diff options
| -rw-r--r-- | src/libnsgif.c | 83 |
1 files changed, 60 insertions, 23 deletions
diff --git a/src/libnsgif.c b/src/libnsgif.c index 7d48b7e..6046769 100644 --- a/src/libnsgif.c +++ b/src/libnsgif.c @@ -247,21 +247,56 @@ static gif_result gif__recover_frame( return GIF_OK; } -static uint32_t gif_interlaced_line(int height, int y) +/** + * Get the next line for GIF decode. + * + * Note that the step size must be initialised to 24 at the start of the frame + * (when y == 0). This is because of the first two passes of the frame have + * the same step size of 8, and the step size is used to determine the current + * pass. + * + * \param[in] height Frame height in pixels. + * \param[in,out] y Current row, starting from 0, updated on exit. + * \param[in,out] step Current step starting with 24, updated on exit. + * \return true if there is a row to process, false at the end of the frame. + */ +static inline bool gif__deinterlace(uint32_t height, uint32_t *y, uint8_t *step) { - if ((y << 3) < height) { - return (y << 3); + *y += *step & 0xf; + + if (*y < height) return true; + + switch (*step) { + case 24: *y = 4; *step = 8; if (*y < height) return true; + /* Fall through. */ + case 8: *y = 2; *step = 4; if (*y < height) return true; + /* Fall through. */ + case 4: *y = 1; *step = 2; if (*y < height) return true; + /* Fall through. */ + default: + break; } - y -= ((height + 7) >> 3); - if ((y << 3) < (height - 4)) { - return (y << 3) + 4; - } - y -= ((height + 3) >> 3); - if ((y << 2) < (height - 2)) { - return (y << 2) + 2; + + return false; +} + +/** + * Get the next line for GIF decode. + * + * \param[in] interlace Non-zero if the frame is not interlaced. + * \param[in] height Frame height in pixels. + * \param[in,out] y Current row, starting from 0, updated on exit. + * \param[in,out] step Current step starting with 24, updated on exit. + * \return true if there is a row to process, false at the end of the frame. + */ +static inline bool gif__next_row(uint32_t interlace, + uint32_t height, uint32_t *y, uint8_t *step) +{ + if (!interlace) { + return (++*y != height); + } else { + return gif__deinterlace(height, y, step); } - y -= ((height + 1) >> 2); - return (y << 1) + 1; } static gif_result gif__decode_complex( @@ -276,9 +311,15 @@ static gif_result gif__decode_complex( uint32_t *restrict frame_data, uint32_t *restrict colour_table) { - uint32_t available = 0; - gif_result ret = GIF_OK; lzw_result res; + gif_result ret = GIF_OK; + uint32_t available = 0; + uint8_t step = 24; + uint32_t y = 0; + + if (height == 0) { + return GIF_OK; + } /* Initialise the LZW decoding */ res = lzw_decode_init(gif->lzw_ctx, data[0], @@ -288,17 +329,12 @@ static gif_result gif__decode_complex( return gif_error_from_lzw(res); } - for (uint32_t y = 0; y < height; y++) { + do { uint32_t x; - uint32_t decode_y; uint32_t *frame_scanline; - if (interlace) { - decode_y = gif_interlaced_line(height, y) + offset_y; - } else { - decode_y = y + offset_y; - } - frame_scanline = frame_data + offset_x + (decode_y * gif->width); + frame_scanline = frame_data + offset_x + + (y + offset_y) * gif->width; x = width; while (x > 0) { @@ -338,7 +374,8 @@ static gif_result gif__decode_complex( } } } - } + } while (gif__next_row(interlace, height, &y, &step)); + return ret; } |