examples: Added renderer/14-viewport

(This intentionally skips a few example numbers, as I intend to fill in some
other topics before this, later.)

examples/CMakeLists.txt

@@ -131,6 +131,7 @@ add_sdl_example_executable(renderer-rotating-textures SOURCES renderer/08-rotati
 add_sdl_example_executable(renderer-scaling-textures SOURCES renderer/09-scaling-textures/scaling-textures.c DATAFILES ${CMAKE_CURRENT_SOURCE_DIR}/../test/sample.bmp)
 add_sdl_example_executable(renderer-geometry SOURCES renderer/10-geometry/geometry.c DATAFILES ${CMAKE_CURRENT_SOURCE_DIR}/../test/sample.bmp)
 add_sdl_example_executable(renderer-color-mods SOURCES renderer/11-color-mods/color-mods.c DATAFILES ${CMAKE_CURRENT_SOURCE_DIR}/../test/sample.bmp)
+add_sdl_example_executable(renderer-viewport SOURCES renderer/14-viewport/viewport.c DATAFILES ${CMAKE_CURRENT_SOURCE_DIR}/../test/sample.bmp)
 add_sdl_example_executable(audio-simple-playback SOURCES audio/01-simple-playback/simple-playback.c)
 add_sdl_example_executable(audio-simple-playback-callback SOURCES audio/02-simple-playback-callback/simple-playback-callback.c)
 add_sdl_example_executable(audio-load-wav SOURCES audio/03-load-wav/load-wav.c DATAFILES ${CMAKE_CURRENT_SOURCE_DIR}/../test/sample.wav)

examples/renderer/14-viewport/README.txt

@@ -0,0 +1,4 @@
+This example creates an SDL window and renderer, loads a texture
+from a .bmp file, and then draws it a few times each frame, adjusting
+the viewport before each draw.
+

examples/renderer/14-viewport/viewport.c

@@ -0,0 +1,134 @@
+/*
+ * This example creates an SDL window and renderer, and then draws some
+ * textures to it every frame, adjusting the viewport.
+ *
+ * This code is public domain. Feel free to use it for any purpose!
+ */
+
+#define SDL_MAIN_USE_CALLBACKS 1  /* use the callbacks instead of main() */
+#include <SDL3/SDL.h>
+#include <SDL3/SDL_main.h>
+
+/* We will use this renderer to draw into this window every frame. */
+static SDL_Window *window = NULL;
+static SDL_Renderer *renderer = NULL;
+static SDL_Texture *texture = NULL;
+static int texture_width = 0;
+static int texture_height = 0;
+
+#define WINDOW_WIDTH 640
+#define WINDOW_HEIGHT 480
+
+/* This function runs once at startup. */
+SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
+{
+    SDL_Surface *surface = NULL;
+    char *bmp_path = NULL;
+
+    if (!SDL_Init(SDL_INIT_VIDEO)) {
+        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't initialize SDL!", SDL_GetError(), NULL);
+        return SDL_APP_FAILURE;
+    }
+
+    if (!SDL_CreateWindowAndRenderer("examples/renderer/viewport", WINDOW_WIDTH, WINDOW_HEIGHT, 0, &window, &renderer)) {
+        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create window/renderer!", SDL_GetError(), NULL);
+        return SDL_APP_FAILURE;
+    }
+
+    /* Textures are pixel data that we upload to the video hardware for fast drawing. Lots of 2D
+       engines refer to these as "sprites." We'll do a static texture (upload once, draw many
+       times) with data from a bitmap file. */
+
+    /* SDL_Surface is pixel data the CPU can access. SDL_Texture is pixel data the GPU can access.
+       Load a .bmp into a surface, move it to a texture from there. */
+    SDL_asprintf(&bmp_path, "%ssample.bmp", SDL_GetBasePath());  /* allocate a string of the full file path */
+    surface = SDL_LoadBMP(bmp_path);
+    if (!surface) {
+        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't load bitmap!", SDL_GetError(), NULL);
+        return SDL_APP_FAILURE;
+    }
+
+    SDL_free(bmp_path);  /* done with this, the file is loaded. */
+
+    texture_width = surface->w;
+    texture_height = surface->h;
+
+    texture = SDL_CreateTextureFromSurface(renderer, surface);
+    if (!texture) {
+        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create static texture!", SDL_GetError(), NULL);
+        return SDL_APP_FAILURE;
+    }
+
+    SDL_DestroySurface(surface);  /* done with this, the texture has a copy of the pixels now. */
+
+    return SDL_APP_CONTINUE;  /* carry on with the program! */
+}
+
+/* This function runs when a new event (mouse input, keypresses, etc) occurs. */
+SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event)
+{
+    if (event->type == SDL_EVENT_QUIT) {
+        return SDL_APP_SUCCESS;  /* end the program, reporting success to the OS. */
+    }
+    return SDL_APP_CONTINUE;  /* carry on with the program! */
+}
+
+/* This function runs once per frame, and is the heart of the program. */
+SDL_AppResult SDL_AppIterate(void *appstate)
+{
+    SDL_FRect dst_rect = { 0, 0, (float) texture_width, (float) texture_height };
+    SDL_Rect viewport;
+
+    /* Setting a viewport has the effect of limiting the area that rendering
+       can happen, and making coordinate (0, 0) live somewhere else in the
+       window. It does _not_ scale rendering to fit the viewport. */
+
+    /* as you can see from this, rendering draws over whatever was drawn before it. */
+    SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);  /* black, full alpha */
+    SDL_RenderClear(renderer);  /* start with a blank canvas. */
+
+    /* Draw once with the whole window as the viewport. */
+    viewport.x = 0;
+    viewport.y = 0;
+    viewport.w = WINDOW_WIDTH / 2;
+    viewport.h = WINDOW_HEIGHT / 2;
+    SDL_SetRenderViewport(renderer, NULL);  /* NULL means "use the whole window" */
+    SDL_RenderTexture(renderer, texture, NULL, &dst_rect);
+
+    /* top right quarter of the window. */
+    viewport.x = WINDOW_WIDTH / 2;
+    viewport.y = WINDOW_HEIGHT / 2;
+    viewport.w = WINDOW_WIDTH / 2;
+    viewport.h = WINDOW_HEIGHT / 2;
+    SDL_SetRenderViewport(renderer, &viewport);
+    SDL_RenderTexture(renderer, texture, NULL, &dst_rect);
+
+    /* bottom 20% of the window. Note it clips the width! */
+    viewport.x = 0;
+    viewport.y = WINDOW_HEIGHT - (WINDOW_HEIGHT / 5);
+    viewport.w = WINDOW_WIDTH / 5;
+    viewport.h = WINDOW_HEIGHT / 5;
+    SDL_SetRenderViewport(renderer, &viewport);
+    SDL_RenderTexture(renderer, texture, NULL, &dst_rect);
+
+    /* what happens if you try to draw above the viewport? It should clip! */
+    viewport.x = 100;
+    viewport.y = 200;
+    viewport.w = WINDOW_WIDTH;
+    viewport.h = WINDOW_HEIGHT;
+    SDL_SetRenderViewport(renderer, &viewport);
+    dst_rect.y = -50;
+    SDL_RenderTexture(renderer, texture, NULL, &dst_rect);
+
+    SDL_RenderPresent(renderer);  /* put it all on the screen! */
+
+    return SDL_APP_CONTINUE;  /* carry on with the program! */
+}
+
+/* This function runs once at shutdown. */
+void SDL_AppQuit(void *appstate)
+{
+    SDL_DestroyTexture(texture);
+    /* SDL will clean up the window/renderer for us. */
+}
+