add a decoding option to flip image vertically

New API options: WebPDecoderOptions.flip and 'dwebp -flip ...'

it uses negative stride trick.

Also changed the decoder code to support user-supplied
buffers with negative stride, independently of the
WebPDecoderOptions.flip value.

Change-Id: I4dc0d06f0c87e51a3f3428be4fee2d6b5ad76053

README

@@ -262,19 +262,20 @@ Use following options to convert into alternate image formats:
   -yuv ......... save the raw YUV samples in flat layout
 
  Other options are:
-  -version  .... print version number and exit.
+  -version  .... print version number and exit
-  -nofancy ..... don't use the fancy YUV420 upscaler.
+  -nofancy ..... don't use the fancy YUV420 upscaler
-  -nofilter .... disable in-loop filtering.
+  -nofilter .... disable in-loop filtering
-  -nodither .... disable dithering.
+  -nodither .... disable dithering
   -dither <d> .. dithering strength (in 0..100)
   -mt .......... use multi-threading
   -crop <x> <y> <w> <h> ... crop output with the given rectangle
   -scale <w> <h> .......... scale the output (*after* any cropping)
-  -alpha ....... only save the alpha plane.
+  -flip ........ flip the output vertically
+  -alpha ....... only save the alpha plane
   -incremental . use incremental decoding (useful for tests)
-  -h     ....... this help message.
+  -h     ....... this help message
   -v     ....... verbose (e.g. print encoding/decoding times)
-  -noasm ....... disable all assembly optimizations.
+  -noasm ....... disable all assembly optimizations
 
 Visualization tool:
 ===================

examples/dwebp.c

@@ -552,20 +552,21 @@ static void Help(void) {
          "  -yuv ......... save the raw YUV samples in flat layout\n"
          "\n"
          " Other options are:\n"
-         "  -version  .... print version number and exit.\n"
+         "  -version  .... print version number and exit\n"
-         "  -nofancy ..... don't use the fancy YUV420 upscaler.\n"
+         "  -nofancy ..... don't use the fancy YUV420 upscaler\n"
-         "  -nofilter .... disable in-loop filtering.\n"
+         "  -nofilter .... disable in-loop filtering\n"
-         "  -nodither .... disable dithering.\n"
+         "  -nodither .... disable dithering\n"
          "  -dither <d> .. dithering strength (in 0..100)\n"
          "  -mt .......... use multi-threading\n"
          "  -crop <x> <y> <w> <h> ... crop output with the given rectangle\n"
          "  -scale <w> <h> .......... scale the output (*after* any cropping)\n"
-         "  -alpha ....... only save the alpha plane.\n"
+         "  -flip ........ flip the output vertically\n"
+         "  -alpha ....... only save the alpha plane\n"
          "  -incremental . use incremental decoding (useful for tests)\n"
-         "  -h     ....... this help message.\n"
+         "  -h     ....... this help message\n"
          "  -v     ....... verbose (e.g. print encoding/decoding times)\n"
 #ifndef WEBP_DLL
-         "  -noasm ....... disable all assembly optimizations.\n"
+         "  -noasm ....... disable all assembly optimizations\n"
 #endif
         );
 }
@@ -641,6 +642,8 @@ int main(int argc, const char *argv[]) {
       config.options.use_scaling = 1;
       config.options.scaled_width  = strtol(argv[++c], NULL, 0);
       config.options.scaled_height = strtol(argv[++c], NULL, 0);
+    } else if (!strcmp(argv[c], "-flip")) {
+      config.options.flip = 1;
     } else if (!strcmp(argv[c], "-v")) {
       verbose = 1;
 #ifndef WEBP_DLL

man/dwebp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH DWEBP 1 "December 12, 2013"
+.TH DWEBP 1 "January 11, 2014"
 .SH NAME
 dwebp \- decompress a WebP file to an image file
 .SH SYNOPSIS
@@ -76,6 +76,9 @@ The top-left corner will be snapped to even coordinates if needed.
 This option is meant to reduce the memory needed for cropping large images.
 Note: the cropping is applied \fIbefore\fP any scaling.
 .TP
+.B \-flip
+Flip decoded image vertically (can be useful for OpenGL textures for instance).
+.TP
 .BI \-scale " width height
 Rescale the decoded picture to dimension \fBwidth\fP x \fBheight\fP. This
 option is mostly intended to reducing the memory needed to decode large images,

