[PATCH v2 0/1] MR11149: windowscodecs: Decode GIF frames on demand.

[zhengyong chen (＠chenzhengyong)]

The GIF decoder currently calls DGifSlurp() during initialization, which decodes and stores pixel data for every frame in the image.

For animated GIFs with a large number of frames this can result in significant startup time and memory consumption, even when only a single frame is requested by the application.

Change the decoder to scan GIF headers during initialization and defer frame pixel decoding until CopyPixels() is called for a specific frame. This avoids eagerly decoding unused frames while preserving existing decoder behavior.

Signed-off-by: chenzhengyong [e-mail]

tested on deepin v25 & UOS V20.

--
  v2: windowscodecs: Decode GIF frames on demand.
 
https://gitlab.winehq.org/wine/wine/-/merge_requests/11149

[PATCH v2 1/1] windowscodecs: Decode GIF frames on demand.

[chenzhengyong]

From: chenzhengyong <[e-mail]>

The GIF decoder currently calls DGifSlurp() during initialization,
which decodes and stores pixel data for every frame in the image.

For animated GIFs with a large number of frames this can result in
significant startup time and memory consumption, even when only a
single frame is requested by the application.

Change the decoder to scan GIF headers during initialization and defer
frame pixel decoding until CopyPixels() is called for a specific frame.
This avoids eagerly decoding unused frames while preserving existing
decoder behavior.

Signed-off-by: chenzhengyong <[e-mail]>
---
 dlls/windowscodecs/gifformat.c |  38 +++++++-
 dlls/windowscodecs/ungif.c     | 171 +++++++++++++++++++++++++++++++++
 dlls/windowscodecs/ungif.h     |   8 ++
 3 files changed, 213 insertions(+), 4 deletions(-)

diff --git a/dlls/windowscodecs/gifformat.c b/dlls/windowscodecs/gifformat.c
index 287a49f83e9..8a33e87dceb 100644
--- a/dlls/windowscodecs/gifformat.c
+++ b/dlls/windowscodecs/gifformat.c
@@ -37,6 +37,7 @@ struct gif_decoder
 {
     struct decoder decoder;
     GifFileType *gif;
+    IStream *stream;  /* kept to allow on-demand frame decoding */
 };
 
 static inline struct gif_decoder *impl_from_decoder(struct decoder *iface)
@@ -646,6 +647,13 @@ static int _gif_inputfunc(GifFileType *gif, GifByteType *data, int len) {
     return bytesread;
 }
 
+static void gif_seek_stream(GifFileType *gif, int offset)
+{
+    LARGE_INTEGER seek;
+    seek.QuadPart = offset;
+    IStream_Seek((IStream *)gif->UserData, seek, STREAM_SEEK_SET, NULL);
+}
+
 static HRESULT CDECL gif_decoder_initialize(struct decoder *iface, IStream *stream, struct decoder_stat *st)
 {
     struct gif_decoder *decoder = impl_from_decoder(iface);
@@ -656,17 +664,29 @@ static HRESULT CDECL gif_decoder_initialize(struct decoder *iface, IStream *stre
     seek.QuadPart = 0;
     IStream_Seek(stream, seek, STREAM_SEEK_SET, NULL);
 
+    /* keep stream for on-demand frame decoding */
+    IStream_AddRef(stream);
+    decoder->stream = stream;
+
     /* read all data from the stream */
     decoder->gif = DGifOpen((void *)stream, _gif_inputfunc);
     if (!decoder->gif)
+    {
+        IStream_Release(stream);
         return E_FAIL;
+    }
+
+    /* Set up seek function for on-demand frame decoding */
+    decoder->gif->seekFunc = gif_seek_stream;
 
