Add jpeg_skip_scanlines() API to libjpeg-turbo

djpeg.c

Index: djpeg.c
diff --git a/djpeg.c b/djpeg.c
index 589c5805cb54a1a39b2ffe4f2edb707062876c70..925ca8a8d71aaf90ee726bfec9478b0d3009cc54 100644
--- a/djpeg.c
+++ b/djpeg.c
@@ -4,7 +4,8 @@
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1991-1997, Thomas G. Lane.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2010-2011, 2013, D. R. Commander.
+ * Copyright (C) 2010-2011, 2013-2015, D. R. Commander.
+ * Copyright (C) 2015, Google, Inc.
  * For conditions of distribution and use, see the accompanying README file.
  *
  * This file contains a command-line user interface for the JPEG decompressor.
@@ -88,6 +89,8 @@ static IMAGE_FORMATS requested_fmt;
 static const char * progname;	/* program name for error messages */
 static char * outfilename;	/* for -outfile switch */
 boolean memsrc;  /* for -memsrc switch */
+boolean strip, skip;
+JDIMENSION startY, endY;
 #define INPUT_BUF_SIZE  4096
 
 
@@ -163,6 +166,8 @@ usage (void)
   fprintf(stderr, "  -memsrc        Load input file into memory before decompressing\n");
 #endif
 
+  fprintf(stderr, "  -skip Y0,Y1    Decode all rows except those between Y0 and Y1 (inclusive)\n");
+  fprintf(stderr, "  -strip Y0,Y1   Decode only rows between Y0 and Y1 (inclusive)\n");
   fprintf(stderr, "  -verbose  or  -debug   Emit debug output\n");
   exit(EXIT_FAILURE);
 }
@@ -187,6 +192,8 @@ parse_switches (j_decompress_ptr cinfo, int argc, char **argv,
   requested_fmt = DEFAULT_FMT;	/* set default output file format */
   outfilename = NULL;
   memsrc = FALSE;
+  strip = FALSE;
+  skip = FALSE;
   cinfo->err->trace_level = 0;
 
   /* Scan command line options, adjust parameters */
@@ -350,7 +357,7 @@ parse_switches (j_decompress_ptr cinfo, int argc, char **argv,
       /* RLE output format. */
       requested_fmt = FMT_RLE;
 
-    } else if (keymatch(arg, "scale", 1)) {
+    } else if (keymatch(arg, "scale", 2)) {
       /* Scale the output image by a fraction M/N. */
       if (++argn >= argc)	/* advance to next argument */
 	usage();
@@ -358,6 +365,20 @@ parse_switches (j_decompress_ptr cinfo, int argc, char **argv,
 		 &cinfo->scale_num, &cinfo->scale_denom) != 2)
 	usage();
 
+    } else if (keymatch(arg, "strip", 2)) {
+      if (++argn >= argc)
+        usage();
+      if (sscanf(argv[argn], "%d,%d", &startY, &endY) != 2 || startY > endY)
+        usage();
+      strip = TRUE;
+
+    } else if (keymatch(arg, "skip", 2)) {
+      if (++argn >= argc)
+        usage();
+      if (sscanf(argv[argn], "%d,%d", &startY, &endY) != 2 || startY > endY)
+        usage();
+      skip = TRUE;
+
     } else if (keymatch(arg, "targa", 1)) {
       /* Targa output format. */
       requested_fmt = FMT_TARGA;
@@ -628,14 +649,64 @@ main (int argc, char **argv)
   /* Start decompressor */
   (void) jpeg_start_decompress(&cinfo);
 
-  /* Write output file header */
-  (*dest_mgr->start_output) (&cinfo, dest_mgr);
+  /* Strip decode */
+  if (strip || skip) {
+    JDIMENSION tmp;
+
+    /* Check for valid endY.  We cannot check this value until after
+     * jpeg_start_decompress() is called.  Note that we have already verified
+     * that startY <= endY.
+     */
+    if (endY > cinfo.output_height - 1) {
+      fprintf(stderr, "%s: strip %d-%d exceeds image height %d\n", progname,
+              startY, endY, cinfo.output_height);
+      exit(EXIT_FAILURE);
+    }
+
+    /* Write output file header.  This is a hack to ensure that the destination
+     * manager creates an image of the proper size for the partial decode.
+     */
+    tmp = cinfo.output_height;
+    cinfo.output_height = endY - startY + 1;
+    if (skip)
+      cinfo.output_height = tmp - cinfo.output_height;
+    (*dest_mgr->start_output) (&cinfo, dest_mgr);
+    cinfo.output_height = tmp;
+
+    /* Process data */
+    if (skip) {
+      while (cinfo.output_scanline < startY) {
+        num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,
+                                            dest_mgr->buffer_height);
+        (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
+      }
+      jpeg_skip_scanlines(&cinfo, endY - startY + 1);
+      while (cinfo.output_scanline < cinfo.output_height) {
+        num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,
+                                            dest_mgr->buffer_height);
+        (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
+      }
+    } else {
+      jpeg_skip_scanlines(&cinfo, startY);
+      while (cinfo.output_scanline <= endY) {
+        num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,
+                                            dest_mgr->buffer_height);
+        (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
+      }
+      jpeg_skip_scanlines(&cinfo, cinfo.output_height - endY + 1);
+    }
 
-  /* Process data */
-  while (cinfo.output_scanline < cinfo.output_height) {
-    num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,
-					dest_mgr->buffer_height);
-    (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
+  /* Normal full image decode */
+  } else {
+    /* Write output file header */
+    (*dest_mgr->start_output) (&cinfo, dest_mgr);
+
+    /* Process data */
+    while (cinfo.output_scanline < cinfo.output_height) {
+      num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,
+                                          dest_mgr->buffer_height);
+      (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
+    }
   }
 
 #ifdef PROGRESS_REPORT