Fix double height character rendering logic

src/teletext/renderer.py

@@ -215,13 +215,27 @@ class TeletextCanvas(QWidget):
             if 0 <= p.row <= 25:
                 grid[p.row] = p
                 
+        # Pass 1: Backgrounds
+        occlusion_mask = [False] * 40
         for r in range(25):
             packet = grid[r]
-            self.draw_row(painter, r, packet)
+            occlusion_mask = self.draw_row(painter, r, packet, draw_bg=True, draw_fg=False, occlusion_mask=occlusion_mask)
+
+        # Pass 2: Foregrounds
+        occlusion_mask = [False] * 40
+        for r in range(25):
+            packet = grid[r]
+            occlusion_mask = self.draw_row(painter, r, packet, draw_bg=False, draw_fg=True, occlusion_mask=occlusion_mask)
             
         painter.end()
 
-    def draw_row(self, painter, row, packet):
+    def draw_row(self, painter, row, packet, draw_bg=True, draw_fg=True, occlusion_mask=None):
+        if occlusion_mask is None:
+            occlusion_mask = [False] * 40
+            
+        # Output mask for the next row
+        next_occlusion_mask = [False] * 40
+
         # Default State at start of row
         fg = COLORS[7] # White
         bg = COLORS[0] # Black
@@ -229,6 +243,7 @@ class TeletextCanvas(QWidget):
         contiguous = True # Mosaic
         hold_graphics = False
         held_char = 0x20 # Space
+        double_height = False
         
         y = row * self.cell_h
         
@@ -248,6 +263,24 @@ class TeletextCanvas(QWidget):
         for c in range(40):
             x = c * self.cell_w
             
+            # If this cell is occluded by the row above, skip drawing and attribute processing?
+            # Spec says "The characters in the row below are ignored."
+            # Ideally we shouldn't even process attributes, but for simple renderer we just skip draw.
+            # However, if we skip attribute processing, state (fg/bg) won't update. 
+            # Teletext attributes are serial.
+            # BUT, if the row above covers it, the viewer sees the row above.
+            # Does the hidden content affect the *rest* of the row?
+            # Likely yes, attributes usually propagate. 
+            # But the spec says "ignored". Let's assume we skip *everything* for this cell visually,
+            # but maybe we should technically maintain state?
+            # For "Double Height" visual correctness, skipping drawing is the key.
+            # We will Process attributes (to keep state consistent) but Skip Drawing if occluded.
+            
+            # Wait, if we process attributes, we might set double_height=True for the NEXT row?
+            # If this cell is occluded, it shouldn't trigger DH for the next row.
+            
+            is_occluded = occlusion_mask[c]
+            
             # Decide byte value
             if row == 0 and c < 8:
                 # Use generated header prefix
@@ -274,9 +307,9 @@ class TeletextCanvas(QWidget):
                 elif byte_val == 0x1D: # New BG
                     bg = fg
                 elif byte_val == 0x0C: # Normal Height
-                    pass 
+                    double_height = False
                 elif byte_val == 0x0D: # Double Height
-                    pass # Not implemented yet
+                    double_height = True
                 elif byte_val == 0x19: # Contiguous Graphics
                     contiguous = True
                 elif byte_val == 0x1A: # Separated Graphics
@@ -286,14 +319,33 @@ class TeletextCanvas(QWidget):
                 elif byte_val == 0x1F: # Release Graphics
                     hold_graphics = False
             
+            # Record Double Height for next row
+            if double_height and not is_occluded:
+                next_occlusion_mask[c] = True
+
+            # If occluded, do not draw anything for this cell
+            if is_occluded:
+                continue
+
             # Draw Background
-            painter.fillRect(x, y, self.cell_w, self.cell_h, bg)
+            if draw_bg:
+                # If double height, draw taller background
+                h_bg = self.cell_h * 2 if double_height else self.cell_h
+                painter.fillRect(x, y, self.cell_w, h_bg, bg)
             
