STY: Increase readability (py-pdf#2775)

Add comments and remove noisy comments.

pypdf/filters.py

@@ -190,29 +190,29 @@ def _decode_png_prediction(data: bytes, columns: int, rowlength: int) -> bytes:
  filter_byte = rowdata[0]
 
  if filter_byte == 0:
+ # PNG None Predictor
  pass
  elif filter_byte == 1:
+ # PNG Sub Predictor
  for i in range(bpp + 1, rowlength):
  rowdata[i] = (rowdata[i] + rowdata[i - bpp]) % 256
  elif filter_byte == 2:
+ # PNG Up Predictor
  for i in range(1, rowlength):
  rowdata[i] = (rowdata[i] + prev_rowdata[i]) % 256
  elif filter_byte == 3:
+ # PNG Average Predictor
  for i in range(1, bpp + 1):
- # left = 0
  floor = prev_rowdata[i] // 2
  rowdata[i] = (rowdata[i] + floor) % 256
  for i in range(bpp + 1, rowlength):
  left = rowdata[i - bpp]
  floor = (left + prev_rowdata[i]) // 2
  rowdata[i] = (rowdata[i] + floor) % 256
  elif filter_byte == 4:
+ # PNG Paeth Predictor
  for i in range(1, bpp + 1):
- # left = 0
- up = prev_rowdata[i]
- # up_left = 0
- paeth = up
- rowdata[i] = (rowdata[i] + paeth) % 256
+ rowdata[i] = (rowdata[i] + prev_rowdata[i]) % 256
  for i in range(bpp + 1, rowlength):
  left = rowdata[i - bpp]
  up = prev_rowdata[i]