-    ret = DGifSlurp(decoder->gif);
+    /* Only scan frame headers, skip pixel data for on-demand decoding */
+    ret = DGifSlurpHeaders(decoder->gif);
     if (ret == GIF_ERROR)
+    {
+        DGifCloseFile(decoder->gif);
+        IStream_Release(stream);
         return E_FAIL;
-
-    /* make sure we don't use the stream after this method returns */
-    decoder->gif->UserData = NULL;
+    }
 
     st->flags = WICBitmapDecoderCapabilityCanDecodeAllImages |
                 WICBitmapDecoderCapabilityCanDecodeSomeImages |
@@ -759,6 +779,14 @@ static HRESULT CDECL gif_decoder_copy_pixels(struct decoder *iface, UINT frame,
 
     image = &decoder->gif->SavedImages[frame];
 
+    /* Decode this frame's pixel data on demand */
+    if (!image->RasterBits)
+    {
+        decoder->gif->UserData = (void *)decoder->stream;
+        if (DGifDecodeFrame(decoder->gif, frame) == GIF_ERROR)
+            return E_FAIL;
+    }
+
     if (image->ImageDesc.Interlace)
         return copy_interlaced_pixels(image->RasterBits, image->ImageDesc.Width,
                 image->ImageDesc.Height, image->ImageDesc.Width, prc, stride, buffersize, buffer);
@@ -871,6 +899,8 @@ static void CDECL gif_decoder_destroy(struct decoder *iface)
 {
     struct gif_decoder *decoder = impl_from_decoder(iface);
 
+    if (decoder->stream)
+        IStream_Release(decoder->stream);
     DGifCloseFile(decoder->gif);
     free(decoder);
 }
diff --git a/dlls/windowscodecs/ungif.c b/dlls/windowscodecs/ungif.c
index aa0552c04ce..98584e016ba 100644
--- a/dlls/windowscodecs/ungif.c
+++ b/dlls/windowscodecs/ungif.c
@@ -590,6 +590,24 @@ DGifGetCodeNext(GifFileType * GifFile,
     return GIF_OK;
 }
 
+/******************************************************************************
+ * Skip the LZW image sub-blocks without decoding any pixels.
+ * Called after DGifSetupDecompress has read the code size byte.
+ *****************************************************************************/
+static int
+DGifSkipImageData(GifFileType *GifFile)
+{
+    GifByteType *CodeBlock;
+    do {
+        if (DGifGetCodeNext(GifFile, &CodeBlock) == GIF_ERROR)
+        {
+            WARN("GIF is not properly terminated\n");
+            break;
+        }
+    } while (CodeBlock != NULL);
+    return GIF_OK;
+}
+
 /******************************************************************************
  * Setup the LZ decompression for this image:
  *****************************************************************************/
@@ -975,6 +993,159 @@ DGifSlurp(GifFileType * GifFile) {
     return (GIF_OK);
 }
 
