Add PNG support

This is a first stab at adding PNG support. For now there are no
filters, so only indexed images will work.

Author: deshipu

adafruit_imageload/__init__.py

@@ -80,4 +80,8 @@ def load(
             from . import gif
 
             return gif.load(file, bitmap=bitmap, palette=palette)
+        if header.startswith(b'\x89PN'):
+            from . import png
+
+            return png.load(file, bitmap=bitmap, palette=palette)
         raise RuntimeError("Unsupported image format")

adafruit_imageload/png.py

@@ -0,0 +1,81 @@
+# SPDX-FileCopyrightText: 2022 Radomir Dopieralski
+#
+# SPDX-License-Identifier: MIT
+
+"""
+`adafruit_imageload.png`
+====================================================
+
+Load pixel values (indices or colors) into a bitmap and colors into a palette
+from a PNG file.
+
+* Author(s): Radomir Dopieralski
+
+"""
+
+import struct
+import zlib
+
+
+__version__ = "0.0.0-auto.0"
+__repo__ = "https://github.com/adafruit/Adafruit_CircuitPython_ImageLoad.git"
+
+
+def load(file, *, bitmap, palette=None):  # pylint: disable=too-many-locals,too-many-branches
+    """Loads a PNG image from the open ``file``.
+
+    Returns tuple of bitmap object and palette object.
+
+    :param file: The *.png file being loaded
+    :param object bitmap: Type to store bitmap data. Must have API similar to
+      `displayio.Bitmap`.
+    :param object palette: Type to store the palette. Must have API similar to
+      `displayio.Palette`. Will be skipped if None"""
+    header = file.read(8)
+    if header != b'\x89PNG\r\n\x1a\n':
+        raise ValueError("Not a PNG file")
+    del header
+    data = bytearray()
+    pal = None
+    mode = None
+    depth = None
+    while True:
+        size, chunk = struct.unpack(">I4s", file.read(8))
+        if chunk == b'IHDR':
+            (width, height, depth, mode, compression, filters,
+                    interlaced) = struct.unpack(">IIBBBBB", file.read(13))
+            if interlaced:
+                raise NotImplementedError("Interlaced images unsupported")
+            # compression and filters must be 0 with current spec
+            assert compression == 0
+            assert filters == 0
+        elif chunk == b'PLTE':
+            if palette is None:
+                file.seek(size, 1)
+            else:
+                if mode != 3:
+                    raise NotImplementedError("Palette in non-indexed image")
+                pal_size = size // 3
+                pal = palette(pal_size)
+                for i in range(pal_size):
+                    pal[i] = file.read(3)
+        elif chunk == b'IDAT':
+            data.extend(file.read(size))
+        elif chunk == b'IEND':
+            break
+        else:
+            file.seek(size, 1)  # skip unknown chunks
+        file.seek(4, 1)  # skip CRC
+    data = zlib.decompress(data)
+    bmp = bitmap(width, height, 1 << depth)
+    scanline = (width * depth + 7) // 8
+    mem = memoryview(bmp)
+    for y in range(height):
+        dst = y * scanline
+        src = y * (scanline + 1) + 1
+        filter_ = data[src - 1]
+        if filter_ == 0:
+            mem[dst:dst + scanline] = data[src:src + scanline]
+        else:
+            raise NotImplementedError("Filters not supported")
+    return bmp, pal