Added viewport test, contributed by Andreas Schiffler and improved by me and Ryan Gordon.

If it works correctly you should see a square moving from the upper left to the lower right, with a little yellow box at the top of the moving square.

You can pass --target as a command line option to have it use a render target instead of rendering directly to the screen.  The output should be identical in this case.

test/Makefile.in

@@ -49,6 +49,7 @@ TARGETS = \
 	teststreaming$(EXE) \
 	testtimer$(EXE) \
 	testver$(EXE) \
+	testviewport$(EXE) \
 	testwm2$(EXE) \
 	torturethread$(EXE) \
 	testrendercopyex$(EXE) \
@@ -218,6 +219,9 @@ testtimer$(EXE): $(srcdir)/testtimer.c
 testver$(EXE): $(srcdir)/testver.c
 	$(CC) -o $@ $^ $(CFLAGS) $(LIBS)
 
+testviewport$(EXE): $(srcdir)/testviewport.c
+	$(CC) -o $@ $^ $(CFLAGS) $(LIBS)
+
 testwm2$(EXE): $(srcdir)/testwm2.c
 	$(CC) -o $@ $^ $(CFLAGS) $(LIBS)
 

test/testviewport.c

@@ -0,0 +1,184 @@
+/*
+  Copyright (C) 1997-2013 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely.
+*/
+/* Simple program:  Check viewports */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <time.h>
+
+#include "SDL_test.h"
+#include "SDL_test_common.h"
+
+
+static SDLTest_CommonState *state;
+
+/* Call this instead of exit(), so we can clean up SDL: atexit() is evil. */
+static void
+quit(int rc)
+{
+    SDLTest_CommonQuit(state);
+    exit(rc);
+}
+
+void
+DrawOnViewport(SDL_Renderer * renderer, SDL_Rect viewport)
+{    
+    SDL_Rect rect;
+
+    /* Set the viewport */
+    SDL_RenderSetViewport(renderer, &viewport);
+    
+    /* Draw a gray background */
+    SDL_SetRenderDrawColor(renderer, 0x80, 0x80, 0x80, 0xFF);
+    SDL_RenderClear(renderer);
+
+    /* Test inside points */
+    SDL_SetRenderDrawColor(renderer, 0xFF, 0xFF, 0xF, 0xFF);
+    SDL_RenderDrawPoint(renderer, viewport.h/2 + 10, viewport.w/2);
+    SDL_RenderDrawPoint(renderer, viewport.h/2 - 10, viewport.w/2);
+    SDL_RenderDrawPoint(renderer, viewport.h/2     , viewport.w/2 - 10);
+    SDL_RenderDrawPoint(renderer, viewport.h/2     , viewport.w/2 + 10);
+
+    /* Test horizontal and vertical lines */
+    SDL_SetRenderDrawColor(renderer, 0x00, 0xFF, 0x00, 0xFF);
+    SDL_RenderDrawLine(renderer, 1, 0, viewport.w-2, 0);
+    SDL_RenderDrawLine(renderer, 1, viewport.h-1, viewport.w-2, viewport.h-1);
+    SDL_RenderDrawLine(renderer, 0, 1, 0, viewport.h-2);
+    SDL_RenderDrawLine(renderer, viewport.w-1, 1, viewport.w-1, viewport.h-2);
+
+    /* Test diagonal lines */
+    SDL_SetRenderDrawColor(renderer, 0x00, 0x00, 0xFF, 0xFF);
+    SDL_RenderDrawLine(renderer, 0, 0,
+                       viewport.w-1, viewport.h-1);
+    SDL_RenderDrawLine(renderer, viewport.w-1, 0,
+                       0, viewport.h-1);                      
+
+    /* Test outside points */
+    SDL_SetRenderDrawColor(renderer, 0xFF, 0xFF, 0xF, 0xFF);
+    SDL_RenderDrawPoint(renderer, viewport.h/2 + viewport.h, viewport.w/2);
+    SDL_RenderDrawPoint(renderer, viewport.h/2 - viewport.h, viewport.w/2);
+    SDL_RenderDrawPoint(renderer, viewport.h/2     , viewport.w/2 - viewport.w);
+    SDL_RenderDrawPoint(renderer, viewport.h/2     , viewport.w/2 + viewport.w);
+
+    /* Add a box at the top */
+    rect.w = 8;
+    rect.h = 8;
+    rect.x = (viewport.w - rect.w) / 2;
