dstrect of SDL_RenderCopy of SDL2 is broken on HTML5

[bottle2]

hey pals

Documentation for parameter dstrect of SDL_RenderCopy in https://wiki.libsdl.org/SDL2/SDL_RenderCopy: "the destination SDL_Rect structure or NULL for the entire rendering target; the texture will be stretched to fill the given rectangle"

on Windows, it works appropriately. however, on HTML5, NULL yields strange results.

$ uname -a
MINGW64_NT-10.0-19045 DESKTOP-QIBT7ND 3.5.4-0bc1222b.x86_64 2024-12-05 09:27 UTC x86_64 Msys
$ emcc -v
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.74-git (1092ec30a3fb1d46b
1782ff1b4db5094d3d06ae5)
clang version 20.0.0git
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: C:/msys64/clang64/opt/emscripten-llvm/bin
$ pacman -Qi mingw-w64-clang-x86_64-SDL2
Name            : mingw-w64-clang-x86_64-SDL2
Version         : 2.30.11-1
Description     : A library for portable low-level access to a video framebuffer, audio output,
                  mouse, and keyboard (Version 2) (mingw-w64)
Architecture    : any
URL             : https://libsdl.org/
Licenses        : spdx:Zlib
Groups          : None
Provides        : None
Depends On      : mingw-w64-clang-x86_64-gcc-libs  mingw-w64-clang-x86_64-libiconv
                  mingw-w64-clang-x86_64-vulkan
Optional Deps   : None
Required By     : mingw-w64-clang-x86_64-SDL2_image  mingw-w64-clang-x86_64-SDL2_net
                  mingw-w64-clang-x86_64-SDL2_ttf  mingw-w64-clang-x86_64-ffmpeg
Optional For    : mingw-w64-clang-x86_64-libde265
Conflicts With  : None
Replaces        : None
Installed Size  : 7.73 MiB
Packager        : CI (msys2/msys2-autobuild/35ff0b71/12593557521)
Build Date      : Fri Jan 3 03:09:42 2025
Install Date    : Thu Jan 9 11:44:37 2025
Install Reason  : Explicitly installed
Install Script  : No
Validated By    : SHA-256 Sum  Signature

further disclaimer: I've run emcc --clear-cache before running the tests.

MRE:

#include <stdbool.h>
#include <stdio.h>

#include <SDL.h>

#ifdef __EMSCRIPTEN__
#include <emscripten.h>
#define LOOP(ITER) emscripten_set_main_loop((ITER), 0, 1)
#define LOOP_END emscripten_cancel_main_loop(); return
#else
static bool is_playing = true;
#define LOOP(ITER) while (is_playing) (ITER)()
#define LOOP_END is_playing = false; return
#endif

#define TRY(IT) \
if ((IT)) { SDL_LogError(SDL_LOG_CATEGORY_ERROR, \
            __FILE__ ":%d: %s\n", __LINE__, SDL_GetError()); \
            exit(EXIT_FAILURE); } else (void)0

static SDL_threadID tid;
static SDL_Window *window;
static SDL_Renderer *renderer;
static SDL_Texture *texture;
static SDL_Rect rect = {0, 0, 800, 600};
static SDL_PixelFormat *pf = NULL;
static bool use_rect = false;
static uint32_t pixel_format;

static void resize(int width, int height)
{
    int w;
    int h;
    SDL_GetRendererOutputSize(renderer, &w, &h);
    SDL_Log("from event: %d %d from output: %d %d\n", width, height, w, h);

    if (rect.w != width || rect.h != height)
    {
        rect.w = width;
        rect.h = height;
    }
}

static void render(void)
{
    SDL_RenderClear(renderer);

    void *pixels;
    int pitch;
    TRY(SDL_LockTexture(texture, &rect, &pixels, &pitch));

    int x = rect.w / 2;
    int y = rect.h / 2;
    int radius = (rect.w < rect.h ? rect.w : rect.h) * 0.8 / 2;

    for (int j = 0; j < rect.h; j++)
        for (int i = 0; i < rect.w; i++)
        {
            uint8_t c = (j-y)*(j-y) + (i-x)*(i-x) < radius * radius ? 255 : 0;
            ((uint32_t *)(((unsigned char *)pixels) + j * pitch))[i]
                    = SDL_MapRGB(pf, c, c, c);
        }

