cannot work out why SDL2 streaming texture is black - c++

Question

I am trying to draw individual pixels to an SDL2 streaming texture. I am attempting to store my pixels in a std::vector and then lock my texture, update the texture then unlock, pass it to the renderer and the present it.
However, I am always getting a black screen. I have tried several permutations of the below code but I am at a loss.
#include "pch.h"
#include "Window.h"
Window::Window(unsigned int width, unsigned int height) :
m_width(width), m_height(height)
{
if (SDL_Init(SDL_INIT_VIDEO || SDL_INIT_TIMER) < 0)
{
std::cout << "SDL failed to initilize correct. Error: "
<< SDL_GetError << std::endl;
}
m_window = SDL_CreateWindow("Cotton Rasterizer - Hardeep Bahia 2021", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, m_width, m_height, 0);
if (m_window == NULL)
{
std::cout << "Window failed to initilize: " << SDL_GetError << std::endl;
}
m_renderer = SDL_CreateRenderer(m_window, -1, SDL_RENDERER_TARGETTEXTURE);
if (m_renderer == NULL)
{
std::cout << "Renderer failed to initilize: " << SDL_GetError << std::endl;
}
m_texture = SDL_CreateTexture(m_renderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_STREAMING, m_width, m_height);
if (m_texture == NULL)
{
std::cout << "Texture failed to initilize: " << SDL_GetError << std::endl;
}
auto p_format = SDL_GetWindowPixelFormat(m_window);
m_pixel_format = SDL_AllocFormat(p_format);
}
Window::~Window()
{
SDL_FreeFormat(m_pixel_format);
SDL_DestroyTexture(m_texture);
SDL_DestroyRenderer(m_renderer);
SDL_DestroyWindow(m_window);
}
void Window::test()
{
SDL_Rect filledRect = { m_width / 4, m_height / 4, m_width / 2, m_height / 2 };
SDL_SetRenderDrawColor(m_renderer, 255, 0, 0, 255);
SDL_RenderFillRect(m_renderer, &filledRect);
}
void Window::present()
{
SDL_RenderPresent(m_renderer);
}
void Window::clear()
{
SDL_SetRenderDrawColor(m_renderer, 0.f, 0.f, 0.f, 255.f);
SDL_RenderClear(m_renderer);
}
void Window::draw(const std::vector<Pixel>& i_pixels)
{
int pitch {(m_width * 4)};
std::vector<Uint32> pixels(m_width * m_height * 4, 0);
Uint32 color = SDL_MapRGBA(m_pixel_format, 255, 255, 255, 255);
for (auto& a : i_pixels)
{
unsigned int offset = (m_width * 4 * a.m_position.y) + a.m_position.x * 4;
pixels[offset + 0] = 255;
pixels[offset + 1] = 255;
pixels[offset + 2] = 255;
pixels[offset + 3] = 255;
}
unsigned char* locked_pixels{};
SDL_LockTexture(m_texture, NULL, reinterpret_cast<void **> (&locked_pixels) , &pitch);
std::memcpy(locked_pixels, pixels.data(), pixels.size());
SDL_UpdateTexture(m_texture, NULL, pixels.data(), pitch);
SDL_UnlockTexture(m_texture);
SDL_RenderCopy(m_renderer, m_texture, NULL, NULL);
}
My test function does draw, and is visible on the screen but it does not use the texture.

Answer 1

Resolved it by changing the draw function to as follow:
void Window::draw(const std::vector<Pixel>& i_pixels)
{
std::vector<unsigned char>pixels(m_width * m_height * 4, 0);
for (auto& i : i_pixels)
{
unsigned int x = i.m_position.x;
unsigned int y = i.m_position.y;
const unsigned int offset = (m_width * 4 * y) + (x * 4);
pixels[offset] = 255;
pixels[offset + 1] = 255;
pixels[offset + 2] = 255;
pixels[offset + 3] = 255;
}
unsigned char* lockedpixels = nullptr;
int pitch = (m_width * 4);
SDL_LockTexture(m_texture, NULL, reinterpret_cast<void**>(&lockedpixels), &pitch);
std::memcpy(lockedpixels, pixels.data(), pixels.size());
SDL_UpdateTexture(m_texture, NULL, pixels.data(), m_width * 4);
SDL_UnlockTexture(m_texture);
SDL_RenderCopy(m_renderer, m_texture, NULL, NULL);
}