src/dec/buffer.c

@@ -42,29 +42,34 @@ static VP8StatusCode CheckDecBuffer(const WebPDecBuffer* const buffer) {
     ok = 0;
   } else if (!WebPIsRGBMode(mode)) {   // YUV checks
     const WebPYUVABuffer* const buf = &buffer->u.YUVA;
-    const uint64_t y_size = (uint64_t)buf->y_stride * height;
+    const int y_stride = abs(buf->y_stride);
-    const uint64_t u_size = (uint64_t)buf->u_stride * ((height + 1) / 2);
+    const int u_stride = abs(buf->u_stride);
-    const uint64_t v_size = (uint64_t)buf->v_stride * ((height + 1) / 2);
+    const int v_stride = abs(buf->v_stride);
-    const uint64_t a_size = (uint64_t)buf->a_stride * height;
+    const int a_stride = abs(buf->a_stride);
+    const uint64_t y_size = (uint64_t)y_stride * height;
+    const uint64_t u_size = (uint64_t)u_stride * ((height + 1) / 2);
+    const uint64_t v_size = (uint64_t)v_stride * ((height + 1) / 2);
+    const uint64_t a_size = (uint64_t)a_stride * height;
     ok &= (y_size <= buf->y_size);
     ok &= (u_size <= buf->u_size);
     ok &= (v_size <= buf->v_size);
-    ok &= (buf->y_stride >= width);
+    ok &= (y_stride >= width);
-    ok &= (buf->u_stride >= (width + 1) / 2);
+    ok &= (u_stride >= (width + 1) / 2);
-    ok &= (buf->v_stride >= (width + 1) / 2);
+    ok &= (v_stride >= (width + 1) / 2);
     ok &= (buf->y != NULL);
     ok &= (buf->u != NULL);
     ok &= (buf->v != NULL);
     if (mode == MODE_YUVA) {
-      ok &= (buf->a_stride >= width);
+      ok &= (a_stride >= width);
       ok &= (a_size <= buf->a_size);
       ok &= (buf->a != NULL);
     }
   } else {    // RGB checks
     const WebPRGBABuffer* const buf = &buffer->u.RGBA;
-    const uint64_t size = (uint64_t)buf->stride * height;
+    const int stride = abs(buf->stride);
+    const uint64_t size = (uint64_t)stride * height;
     ok &= (size <= buf->size);
-    ok &= (buf->stride >= width * kModeBpp[mode]);
+    ok &= (stride >= width * kModeBpp[mode]);
     ok &= (buf->rgba != NULL);
   }
   return ok ? VP8_STATUS_OK : VP8_STATUS_INVALID_PARAM;
@@ -131,9 +136,35 @@ static VP8StatusCode AllocateBuffer(WebPDecBuffer* const buffer) {
   return CheckDecBuffer(buffer);
 }
 
+VP8StatusCode WebPFlipBuffer(WebPDecBuffer* const buffer) {
+  if (buffer == NULL) {
+    return VP8_STATUS_INVALID_PARAM;
+  }
+  if (WebPIsRGBMode(buffer->colorspace)) {
+    WebPRGBABuffer* const buf = &buffer->u.RGBA;
+    buf->rgba += (buffer->height - 1) * buf->stride;
+    buf->stride = -buf->stride;
+  } else {
+    WebPYUVABuffer* const buf = &buffer->u.YUVA;
+    const int H = buffer->height;
+    buf->y += (H - 1) * buf->y_stride;
+    buf->y_stride = -buf->y_stride;
+    buf->u += ((H - 1) >> 1) * buf->u_stride;
+    buf->u_stride = -buf->u_stride;
+    buf->v += ((H - 1) >> 1) * buf->v_stride;
+    buf->v_stride = -buf->v_stride;
+    if (buf->a != NULL) {
+      buf->a += (H - 1) * buf->a_stride;
+      buf->a_stride = -buf->a_stride;
+    }
+  }
+  return VP8_STATUS_OK;
+}
+
 VP8StatusCode WebPAllocateDecBuffer(int w, int h,
                                     const WebPDecoderOptions* const options,
                                     WebPDecBuffer* const out) {
+  VP8StatusCode status;
   if (out == NULL || w <= 0 || h <= 0) {
     return VP8_STATUS_INVALID_PARAM;
   }
@@ -160,8 +191,15 @@ VP8StatusCode WebPAllocateDecBuffer(int w, int h,
   out->width = w;
   out->height = h;
 
-  // Then, allocate buffer for real
+  // Then, allocate buffer for real.
-  return AllocateBuffer(out);
+  status = AllocateBuffer(out);
+  if (status != VP8_STATUS_OK) return status;
+
+  // Use the stride trick if vertical flip is needed.
+  if (options != NULL && options->flip) {
+    status = WebPFlipBuffer(out);
+  }
+  return status;
 }
 
