c++ - 窗口渲染上的 SDL2.0 屏幕 nullptr

Question

嘿，我对 SDL 库还比较陌生，只是想掌握它。

我找到了 Minecraft4k 的 C++ 转换，但它基于 SDL1.x，所以我试图将它转换为 SDL2.0

目前构建成功，但是当它到达时；

plot(x, y, rgbmul(col, fxmul(br, ddist)));

它抛出读取访问冲突异常:

screen was nullptr

这是我的代码；

// C++ port of Minecraft 4k JS (http://jsdo.it/notch/dB1E)
// By The8BitPimp
// See: the8bitpimp.wordpress.com

#include <SDL.h>
#include <math.h>
#include <windows.h>
#include <tchar.h>
#include "plot.h"
#include "llist.h"

const int w = 320;
const int h = 240;

SDL_Surface *screen = nullptr;

const float math_pi = 3.14159265359f;

static inline float math_sin(float x) {
    return sinf(x);
}

static inline float math_cos(float x) {
    return cosf(x);
}

// the texture map
int texmap[16 * 16 * 16 * 3];

// the voxel map
char map[64 * 64 * 64];

static inline int random(int max) {
 return (rand() ^ (rand() << 16)) % max;
}

static inline void plot(int x, int y, int c) {
int *p = (int*)screen->pixels;
p[y * w + x] = c;
}

static void makeTextures(void) {

// each texture
for (int j = 0; j<16; j++) {

    int k = 255 - random(96);

    // each pixel in the texture
    for (int m = 0; m<16 * 3; m++)
        for (int n = 0; n<16; n++) {

            int i1 = 0x966C4A;
            int i2 = 0;
            int i3 = 0;

            if (j == 4)
                i1 = 0x7F7F7F;
            if ((j != 4) || (random(3) == 0))
                k = 255 - random(96);
            if (j == 1)
            {
                if (m < (((n * n * 3 + n * 81) >> 2) & 0x3) + 18)
                    i1 = 0x6AAA40;
                else if (m < (((n * n * 3 + n * 81) >> 2) & 0x3) + 19)
                    k = k * 2 / 3;
            }
            if (j == 7)
            {
                i1 = 0x675231;
                if ((n > 0) && (n < 15) && (((m > 0) && (m < 15)) || ((m > 32) && (m < 47))))
                {
                    i1 = 0xBC9862;
                    i2 = n - 7;
                    i3 = (m & 0xF) - 7;
                    if (i2 < 0)
                        i2 = 1 - i2;

                    if (i3 < 0)
                        i3 = 1 - i3;

                    if (i3 > i2)
                        i2 = i3;

                    k = 196 - random(32) + i2 % 3 * 32;
                }
                else if (random(2) == 0)
                    k = k * (150 - (n & 0x1) * 100) / 100;
            }
            if (j == 5)
            {
                i1 = 0xB53A15;
                if (((n + m / 4 * 4) % 8 == 0) || (m % 4 == 0))
                    i1 = 0xBCAFA5;
            }
            i2 = k;
            if (m >= 32)
                i2 /= 2;
            if (j == 8)
            {
                i1 = 5298487;
                if (random(2) == 0)
                {
                    i1 = 0;
                    i2 = 255;
                }
            }

            // fixed point colour multiply between i1 and i2
            i3 =
                ((((i1 >> 16) & 0xFF) * i2 / 255) << 16) |
                ((((i1 >> 8) & 0xFF) * i2 / 255) << 8) |
                ((i1 & 0xFF) * i2 / 255);
            // pack the colour away
            texmap[n + m * 16 + j * 256 * 3] = i3;
        }
}
}

static void makeMap(void) {
// add random blocks to the map
for (int x = 0; x < 64; x++) {
    for (int y = 0; y < 64; y++) {
        for (int z = 0; z < 64; z++) {
            int i = (z << 12) | (y << 6) | x;
            float yd = (y - 32.5) * 0.4;
            float zd = (z - 32.5) * 0.4;
            map[i] = random(16);

            float th = random(256) / 256.0f;

            if (th > sqrtf(sqrtf(yd * yd + zd * zd)) - 0.8f)
                map[i] = 0;
        }
    }
}
}

static void init(void) {
  makeTextures();
  makeMap();
}

// fixed point byte byte multiply
static inline int fxmul(int a, int b) {
return (a*b) >> 8;
}

// fixed point 8bit packed colour multiply
static inline int rgbmul(int a, int b) {
int _r = (((a >> 16) & 0xff) * b) >> 8;
int _g = (((a >> 8) & 0xff) * b) >> 8;
int _b = (((a)& 0xff) * b) >> 8;
return (_r << 16) | (_g << 8) | _b;
}

static void render(void) {

float now = (float)(SDL_GetTicks() % 10000) / 10000.f;

float xRot = math_sin(now * math_pi * 2) * 0.4 + math_pi / 2;
float yRot = math_cos(now * math_pi * 2) * 0.4;
float yCos = math_cos(yRot);
float ySin = math_sin(yRot);
float xCos = math_cos(xRot);
float xSin = math_sin(xRot);

float ox = 32.5 + now * 64.0;
float oy = 32.5;
float oz = 32.5;