+/******************************************************************************
+ * Like DGifSlurp() but skips pixel data of each frame.
+ * Only reads frame headers (ImageDesc, extension blocks) so it is fast.
+ * Use DGifDecodeFrame() to decode individual frames on demand.
+ *****************************************************************************/
+int
+DGifSlurpHeaders(GifFileType *GifFile)
+{
+    GifRecordType RecordType;
+    SavedImage *sp;
+    GifByteType *ExtData;
+    Extensions temp_save;
+
+    temp_save.ExtensionBlocks = NULL;
+    temp_save.ExtensionBlockCount = 0;
+
+    do {
+        if (DGifGetRecordType(GifFile, &RecordType) == GIF_ERROR)
+            return GIF_ERROR;
+
+        switch (RecordType) {
+          case IMAGE_DESC_RECORD_TYPE:
+              if (DGifGetImageDesc(GifFile) == GIF_ERROR)
+                  return GIF_ERROR;
+
+              sp = &GifFile->SavedImages[GifFile->ImageCount - 1];
+              /* GetFileOffset is past the code size byte read by DGifSetupDecompress
+               * inside DGifGetImageDesc, so subtract 1 to point at the code size byte
+               * itself, so DGifDecodeFrame can re-read it correctly after seeking. */
+              sp->LZWDataOffset = GetFileOffset(GifFile) - 1;
+
+              /* Skip LZW image sub-blocks without decoding */
+              if (DGifSkipImageData(GifFile) == GIF_ERROR)
+                  return GIF_ERROR;
+
+              /* Attach pending extension to this frame */
+              if (temp_save.ExtensionBlocks) {
+                  sp->Extensions.ExtensionBlocks = temp_save.ExtensionBlocks;
+                  sp->Extensions.ExtensionBlockCount = temp_save.ExtensionBlockCount;
+                  temp_save.ExtensionBlocks = NULL;
+                  temp_save.ExtensionBlockCount = 0;
+                  sp->Extensions.Function = sp->Extensions.ExtensionBlocks[0].Function;
+              }
+              break;
+
+          case EXTENSION_RECORD_TYPE:
+          {
+              int Function;
+              Extensions *Extensions;
+
+              if (DGifGetExtension(GifFile, &Function, &ExtData) == GIF_ERROR)
+                  return GIF_ERROR;
+
+              if (GifFile->ImageCount || Function == GRAPHICS_EXT_FUNC_CODE)
+                  Extensions = &temp_save;
+              else
+                  Extensions = &GifFile->Extensions;
+
+              Extensions->Function = Function;
+
+              if (ExtData)
+              {
+                  if (AddExtensionBlock(Extensions, ExtData[0], &ExtData[1]) == GIF_ERROR)
+                      return GIF_ERROR;
+              }
+              else
+              {
+                  if (AddExtensionBlock(Extensions, 0, NULL) == GIF_ERROR)
+                      return GIF_ERROR;
+              }
+
+              while (ExtData != NULL) {
+                  int Len;
+                  GifByteType *Data;
+
+                  if (DGifGetExtensionNext(GifFile, &ExtData) == GIF_ERROR)
+                      return GIF_ERROR;
+
+                  if (ExtData)
+                  {
+                      Len = ExtData[0];
+                      Data = &ExtData[1];
+                  }
+                  else
+                  {
+                      Len = 0;
+                      Data = NULL;
+                  }
+
+                  if (AppendExtensionBlock(Extensions, Len, Data) == GIF_ERROR)
+                      return GIF_ERROR;
+              }
+              break;
+          }
+
+          case TERMINATE_RECORD_TYPE:
+              break;
+
+          default:
+              break;
+        }
+    } while (RecordType != TERMINATE_RECORD_TYPE);
+
+    if (temp_save.ExtensionBlocks)
+        FreeExtension(&temp_save);
+
+    return GIF_OK;
+}
+
+/******************************************************************************
+ * Decode a specific frame on demand.
+ * Requires DGifSlurpHeaders() to have been called first to populate SavedImages.
+ * On success the frame's RasterBits are allocated and filled; subsequent calls
+ * return immediately if the frame is already decoded.
+ *****************************************************************************/
+int
+DGifDecodeFrame(GifFileType *GifFile, int frame)
+{
+    GifFilePrivateType *Private = GifFile->Private;
+    SavedImage *sp;
+    int ImageSize;
+
+    if (frame < 0 || frame >= GifFile->ImageCount)
+        return GIF_ERROR;
+
+    sp = &GifFile->SavedImages[frame];
+
+    /* Already decoded */
+    if (sp->RasterBits)
+        return GIF_OK;
+
+    /* Seek to this frame's LZW data in the stream */
+    if (GifFile->seekFunc)
+    {
+        ((SeekFunc)GifFile->seekFunc)(GifFile, sp->LZWDataOffset);
+        /* Reset the decoder's byte offset counter so subsequent READ() calls
+         * start from the correct position */
+        Private->Offset = sp->LZWDataOffset;
+    }
+
+    ImageSize = sp->ImageDesc.Width * sp->ImageDesc.Height;
+
+    /* Re-initialize LZW decompress state (reads the code size byte) */
+    Private->PixelCount = ImageSize;
+    DGifSetupDecompress(GifFile);
+
+    sp->RasterBits = malloc(ImageSize * sizeof(GifPixelType));
+    if (!sp->RasterBits)
+        return GIF_ERROR;
+
+    return DGifGetLine(GifFile, sp->RasterBits, ImageSize);
+}
+
 /******************************************************************************
  * GifFileType constructor with user supplied input function (TVT)
  *****************************************************************************/
diff --git a/dlls/windowscodecs/ungif.h b/dlls/windowscodecs/ungif.h
index c402109bc67..4c91c4c4881 100644
--- a/dlls/windowscodecs/ungif.h
+++ b/dlls/windowscodecs/ungif.h
@@ -124,6 +124,7 @@ typedef struct GifFileType {
     struct SavedImage *SavedImages; /* Use this to accumulate file state */
     void *UserData;             /* hook to attach user data (TVT) */
     void *Private;              /* Don't mess with this! */
+    void *seekFunc;             /* SeekFunc: seek to absolute byte offset for on-demand decoding */
 } GifFileType;
 