             # Draw Foreground
+            if draw_fg:
+                # Calculate height
+                # For Mosaics, we use the height param.
+                # For Alphanumerics, we scale the painter.
+                
                 if is_control:
                     # "Set-at" spacing attribute? Teletext control codes occupy a space
                     # unless "Hold Graphics" replaces it with previous graphic char.
                     if hold_graphics and graphics_mode:
+                        if double_height:
+                             self.draw_mosaic(painter, x, y, held_char, fg, contiguous, height=self.cell_h * 2)
+                        else:
                              self.draw_mosaic(painter, x, y, held_char, fg, contiguous)
                     else:
                         # Draw space (nothing, since we filled BG)
@@ -301,28 +353,45 @@ class TeletextCanvas(QWidget):
                 else:
                     if graphics_mode:
                         # Mosaic Graphics
+                        h_mos = self.cell_h * 2 if double_height else self.cell_h
                         if (0x20 <= byte_val <= 0x3F) or (0x60 <= byte_val <= 0x7F):
-                        self.draw_mosaic(painter, x, y, byte_val, fg, contiguous)
+                            self.draw_mosaic(painter, x, y, byte_val, fg, contiguous, height=h_mos)
                             held_char = byte_val
                         else:
                             # Capital letter in graphics mode? Usually shows char?
                             char = get_char(byte_val, self.subset_idx)
                             painter.setPen(fg)
+                            if double_height:
+                                painter.save()
+                                painter.translate(x, y)
+                                painter.scale(1, 2)
+                                painter.drawText(QRect(0, 0, self.cell_w, self.cell_h), Qt.AlignmentFlag.AlignCenter, char)
+                                painter.restore()
+                            else:
                                 painter.drawText(QRect(x, y, self.cell_w, self.cell_h), Qt.AlignmentFlag.AlignCenter, char)
                             held_char = 0x20
                     else:
                         # Alphanumeric
                         char = get_char(byte_val, self.subset_idx)
                         painter.setPen(fg)
+                        if double_height:
+                            painter.save()
+                            painter.translate(x, y)
+                            painter.scale(1, 2)
+                            painter.drawText(QRect(0, 0, self.cell_w, self.cell_h), Qt.AlignmentFlag.AlignCenter, char)
+                            painter.restore()
+                        else:
                             painter.drawText(QRect(x, y, self.cell_w, self.cell_h), Qt.AlignmentFlag.AlignCenter, char)
             # Draw Cursor
             # Invert the cell at cursor position
-            if self.cursor_visible and c == self.cursor_x and row == self.cursor_y:
+            if draw_fg and self.cursor_visible and c == self.cursor_x and row == self.cursor_y:
                  painter.setCompositionMode(QPainter.CompositionMode.CompositionMode_Difference)
                  # Difference with white creates inversion
+                 # Note: Cursor follows double height? Probably just the active cell.
                  painter.fillRect(x, y, self.cell_w, self.cell_h, QColor(255, 255, 255))
                  painter.setCompositionMode(QPainter.CompositionMode.CompositionMode_SourceOver)
         
+        return next_occlusion_mask
     def mousePressEvent(self, event):
         self.setFocus()
         # Calculate cell from mouse position
@@ -360,7 +429,10 @@ class TeletextCanvas(QWidget):
              row = int(my / (self.cell_h * scale))
              self.set_cursor(col, row)
              
-    def draw_mosaic(self, painter, x, y, char_code, color, contiguous):
+    def draw_mosaic(self, painter, x, y, char_code, color, contiguous, height=None):
+        if height is None:
+            height = self.cell_h
+
         val = char_code & 0x7F
         bits = 0
         if val >= 0x20:
@@ -368,7 +440,7 @@ class TeletextCanvas(QWidget):
             
         # Grid definitions for 2x3 grid
         x_splits = [0, int(self.cell_w / 2), self.cell_w]
-        y_splits = [0, int(self.cell_h / 3), int(2 * self.cell_h / 3), self.cell_h]
+        y_splits = [0, int(height / 3), int(2 * height / 3), height]
         
         # Block indices (col, row) for the 6 bits
         block_indices = [