Leptonica: learn to despeckle 1bpp images

src/ocrmypdf/leptonica.py

@@ -143,7 +143,7 @@ class LeptonicaObject:
 
     def __init__(self, cdata):
         if not cdata:
-            raise ValueError('NULL cdata object')
+            raise ValueError('Tried to wrap a NULL ' + self.LEPTONICA_TYPENAME)
         self._cdata = ffi.gc(cdata, self._destroy)
 
     @classmethod
@@ -570,6 +570,35 @@ class Pix(LeptonicaObject):
                 right, bottom = box.x + box.w, box.y + box.h
                 yield (decoded, (left, top, right, bottom))
 
+    def despeckle(self, size):
+        if size == 2:
+            speckle2 = """
+                oooo
+                oC o
+                o  o
+                oooo
+                """
+            sel1 = Sel.from_selstr(speckle2, 'speckle2')
+            sel2 = Sel.create_brick(2, 2, 0, 0, lept.SEL_HIT)
+        elif size == 3:
+            speckle3 = """
+                ooooo
+                oC  o
+                o   o
+                o   o
+                ooooo
+                """
+            sel1 = Sel.from_selstr(speckle3, 'speckle3')
+            sel2 = Sel.create_brick(3, 3, 0, 0, lept.SEL_HIT)
+        else:
+            raise ValueError(size)
+
+        pixhmt = Pix(lept.pixHMT(ffi.NULL, self._cdata, sel1._cdata))
+        pixdilated = Pix(lept.pixDilate(ffi.NULL, pixhmt._cdata, sel2._cdata))
+
+        pixsub = Pix(lept.pixSubtract(ffi.NULL, self._cdata, pixdilated._cdata))
+        return pixsub
+
 
 class CompressedData(LeptonicaObject):
     """Wrapper for L_COMP_DATA - abstract compressed image data"""
@@ -682,6 +711,7 @@ class BoxArray(LeptonicaObject, Sequence):
 
 
 class StringArray(LeptonicaObject, Sequence):
+    """Leptonica SARRAY/string array"""
 
     LEPTONICA_TYPENAME = 'SARRAY'
     cdata_destroy = lept.sarrayDestroy
@@ -695,6 +725,37 @@ class StringArray(LeptonicaObject, Sequence):
         raise IndexError(n)
 
 
+class Sel(LeptonicaObject):
+    """Leptonica 'sel'/selection element for hit-miss transform"""
+
+    LEPTONICA_TYPENAME = 'SEL'
+    cdata_destroy = lept.selDestroy
+
+    @classmethod
+    def from_selstr(cls, selstr, name):
+        lines = [line.strip() for line in selstr.split('\n') if line.strip()]
+        h = len(lines)
+        w = len(lines[0])
+        lengths = set(len(line) for line in lines)
+        if len(lengths) != 1:
+            raise ValueError("All lines in selstr must be same length")
+
+        repacked = ''.join(line.strip() for line in lines)
+        buf_selstr = ffi.from_buffer(repacked.encode('ascii'))
+        buf_name = ffi.from_buffer(name.encode('ascii'))
+        sel = lept.selCreateFromString(buf_selstr, h, w, buf_name)
+        return cls(sel)
+
+    @classmethod
+    def create_brick(cls, h, w, cy, cx, type_):
+        sel = lept.selCreateBrick(h, w, cy, cx, type_)
+        return cls(sel)
+
+    def __repr__(self):
+        selstr = ffi.gc(lept.selPrintToString(self._cdata), lept.lept_free)
+        return '<Sel \n' + ffi.string(selstr).decode('ascii') + '\n>'
+
+
 @lru_cache(maxsize=1)
 def get_leptonica_version():
     """Get Leptonica version string.

src/ocrmypdf/lib/compile_leptonica.py

@@ -135,6 +135,18 @@ struct L_Compressed_Data
 };
 typedef struct L_Compressed_Data L_COMP_DATA;
 
+/*! Selection */
+struct Sel
+{
+    l_int32       sy;        /*!< sel height                               */
+    l_int32       sx;        /*!< sel width                                */
+    l_int32       cy;        /*!< y location of sel origin                 */
+    l_int32       cx;        /*!< x location of sel origin                 */
+    l_int32     **data;      /*!< {0,1,2}; data[i][j] in [row][col] order  */
+    char         *name;      /*!< used to find sel by name                 */
+};
+typedef struct Sel SEL;
+
 enum {
     REMOVE_CMAP_TO_BINARY = 0,     /*!< remove colormap for conv to 1 bpp  */
     REMOVE_CMAP_TO_GRAYSCALE = 1,  /*!< remove colormap for conv to 8 bpp  */
@@ -184,6 +196,12 @@ enum {
     L_SEVERITY_NONE     = 6    /* Highest severity: print no messages     */
 };
 
+enum {
+    SEL_DONT_CARE  = 0,
+    SEL_HIT        = 1,
+    SEL_MISS       = 2
+};
+
 """)
 
 ffibuilder.cdef("""
@@ -407,12 +425,34 @@ l_generateCIDataForPdf(const char *fname,
                        l_int32 quality,
                        L_COMP_DATA **pcid);
 
+
 BOX *
 boxClone ( BOX *box );
 
 BOX *
 boxaGetBox ( BOXA *boxa, l_int32 index, l_int32 accessflag );
 
+SEL *
+selCreateFromString ( const char *text, l_int32 h, l_int32 w, const char *name );
+
+SEL *
+selCreateBrick ( l_int32 h, l_int32 w, l_int32 cy, l_int32 cx, l_int32 type );
+
+char *
+selPrintToString(SEL  *sel);
+
+PIX *
+pixDilate ( PIX *pixd, PIX *pixs, SEL *sel );
+
+PIX *
+pixErode ( PIX *pixd, PIX *pixs, SEL *sel );
+
+PIX *
+pixHMT ( PIX *pixd, PIX *pixs, SEL *sel );
+
+PIX *
+pixSubtract ( PIX *pixd, PIX *pixs1, PIX *pixs2 );
+
 void
 boxDestroy(BOX  **pbox);
 
@@ -434,8 +474,11 @@ sarrayDestroy(SARRAY  **psa);
 void
 lept_free(void *ptr);
 
+void selDestroy ( SEL **psel );
+
 l_int32
 setMsgSeverity(l_int32 newsev);
+
 """)