vwebp: add a -fit option

This is to make the initial window be rescaled in case the image
dimension is too large to fit the display.

BUG=webp:433

Change-Id: Ib04c12962bc8c26e74c8a6193829214da636ebde

README

@@ -405,6 +405,7 @@ Options are:
   -dither <int>  dithering strength (0..100), default=50
   -noalphadither disable alpha plane dithering
   -usebgcolor .. display background color
+  -fit ......... downscale large images to preserve aspect ratio
   -mt .......... use multi-threading
   -info ........ print info
   -h ........... this help message

examples/vwebp.c

@@ -72,6 +72,7 @@ static struct {
   WebPIterator prev_frame;
   WebPChunkIterator iccp;
   int viewport_width, viewport_height;
+  int fit;  // if true, rescale large images to preserve aspect ratio.
 } kParams;
 
 static void ClearPreviousPic(void) {
@@ -418,8 +419,9 @@ static void HandleDisplay(void) {
 }
 
 static void StartDisplay(void) {
-  const int width = kParams.canvas_width;
+  int width = kParams.canvas_width;
-  const int height = kParams.canvas_height;
+  int height = kParams.canvas_height;
+  int screen_width, screen_height;
   // TODO(webp:365) GLUT_DOUBLE results in flickering / old frames to be
   // partially displayed with animated webp + alpha.
 #if defined(__APPLE__) || defined(_WIN32)
@@ -427,6 +429,18 @@ static void StartDisplay(void) {
 #else
   glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA);
 #endif
+  screen_width = glutGet(GLUT_SCREEN_WIDTH);
+  screen_height = glutGet(GLUT_SCREEN_HEIGHT);
+  if ((width > screen_width || height > screen_height) && kParams.fit) {
+    if (width > screen_width) {
+      height = (height * screen_width + width - 1) / width;
+      width = screen_width;
+    }
+    if (height > screen_height) {
+      width = (width * screen_height + height - 1) / height;
+      height = screen_height;
+    }
+  }
   glutInitWindowSize(width, height);
   glutCreateWindow("WebP viewer");
   glutDisplayFunc(HandleDisplay);
@@ -454,6 +468,7 @@ static void Help(void) {
       "  -dither <int>  dithering strength (0..100), default=50\n"
       "  -noalphadither disable alpha plane dithering\n"
       "  -usebgcolor .. display background color\n"
+      "  -fit ......... downscale large images to preserve aspect ratio\n"
       "  -mt .......... use multi-threading\n"
       "  -info ........ print info\n"
       "  -h ........... this help message\n"
@@ -482,6 +497,8 @@ int main(int argc, char *argv[]) {
   kParams.use_color_profile = 1;
   // Background color hidden by default to see transparent areas.
   kParams.draw_anim_background_color = 0;
+  // By default don't rescale too-large input images to preserve aspect ratio.
+  kParams.fit = 0;
 
   for (c = 1; c < argc; ++c) {
     int parse_error = 0;
@@ -498,6 +515,8 @@ int main(int argc, char *argv[]) {
       config->options.alpha_dithering_strength = 0;
     } else if (!strcmp(argv[c], "-usebgcolor")) {
       kParams.draw_anim_background_color = 1;
+    } else if (!strcmp(argv[c], "-fit")) {
+      kParams.fit = 1;
     } else if (!strcmp(argv[c], "-dither") && c + 1 < argc) {
       config->options.dithering_strength =
           ExUtilGetInt(argv[++c], 0, &parse_error);

man/vwebp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH VWEBP 1 "July 20, 2018"
+.TH VWEBP 1 "May 28, 2019"
 .SH NAME
 vwebp \- decompress a WebP file and display it in a window
 .SH SYNOPSIS
@@ -42,6 +42,10 @@ to smooth the gradients. This flag will prevent this dithering.
 Fill transparent areas with the bitstream's own background color instead of
 checkerboard only. Default is white for non-animated images.
 .TP
+.B \-fit
+If an image is larger than the display dimension, rescale it to preserve
+the aspect ratio.
+.TP
 .B \-mt
 Use multi-threading for decoding, if possible.
 .TP