pixel manipulation: SDL_LockTexture and SDL_MapRGBA sample on Emscripten.

SDL_Emscripten_pixel manipulation_SDL_LockTexture_SDL_MapRGBA

pixel manipulation: SDL_LockTexture and SDL_MapRGBA sample on Emscripten.

# https://gamedev.stackexchange.com/questions/98641/how-do-i-modify-textures-in-sdl-with-direct-pixel-access
bellow code dont run.
SDL_PixelFormat pixelFormat;
pixelFormat.format = format;

# rewrite my code.
ctx->pixelFormat = SDL_AllocFormat(SDL_PIXELFORMAT_RGBA8888);
*p = SDL_MapRGBA(ctx->pixelFormat, 255, 0, 0, 255);

https://zestedesavoir.com/tutoriels/1014/utiliser-la-sdl-en-langage-c/modification-pixels-par-pixels/

test_pixel3.c

#include <stdbool.h>
#include <stdint.h>

#include <SDL2/SDL.h>
#ifdef __EMSCRIPTEN__
#include <emscripten.h>
#endif

typedef struct {
  SDL_Window *window;
  SDL_Renderer *renderer;
  SDL_Texture *tex;
  uint32_t *pixels;
  SDL_PixelFormat *pixelFormat;
} Context;

void Cleanup(Context *ctx) {
  if (ctx) {
    if (ctx->pixelFormat)
      free(ctx->pixelFormat);
    if (ctx->pixels)
      free(ctx->pixels);
    if (ctx->tex)
      SDL_DestroyTexture(ctx->tex);
    if (ctx->renderer)
      SDL_DestroyRenderer(ctx->renderer);
    if (ctx->window)
      SDL_DestroyWindow(ctx->window);
  }
}

uint32_t Init(Context *ctx) {
  if (SDL_Init(SDL_INIT_VIDEO) != 0) {
    fprintf(stderr, "Error: %s", SDL_GetError());
    return -1;
  }
  ctx->window =
      SDL_CreateWindow("sandbox2", SDL_WINDOWPOS_UNDEFINED,
                       SDL_WINDOWPOS_UNDEFINED, 640, 480, SDL_WINDOW_SHOWN);
  if (ctx->window == NULL) {
    fprintf(stderr, "Error: %s", SDL_GetError());
    return -1;
  }

  ctx->renderer = SDL_CreateRenderer(
      ctx->window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
  if (ctx->renderer == NULL) {
    fprintf(stderr, "Error: %s", SDL_GetError());
    return -1;
  }

  ctx->tex = SDL_CreateTexture(ctx->renderer, SDL_PIXELFORMAT_RGBA8888,
                               SDL_TEXTUREACCESS_STREAMING, 64, 32);
  if (ctx->tex == NULL) {
    fprintf(stderr, "Error: %s", SDL_GetError());
    return -1;
  }
  ctx->pixelFormat = SDL_AllocFormat(SDL_PIXELFORMAT_RGBA8888);

  ctx->pixels = (uint32_t *)malloc(sizeof(uint32_t) * 64 * 32);

  SDL_SetRenderDrawColor(ctx->renderer, 225, 240, 200, 255);

  return 0;
}

void Loop(void *arg) {
  Context *ctx = (Context *)arg;

  Uint32 format;
  int w;
  int h; 
  SDL_QueryTexture(ctx->tex, &format, NULL, &w, &h);

  int pitch;
  uint8_t *pixels;
  SDL_LockTexture(ctx->tex, NULL, (void **)&pixels, &pitch);
  for (int y = 0; y < h; y++) {
    Uint32 *p = (Uint32 *)(pixels + pitch*y); // cast for a pointer increments by 4 bytes.(RGBA)
    for (int x = 0; x < w; x++) {
      // *p = 0x00FF0000;
      if (x > 20)
        *p = SDL_MapRGBA(ctx->pixelFormat, 255, 0, 0, 255);
      else if (x > 10)
        *p = SDL_MapRGBA(ctx->pixelFormat, 0, 255, 0, 255);
      else
        *p = SDL_MapRGBA(ctx->pixelFormat, 0, 0, 255, 255);
      
      p++;
    }
  }
  SDL_UnlockTexture(ctx->tex);

  //
  // Draw
  //
  SDL_RenderClear(ctx->renderer);
  SDL_RenderCopy(ctx->renderer, ctx->tex, NULL, &(SDL_Rect){0, 0, 64*2, 32*2}); // Scale * 2

  SDL_RenderPresent(ctx->renderer);
}

int main() {
  Context ctx = {0, 0, 0, 0, 0};
  Init(&ctx);

  Loop(&ctx);
  emscripten_set_main_loop_arg(Loop, &ctx, -1, 1 /*block*/);
  printf("don't run this message by block.");

  return 0;
}

zbuild.bat

emcc test_pixel3.c ^
 -O2 ^
 -Wall -Wextra -pedantic ^
 -s USE_SDL=2 ^
 -s USE_SDL_IMAGE=2 -s SDL2_IMAGE_FORMATS="[""png""]" ^
 -s USE_SDL_TTF=2 ^
 --preload-file assets ^
 -o test_pixel3.html