[base] Accept negative bitmap alignment for bottom-up flow.

* src/base/ftbitmap.c (FT_Bitmap_Convert): Use negative alignment
to produce negative pitch.
* include/freetype/ftbitmap.c (FT_Bitmap_Convert): Document it.

2 files changed, 18 insertions, 18 deletions

diff --git a/include/freetype/ftbitmap.h b/include/freetype/ftbitmap.h
index c3462dadc..862b8a369 100644
--- a/include/freetype/ftbitmap.h
+++ b/include/freetype/ftbitmap.h
@@ -178,8 +178,8 @@ FT_BEGIN_HEADER
    *     The source bitmap.
    *
    *   alignment ::
-   *     The pitch of the bitmap is a multiple of this argument.  Common
-   *     values are 1, 2, or 4.
+   *     The pitch of the target bitmap is a multiple of this argument.
+   *     Common values are 1, 2, or 4.
    *
    * @output:
    *   target ::
@@ -189,16 +189,16 @@ FT_BEGIN_HEADER
    *   FreeType error code.  0~means success.
    *
    * @note:
-   *   It is possible to call @FT_Bitmap_Convert multiple times without
-   *   calling @FT_Bitmap_Done (the memory is simply reallocated).
+   *   This function reallocates the memory in the target bitmap, which has
+   *   to be valid, either initialized by @FT_Bitmap_Init or reused multiple
+   *   times. `source->buffer` and `target->buffer` must neither be equal
+   *   nor overlap.  Use @FT_Bitmap_Done to finally remove the bitmap object.
    *
-   *   Use @FT_Bitmap_Done to finally remove the bitmap object.
+   *   Negative alignment values produce bottom-up bitmaps with negative
+   *   pitch.  Zero alignment is treated as one, i.e., no padding is used.
    *
    *   The `library` argument is taken to have access to FreeType's memory
    *   handling functions.
-   *
-   *   `source->buffer` and `target->buffer` must neither be equal nor
-   *   overlap.
    */
   FT_EXPORT( FT_Error )
   FT_Bitmap_Convert( FT_Library        library,
diff --git a/src/base/ftbitmap.c b/src/base/ftbitmap.c
index 2dcade968..6d2b86fbc 100644
--- a/src/base/ftbitmap.c
+++ b/src/base/ftbitmap.c
@@ -542,7 +542,7 @@
     case FT_PIXEL_MODE_LCD_V:
     case FT_PIXEL_MODE_BGRA:
       {
-        FT_Int  pad, target_pitch;
+        FT_Int  width = (FT_Int)source->width;
 
 
         FT_Bitmap_Done( library, target );
@@ -551,20 +551,20 @@
         target->rows       = source->rows;
         target->width      = source->width;
 
-        pad = 0;
-        if ( alignment > 0 )
+        if ( alignment )
         {
-          pad = (FT_Int)source->width % alignment;
-          if ( pad != 0 )
-            pad = alignment - pad;
-        }
+          FT_Int  rem = width % alignment;
+
 
-        target_pitch = (FT_Int)source->width + pad;
+          if ( rem )
+            width = alignment < 0 ? width - rem - alignment
+                                  : width - rem + alignment;
+        }
 
-        if ( FT_QALLOC_MULT( target->buffer, target->rows, target_pitch ) )
+        if ( FT_QALLOC_MULT( target->buffer, target->rows, width ) )
           return error;
 
-        target->pitch = target->pitch < 0 ? -target_pitch : target_pitch;
+        target->pitch = alignment < 0 ? -width : width;
       }
       break;