    SDL_UnlockTexture(texture);
    SDL_RenderCopy(renderer, texture, &rect, use_rect ? &rect : NULL);

    SDL_RenderPresent(renderer);
}

// https://discourse.libsdl.org/t/is-it-thread-safe-to-render-inside-event-watcher/49409
static int filter(void *userdata, SDL_Event *event)
{
    (void)userdata;

    if (SDL_WINDOWEVENT == event->type)
    {
        switch (event->window.event)
        {
            case SDL_WINDOWEVENT_EXPOSED:
                SDL_assert(SDL_GetThreadID(NULL) == tid);
                render();
                return 0;
            break;
            case SDL_WINDOWEVENT_RESIZED: // Fall through.
            case SDL_WINDOWEVENT_SIZE_CHANGED:
                if (SDL_GetThreadID(NULL) == tid)
                {
                    resize(event->window.data1, event->window.data2);
                    return 0;
                }
            break;
            default:
                // Do nothing.
            break;
        }
    }

    return 1;
}

void iter(void)
{
    for (SDL_Event event; SDL_PollEvent(&event); )
    {
        if (SDL_QUIT == event.type)
        {
            SDL_DestroyTexture(texture);
            SDL_DestroyRenderer(renderer);
            SDL_DestroyWindow(window);
            SDL_QuitSubSystem(SDL_INIT_VIDEO | SDL_INIT_EVENTS);
            LOOP_END;
        }
        else if (SDL_WINDOWEVENT == event.type &&
                (SDL_WINDOWEVENT_RESIZED == event.window.event ||
                 SDL_WINDOWEVENT_SIZE_CHANGED == event.window.event))
        {
            resize(event.window.data1, event.window.data2);
        }
	else if (SDL_KEYDOWN == event.type && 'u' == event.key.keysym.sym)
        {
            use_rect = !use_rect;
        }
    }

    render();
}

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

    atexit(SDL_Quit);

    TRY(SDL_InitSubSystem(SDL_INIT_VIDEO | SDL_INIT_EVENTS));

    tid = SDL_GetThreadID(NULL);

    TRY(!(window = SDL_CreateWindow(
        "Resize, texture and render test",
        SDL_WINDOWPOS_UNDEFINED,
        SDL_WINDOWPOS_UNDEFINED,
        800, 600,
        SDL_WINDOW_RESIZABLE
    )));

    TRY(!(renderer = SDL_CreateRenderer(window, -1, 0)));

    SDL_SetRenderDrawColor(renderer, 0, 0, 0, 1);

    {
        SDL_RendererInfo info = {0};
        TRY(SDL_GetRendererInfo(renderer, &info));
        SDL_assert(info.num_texture_formats > 0);
        pixel_format = info.texture_formats[0];
        TRY(!(pf = SDL_AllocFormat(pixel_format)));
        SDL_assert(32 == pf->BitsPerPixel);
    }

    TRY(!(texture = SDL_CreateTexture(renderer,
                    pixel_format, SDL_TEXTUREACCESS_STREAMING,
                    5000, 5000)));

    SDL_SetEventFilter(filter, NULL);

    LOOP(iter);

    return 0;
}

commands used to compile the native Windows and HTML5 version:

$ cc -std=c99 $(pkg-config --cflags SDL2) -o simple simple.c $(pkg-config --libs SDL2)
$ emcc -std=c99 -Oz -flto -sINITIAL_MEMORY=655360000 -std=c18 simple.c --use-port=sdl2 -o index.html --shell-file=shell.html

file shell.html comes from here: https://github.com/floooh/sokol-samples/blob/3e746d86cd87f634f4f7793c2e8d0ef71a2067ca/webpage/shell.html

visual evidence of it behaving properly on Windows:

HTML5 just after running emrun index.html:

after resizing windows:

after pressing U:

again, after opening for the first time and pressing U without resizing:

pressing U makes the code use some SDL_Rect for parameter dstrect of SDL_RenderCopy. as I write, I now noticed that I never called SDL_GetWindowSize nor SDL_GetRendererOutputSize after window creation. but oh well, this is a different problem.

[icculus]

If someone could confirm this happens on SDL3 also, I'd appreciate it. I'm crunching on 3.2.0 and can't look at it this week if it isn't.