 //------------------------------------------------------------------------------

src/dec/idec.c

@@ -246,6 +246,19 @@ static int CheckMemBufferMode(MemBuffer* const mem, MemBufferMode expected) {
   return 1;
 }
 
+// To be called last.
+static VP8StatusCode FinishDecoding(WebPIDecoder* const idec) {
+  const WebPDecoderOptions* const options = idec->params_.options;
+  WebPDecBuffer* const output = idec->params_.output;
+
+  idec->state_ = STATE_DONE;
+  if (options != NULL && options->flip) {
+    return WebPFlipBuffer(output);
+  } else {
+    return VP8_STATUS_OK;
+  }
+}
+
 //------------------------------------------------------------------------------
 // Macroblock-decoding contexts
 
@@ -476,9 +489,7 @@ static VP8StatusCode DecodeRemaining(WebPIDecoder* const idec) {
     return IDecError(idec, VP8_STATUS_USER_ABORT);
   }
   dec->ready_ = 0;
-  idec->state_ = STATE_DONE;
+  return FinishDecoding(idec);
-
-  return VP8_STATUS_OK;
 }
 
 static VP8StatusCode ErrorStatusLossless(WebPIDecoder* const idec,
@@ -530,9 +541,7 @@ static VP8StatusCode DecodeVP8LData(WebPIDecoder* const idec) {
     return ErrorStatusLossless(idec, dec->status_);
   }
 
-  idec->state_ = STATE_DONE;
+  return FinishDecoding(idec);
-
-  return VP8_STATUS_OK;
 }
 
   // Main decoding loop

src/dec/webp.c

@@ -512,6 +512,10 @@ static VP8StatusCode DecodeInto(const uint8_t* const data, size_t data_size,
   if (status != VP8_STATUS_OK) {
     WebPFreeDecBuffer(params->output);
   }
+
+  if (params->options != NULL && params->options->flip) {
+    status = WebPFlipBuffer(params->output);
+  }
   return status;
 }
 

src/dec/webpi.h

@@ -93,10 +93,15 @@ int WebPIoInitFromOptions(const WebPDecoderOptions* const options,
 // dimension / etc.). If *options is not NULL, also verify that the options'
 // parameters are valid and apply them to the width/height dimensions of the
 // output buffer. This takes cropping / scaling / rotation into account.
+// Also incorporates the options->flip flag to flip the buffer parameters if
+// needed.
 VP8StatusCode WebPAllocateDecBuffer(int width, int height,
                                     const WebPDecoderOptions* const options,
                                     WebPDecBuffer* const buffer);
 
+// Flip buffer vertically by negating the various strides.
+VP8StatusCode WebPFlipBuffer(WebPDecBuffer* const buffer);
+
 // Copy 'src' into 'dst' buffer, making sure 'dst' is not marked as owner of the
 // memory (still held by 'src').
 void WebPCopyDecBuffer(const WebPDecBuffer* const src,
@@ -105,8 +110,6 @@ void WebPCopyDecBuffer(const WebPDecBuffer* const src,
 // Copy and transfer ownership from src to dst (beware of parameter order!)
 void WebPGrabDecBuffer(WebPDecBuffer* const src, WebPDecBuffer* const dst);
 
-
-
 //------------------------------------------------------------------------------
 
 #ifdef __cplusplus

src/webp/decode.h

@@ -20,7 +20,7 @@
 extern "C" {
 #endif
 
-#define WEBP_DECODER_ABI_VERSION 0x0203    // MAJOR(8b) + MINOR(8b)
+#define WEBP_DECODER_ABI_VERSION 0x0204    // MAJOR(8b) + MINOR(8b)
 
 // Note: forward declaring enumerations is not allowed in (strict) C and C++,
 // the types are left here for reference.
@@ -442,11 +442,12 @@ struct WebPDecoderOptions {
   int scaled_width, scaled_height;    // final resolution
   int use_threads;                    // if true, use multi-threaded decoding
   int dithering_strength;             // dithering strength (0=Off, 100=full)
+  int flip;                           // flip output vertically
 
   // Unused for now:
   int force_rotation;                 // forced rotation (to be applied _last_)
   int no_enhancement;                 // if true, discard enhancement layer
-  uint32_t pad[5];                    // padding for later use
+  uint32_t pad[4];                    // padding for later use
 };
 
 // Main object storing the configuration for advanced decoding.