+    rect.y = 0;
+    SDL_RenderFillRect(renderer, &rect);
+}
+
+int
+main(int argc, char *argv[])
+{
+    int i, j, done;
+    SDL_Event event;
+    Uint32 then, now, frames;
+    SDL_Rect viewport;
+    SDL_bool use_target = SDL_FALSE;
+
+    /* Initialize test framework */
+    state = SDLTest_CommonCreateState(argv, SDL_INIT_VIDEO);
+    if (!state) {
+        return 1;
+    }
+
+    for (i = 1; i < argc;) {
+        int consumed;
+
+        consumed = SDLTest_CommonArg(state, i);
+        if (consumed == 0) {
+            consumed = -1;
+            if (SDL_strcasecmp(argv[i], "--target") == 0) {
+                use_target = SDL_TRUE;
+                consumed = 1;
+            }
+        }
+        if (consumed < 0) {
+            SDL_Log("Usage: %s %s [--target]\n",
+                    argv[0], SDLTest_CommonUsage(state));
+            quit(1);
+        }
+        i += consumed;
+    }
+    if (!SDLTest_CommonInit(state)) {
+        quit(2);
+    }
+
+    if (use_target) {
+        int w, h;
+
+        for (i = 0; i < state->num_windows; ++i) {
+            SDL_GetWindowSize(state->windows[i], &w, &h);
+            state->targets[i] = SDL_CreateTexture(state->renderers[i], SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_TARGET, w, h);
+            SDL_SetRenderTarget(state->renderers[i], state->targets[i]);
+        }
+    }
+
+    for (i = 0; i < state->num_windows; ++i) {
+        SDL_Renderer *renderer = state->renderers[i];
+        SDL_SetRenderDrawColor(renderer, 0xA0, 0xA0, 0xA0, 0xFF);
+        SDL_RenderClear(renderer);
+    }
+
+    /* Main render loop */
+    frames = 0;
+    then = SDL_GetTicks();
+    done = 0;
+    j = 0;
+    while (!done) {
+        /* Check for events */
+        ++frames;
+        while (SDL_PollEvent(&event)) {
+            SDLTest_CommonEvent(state, &event, &done);
+        }
+        
+        /* Move a viewport box in steps around the screen */                
+        viewport.x = j * 100;
+        viewport.y = viewport.x;
+        viewport.w = 100 + j * 50;
+        viewport.h = 100 + j * 50;
+        j = (j + 1) % 4;            
+        SDL_Log("Current Viewport x=%i y=%i w=%i h=%i", viewport.x, viewport.y, viewport.w, viewport.h);
+        
+        for (i = 0; i < state->num_windows; ++i) {
+            if (state->windows[i] == NULL)
+                continue;
+                
+            /* Draw using viewport */        
+            DrawOnViewport(state->renderers[i], viewport);
+
+            /* Update the screen! */
+            if (use_target) {
+                SDL_SetRenderTarget(state->renderers[i], NULL);
+                SDL_RenderCopy(state->renderers[i], state->targets[i], NULL, NULL);
+                SDL_RenderPresent(state->renderers[i]);
+                SDL_SetRenderTarget(state->renderers[i], state->targets[i]);
+            } else {
+                SDL_RenderPresent(state->renderers[i]);
+            }
+        }
+        
+        SDL_Delay(1000);
+    }
+
+    /* Print out some timing information */
+    now = SDL_GetTicks();
+    if (now > then) {
+        double fps = ((double) frames * 1000) / (now - then);
+        SDL_Log("%2.2f frames per second\n", fps);
+    }
+    quit(0);
+    return 0;
+}
+
+/* vi: set ts=4 sw=4 expandtab: */