// for each column
for (int x = 0; x < w; x++) {
    // get the x axis delta
    float ___xd = ((float)x - (float)w / 2.f) / (float)h;
    // for each row
    for (int y = 0; y < h; y++) {
        // get the y axis delta
        float  __yd = ((float)y - (float)h / 2.f) / (float)h;
        float  __zd = 1;
        float ___zd = __zd * yCos + __yd * ySin;
        float   _yd = __yd * yCos - __zd * ySin;
        float   _xd = ___xd * xCos + ___zd * xSin;
        float   _zd = ___zd * xCos - ___xd * xSin;

        int col = 0;
        int br = 255;
        float ddist = 0;

        float closest = 32.f;

        // for each principle axis  x,y,z
        for (int d = 0; d < 3; d++) {
            float dimLength = _xd;
            if (d == 1)
                dimLength = _yd;
            if (d == 2)
                dimLength = _zd;

            float ll = 1.0f / (dimLength < 0.f ? -dimLength : dimLength);
            float xd = (_xd)* ll;
            float yd = (_yd)* ll;
            float zd = (_zd)* ll;

            float       initial = ox - floor(ox);
            if (d == 1) initial = oy - floor(oy);
            if (d == 2) initial = oz - floor(oz);

            if (dimLength > 0) initial = 1 - initial;

            float dist = ll * initial;

            float xp = ox + xd * initial;
            float yp = oy + yd * initial;
            float zp = oz + zd * initial;

            if (dimLength < 0) {
                if (d == 0) xp--;
                if (d == 1) yp--;
                if (d == 2) zp--;
            }

            // while we are concidering a ray that is still closer then the best so far
            while (dist < closest) {

                // quantize to the map grid
                int tex = map[(((int)zp & 63) << 12) | (((int)yp & 63) << 6) | ((int)xp & 63)];

                // if this voxel has a texture applied
                if (tex > 0) {

                    // find the uv coordinates of the intersection point
                    int u = ((int)((xp + zp) * 16.f)) & 15;
                    int v = ((int)(yp       * 16.f) & 15) + 16;

                    // fix uvs for alternate directions?
                    if (d == 1) {
                        u = ((int)(xp * 16.f)) & 15;
                        v = (((int)(zp * 16.f)) & 15);
                        if (yd < 0)
                            v += 32;
                    }

                    // find the colour at the intersection point
                    int cc = texmap[u + v * 16 + tex * 256 * 3];

                    // if the colour is not transparent
                    if (cc > 0) {
                        col = cc;
                        ddist = 255 - ((dist / 32 * 255));
                        br = 255 * (255 - ((d + 2) % 3) * 50) / 255;

                        // we now have the closest hit point (also terminates this ray)
                        closest = dist;
                    }
                }

                // advance the ray
                xp += xd;
                yp += yd;
                zp += zd;
                dist += ll;
            }
        }

        plot(x, y, rgbmul(col, fxmul(br, ddist)));
    }
   }
}

int main(int argc, char *argv[]) {

SDL_Init(SDL_INIT_EVERYTHING);
SDL_Window *screen; 
screen = SDL_CreateWindow(
    "Minecraft4k",                  // window title
    SDL_WINDOWPOS_CENTERED,           // initial x position
    SDL_WINDOWPOS_CENTERED,           // initial y position
    320,                               // width, in pixels
    240,                               // height, in pixels
    SDL_WINDOW_OPENGL                  // flags - see below
    );

SDL_Renderer* renderer;
renderer = SDL_CreateRenderer(screen, -1, SDL_RENDERER_ACCELERATED);

if (screen == nullptr) {
    return 1;
}

init();
bool running = true;
while (running) {
    SDL_Event event;
    while (SDL_PollEvent(&event)) {
        running &= (event.type != SDL_QUIT);
    }
    SDL_RenderPresent(renderer);
    render();
}

SDL_DestroyWindow(screen);
SDL_Quit();
return 0;

}

当我实际运行代码时，我确实遇到了黑屏，但调试器在线上着陆了

plot(x, y, rgbmul(col, fxmul(br, ddist)));

在; 静态无效渲染(无效)

这一切都只是为了“好玩”，因此欢迎任何信息或指导。

Answer 1

您定义了 screen 两次(第一次作为全局变量，第二次在您的 main 中)，但您只初始化了一次(在您的 中)主).
因此，全局变量 screen 实际上设置为 nullptr 并且 plot 尝试使用它失败，如错误消息所述。