 typedef enum {
@@ -137,6 +138,9 @@ typedef enum {
 /* func type to read gif data from arbitrary sources (TVT) */
 typedef int (*InputFunc) (GifFileType *, GifByteType *, int);
 
+/* func type to seek to an absolute byte offset in the GIF data source */
+typedef void (*SeekFunc) (GifFileType *, int offset);
+
 /*  GIF89 extension function codes */
 
 #define COMMENT_EXT_FUNC_CODE     0xfe    /* comment */
@@ -171,8 +175,12 @@ int DGifCloseFile(GifFileType * GifFile);
 typedef struct SavedImage {
     GifImageDesc ImageDesc;
     int ImageDescOffset;
+    int LZWDataOffset;   /* byte offset to this frame's LZW sub-blocks in the stream */
     unsigned char *RasterBits;
     Extensions Extensions;
 } SavedImage;
 
+int DGifSlurpHeaders(GifFileType *GifFile);   /* scan frame headers only, skip pixel data */
+int DGifDecodeFrame(GifFileType *GifFile, int frame); /* on-demand decode of a specific frame */
+
 #endif /* _UNGIF_H_ */
-- 
GitLab

https://gitlab.winehq.org/wine/wine/-/merge_requests/11149

[Esme Povirk (＠madewokherd)]

Esme Povirk (@madewokherd) commented about dlls/windowscodecs/gifformat.c:
> seek.QuadPart = 0;
>      IStream_Seek(stream, seek, STREAM_SEEK_SET, NULL);
>
> +    /* keep stream for on-demand frame decoding */
> +    IStream_AddRef(stream);
> +    decoder->stream = stream;
> +
>      /* read all data from the stream */
>      decoder->gif = DGifOpen((void *)stream, _gif_inputfunc);
>      if (!decoder->gif)
> +    {
> +        IStream_Release(stream);

We should set `decoder->stream` to NULL when releasing it, so we don't release it twice when the object is freed.

-- 
https://gitlab.winehq.org/wine/wine/-/merge_requests/11149#note_143158

[Esme Povirk (＠madewokherd)]

Esme Povirk (@madewokherd) commented about dlls/windowscodecs/ungif.c:
> return GIF_OK;
>  }
>
> +/******************************************************************************
> + * Skip the LZW image sub-blocks without decoding any pixels.
> + * Called after DGifSetupDecompress has read the code size byte.
> + *****************************************************************************/
> +static int
> +DGifSkipImageData(GifFileType *GifFile)
> +{
> +    GifByteType *CodeBlock;
> +    do {
> +        if (DGifGetCodeNext(GifFile, &CodeBlock) == GIF_ERROR)

So if I understand this, we're still reading all the image data, but then we're discarding it?

-- 
https://gitlab.winehq.org/wine/wine/-/merge_requests/11149#note_143159