API: Add optional bitmap rowspan callback.

2 files changed, 28 insertions, 3 deletions

diff --git a/include/nsgif.h b/include/nsgif.h
index a02597c..0ea792b 100644
--- a/include/nsgif.h
+++ b/include/nsgif.h
@@ -162,6 +162,11 @@ typedef enum nsgif_bitmap_fmt {
  * but they are owned by a \ref nsgif_t.
  *
  * See \ref nsgif_bitmap_fmt for pixel format information.
+ *
+ * The bitmap may have a row_span greater than the bitmap width, but the
+ * difference between row span and width must be a whole number of pixels
+ * (a multiple of four bytes). If row span is greater than width, the
+ * \ref get_rowspan callback must be provided.
  */
 typedef void nsgif_bitmap_t;
 
@@ -186,7 +191,8 @@ typedef struct nsgif_bitmap_cb_vt {
 	/**
 	 * Get pointer to pixel buffer in a bitmap.
 	 *
-	 * The pixel buffer must be `width * height * sizeof(uint32_t)`.
+	 * The pixel buffer must be `(width + N) * height * sizeof(uint32_t)`.
+	 * Where `N` is any number greater than or equal to 0.
 	 * Note that the returned pointer to uint8_t must be 4-byte aligned.
 	 *
 	 * \param[in]  bitmap  The bitmap.
@@ -218,6 +224,15 @@ typedef struct nsgif_bitmap_cb_vt {
 	 * \param[in]  bitmap  The bitmap.
 	 */
 	void (*modified)(nsgif_bitmap_t *bitmap);
+
+	/**
+	 * Get row span in pixels.
+	 *
+	 * If this callback is not provided, LibNSGIF will use the width.
+	 *
+	 * \param[in]  bitmap  The bitmap.
+	 */
+	uint32_t (*get_rowspan)(nsgif_bitmap_t *bitmap);
 } nsgif_bitmap_cb_vt;
 
 /**
diff --git a/src/gif.c b/src/gif.c
index 84d4209..435d0d9 100644
--- a/src/gif.c
+++ b/src/gif.c
@@ -72,8 +72,11 @@ struct nsgif {
 	uint32_t frame;
 	/** current frame decoded to bitmap */
 	uint32_t decoded_frame;
+
 	/** currently decoded image; stored as bitmap from bitmap_create callback */
 	nsgif_bitmap_t *frame_image;
+	/** Row span of frame_image in pixels. */
+	uint32_t rowspan;
 
 	/** Minimum allowable frame delay. */
 	uint16_t delay_min;
@@ -223,6 +226,11 @@ static inline uint32_t* nsgif__bitmap_get(
 		return NULL;
 	}
 
+	gif->rowspan = gif->info.width;
+	if (gif->bitmap.get_rowspan) {
+		gif->rowspan = gif->bitmap.get_rowspan(gif->frame_image);
+	}
+
 	/* Get the frame data */
 	assert(gif->bitmap.get_buffer);
 	return (void *)gif->bitmap.get_buffer(gif->frame_image);
@@ -463,7 +471,7 @@ static nsgif_error nsgif__decode_complex(
 		uint32_t *frame_scanline;
 
 		frame_scanline = frame_data + offset_x +
-				(y + offset_y) * gif->info.width;
+				(y + offset_y) * gif->rowspan;
 
 		x = width;
 		while (x > 0) {
@@ -594,7 +602,9 @@ static inline nsgif_error nsgif__decode(
 	uint32_t transparency_index = frame->transparency_index;
 	uint32_t *restrict colour_table = gif->colour_table;
 
-	if (interlace == false && width == gif->info.width && offset_x == 0) {
+	if (interlace == false && offset_x == 0 &&
+			width == gif->info.width &&
+			width == gif->rowspan) {
 		ret = nsgif__decode_simple(gif, height, offset_y,
 				data, transparency_index,
 				frame_data, colour_table);