How to get X and Y pixels of a Image in SDL2?

Question

I am trying to create a Voxel Space Engine In SDL2 C++. However the tutorial I am following reads the X and Y pixels of the heightmap image. I can't find a way to do this.

I am following this tutorial: https://github.com/s-macke/VoxelSpace

Code from the tutorial:

def Render(p, height, horizon, scale_height, distance, screen_width, screen_height):
    # Draw from back to the front (high z coordinate to low z coordinate)
    for z in range(distance, 1, -1):
        # Find line on map. This calculation corresponds to a field of view of 90°
        pleft  = Point(-z + p.x, -z + p.y)
        pright = Point( z + p.x, -z + p.y)
        # segment the line
        dx = (pright.x - pleft.x) / screen_width
        # Raster line and draw a vertical line for each segment
        for i in range(0, screen_width):
            height_on_screen = (height - heightmap[pleft.x, pleft.y]) / z * scale_height. + horizon
            DrawVerticalLine(i, height_on_screen, screen_height, colormap[pleft.x, pleft.y])
            pleft.x += dx

# Call the render function with the camera parameters:
# position, height, horizon line position,
# scaling factor for the height, the largest distance, 
# screen width and the screen height parameter
Render( Point(0, 0), 50, 120, 120, 300, 800, 600 )

My code:

#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>
#include <iostream>
using namespace std;

int main(){
        SDL_Init(SDL_INIT_VIDEO);
        IMG_Init(IMG_INIT_PNG);
        SDL_Window* window = SDL_CreateWindow("Voxel Space", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 480, SDL_WINDOW_SHOWN);
        SDL_Renderer* renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);
        int w = 640;
        int h = 480;
        int pixelX = 0;
        int pixelY = 0;
        int renderDistance = 1;
        bool quit = false;
        SDL_Surface* cm = IMG_Load("colormap.png");
        SDL_Surface* hm = IMG_Load("heightmap.png");
        SDL_SetRenderDrawColor(renderer, 0, 0, 0, 0);
        while(quit){
                SDL_Event e;
                while(SDL_PollEvent(&e)){
                        if(e.type == SDL_QUIT){
                                quit == true;
                        }
                }
                SDL_RenderClear(renderer);
                for(int z = 128; z > renderDistance; z--){
                        int pixelLeftX = (-z + pixelX);
                        int pixelLeftY = (-z + pixelY);
                        int pixelRightX = (z + pixelX);
                        int pixelRightY = (-z + pixelY);
                        double dx = (pixelRightX - pixelLeftX) / w;
                        for(int i = 0; i < w; i++){
                                double hm.x = pixelLeftX;
                                double hm.y = pixelLeftY;
                                heightOnScreen = (50 - hm.xy) / z * h + 120;
                                for(int y = 0; y < i; y++){
                                        SDL_SetRenderDrawColor(renderer, 0, 255, 0, 0);
                                        SDL_RenderDrawPoint(renderer, heightOnScreen, w);
                                        pixelLeftX += dx;
                                }
                        }
                }
                SDL_RenderPresent(renderer);
        }
        return 0;
}

How can I read the X and Y pixels of the heightmap image in SDL2?

Answer 1

Fastest way (and the more lightweight one) is to use SDL_LockSurface https://wiki.libsdl.org/SDL_LockSurface over the image SDL_Surface and after executing it checking the pixels property in the surface which should contain a pointer to the pixels. This method is similar for SDL/SDL2/SDL3. Otherwise you can use slower methods like pixel read with SDL_ReadSurfacePixel or similar, but I do not recomend it.

Aditionally I would avoid using doubles and SDL_RenderDrawPoint: instead is better to use SDL_LockSurface for the screen as well.