also, I guess I should be using vectors instead. But how can I cast the vector of vectors of vectors data into a (void *) ?

Answer 1

This line contains multiple bugs:
unsigned char* pixel = &(imageData[(y * height) + x]);
You should multiply x by height and add y. And there's also the fact that each pixel is actually 3 bytes. Some issues that led to this bug in your code (and will lead to to others)
You should also be using std::vector. You can call std::vector::data to get a pointer to the underlying data to interface to C API's.
You should have a class that represents a pixel. This will handle the offsetting correctly and give things names and made the code clearer.
Whenever you are working with a multi dimensional array that you encode into a single dimensional one, you should try to carefully write an access function that takes care of indexing so you can test it separately.
(end bulleted list... oh SO).
struct Pixel {
unsigned char red;
unsigned char blue;
unsigned char green;
};
struct TwoDimPixelArray {
TwoDimArray(int width, int height)
: m_width(width), m_height(height)
{
m_vector.resize(m_width * m_height);
}
Pixel& get(int x, int y) {
return m_vector[x*height + y];
}
Pixel* data() { return m_vector.data(); }
private:
int m_width;
int m_height;
std::vector<Pixel> m_vector;
}
int width = 64;
int height = 64;
TwoDimPixelArray imageData(width, height);
for (int x = 0; x != width ; ++ x) {
for (int y = 0; y != height ; ++y) {
auto& pixel = imageData.get(x, y);
// ... pixel.red = something, pixel.blue = something, etc
}
}
glTexImage2D(target, 0, GL_RGB, 64, 64, 0, GL_RGB, GL_UNSIGNED_BYTE, imageData.data());

Answer 2

You need to use continuous memory for it to work with opengl.
My solution is inspired by previous answers, with a different indexing system
unsigned char* imageData = new unsigned char[width*height*3];
unsigned char r, g, b;
const unsigned int row_size_bytes = width * 3;
for( unsigned int x = 0; x < width; x++ ) {
unsigned int current_row_offset_bytes = x * 3;
for( unsigned int y = 0; y < height; y++ ) {
unsigned int one_dim_offset = y * row_size_bytes + current_row_offset_bytes
unsigned char* pixel = &(imageData[one_dim_offset]);
pixel[0] = r;
pixel[1] = g;
pixel[2] = b;
}
}
Unfortunnately it's untested, but i'm confident assuming sizeof(char) is 1.