[png] Add support for PNG images

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 1007 insertions, 0 deletions

diff --git a/src/image/png.c b/src/image/png.c
new file mode 100644
index 00000000..c1460855
--- /dev/null
+++ b/src/image/png.c
@@ -0,0 +1,1007 @@
+/*
+ * Copyright (C) 2014 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <byteswap.h>
+#include <ipxe/umalloc.h>
+#include <ipxe/pixbuf.h>
+#include <ipxe/deflate.h>
+#include <ipxe/png.h>
+
+/** @file
+ *
+ * Portable Network Graphics (PNG) format
+ *
+ * The PNG format is defined in RFC 2083.
+ */
+
+/** PNG context */
+struct png_context {
+	/** Offset within image */
+	size_t offset;
+
+	/** Pixel buffer */
+	struct pixel_buffer *pixbuf;
+
+	/** Bit depth */
+	unsigned int depth;
+	/** Colour type */
+	unsigned int colour_type;
+	/** Number of channels */
+	unsigned int channels;
+	/** Number of interlace passes */
+	unsigned int passes;
+	/** Palette, in iPXE's pixel buffer format */
+	uint32_t palette[PNG_PALETTE_COUNT];
+
+	/** Decompression buffer for raw PNG data */
+	struct deflate_chunk raw;
+	/** Decompressor */
+	struct deflate deflate;
+};
+
+/** A PNG interlace pass */
+struct png_interlace {
+	/** Pass number */
+	unsigned int pass;
+	/** X starting indent */
+	unsigned int x_indent;
+	/** Y starting indent */
+	unsigned int y_indent;
+	/** X stride */
+	unsigned int x_stride;
+	/** Y stride */
+	unsigned int y_stride;
+	/** Width */
+	unsigned int width;
+	/** Height */
+	unsigned int height;
+};
+
+/** PNG file signature */
+static struct png_signature png_signature = PNG_SIGNATURE;
+
+/** Number of interlacing passes */
+static uint8_t png_interlace_passes[] = {
+	[PNG_INTERLACE_NONE] = 1,
+	[PNG_INTERLACE_ADAM7] = 7,
+};
+
+/**
+ * Transcribe PNG chunk type name (for debugging)
+ *
+ * @v type		Chunk type
+ * @ret name		Chunk type name
+ */
+static const char * png_type_name ( uint32_t type ) {
+	static union {
+		uint32_t type;
+		char name[ sizeof ( uint32_t ) + 1 /* NUL */ ];
+	} u;
+
+	u.type = type;
+	return u.name;
+}
+
+/**
+ * Calculate PNG interlace pass parameters
+ *
+ * @v png		PNG context
+ * @v pass		Pass number (0=first pass)
+ * @v interlace		Interlace pass to fill in
+ */
+static void png_interlace ( struct png_context *png, unsigned int pass,
+			    struct png_interlace *interlace ) {
+	unsigned int grid_width_log2;
+	unsigned int grid_height_log2;
+	unsigned int x_indent;
+	unsigned int y_indent;
+	unsigned int x_stride_log2;
+	unsigned int y_stride_log2;
+	unsigned int x_stride;
+	unsigned int y_stride;
+	unsigned int width;
+	unsigned int height;
+
+	/* Sanity check */
+	assert ( png->passes > 0 );
+
+	/* Store pass number */
+	interlace->pass = pass;
+
+	/* Calculate interlace grid dimensions */
+	grid_width_log2 = ( png->passes / 2 );
+	grid_height_log2 = ( ( png->passes - 1 ) / 2 );
+
+	/* Calculate starting indents */
+	interlace->x_indent = x_indent =
+		( ( pass & 1 ) ?
+		  ( 1 << ( grid_width_log2 - ( pass / 2 ) - 1 ) ) : 0 );
+	interlace->y_indent = y_indent =
+		( ( pass && ! ( pass & 1 ) ) ?
+		  ( 1 << ( grid_height_log2 - ( ( pass - 1 ) / 2 ) - 1 ) ) : 0);
+
+	/* Calculate strides */
+	x_stride_log2 = ( grid_width_log2 - ( pass / 2 ) );
+	y_stride_log2 =
+		( grid_height_log2 - ( pass ? ( ( pass - 1 ) / 2 ) : 0 ) );
+	interlace->x_stride = x_stride = ( 1 << x_stride_log2 );
+	interlace->y_stride = y_stride = ( 1 << y_stride_log2 );
+
+	/* Calculate pass dimensions */
+	width = png->pixbuf->width;
+	height = png->pixbuf->height;
+	interlace->width =
+		( ( width - x_indent + x_stride - 1 ) >> x_stride_log2 );
+	interlace->height =
+		( ( height - y_indent + y_stride - 1 ) >> y_stride_log2 );
+}
+
+/**
+ * Calculate PNG pixel length
+ *
+ * @v png		PNG context
+ * @ret pixel_len	Pixel length
+ */
+static unsigned int png_pixel_len ( struct png_context *png ) {
+
+	return ( ( ( png->channels * png->depth ) + 7 ) / 8 );
+}
+
+/**
+ * Calculate PNG scanline length
+ *
+ * @v png		PNG context
+ * @v interlace		Interlace pass
+ * @ret scanline_len	Scanline length (including filter byte)
+ */
+static size_t png_scanline_len ( struct png_context *png,
+				 struct png_interlace *interlace ) {
+
+	return ( 1 /* Filter byte */ +
+		 ( ( interlace->width * png->channels * png->depth ) + 7 ) / 8);
+}
+
+/**
+ * Handle PNG image header chunk
+ *
+ * @v image		PNG image
+ * @v png		PNG context
+ * @v len		Chunk length
+ * @ret rc		Return status code
+ */
+static int png_image_header ( struct image *image, struct png_context *png,
+			      size_t len ) {
+	struct png_image_header ihdr;
+	struct png_interlace interlace;
+	unsigned int pass;
+
+	/* Sanity check */
+	if ( len != sizeof ( ihdr ) ) {
+		DBGC ( image, "PNG %s invalid IHDR length %zd\n",
+		       image->name, len );
+		return -EINVAL;
+	}
+	if ( png->pixbuf ) {
+		DBGC ( image, "PNG %s duplicate IHDR\n", image->name );
+		return -EINVAL;
+	}
+
+	/* Extract image header */
+	copy_from_user ( &ihdr, image->data, png->offset, len );
+	DBGC ( image, "PNG %s %dx%d depth %d type %d compression %d filter %d "
+	       "interlace %d\n", image->name, ntohl ( ihdr.width ),
+	       ntohl ( ihdr.height ), ihdr.depth, ihdr.colour_type,
+	       ihdr.compression, ihdr.filter, ihdr.interlace );
+
+	/* Sanity checks */
+	if ( ihdr.compression >= PNG_COMPRESSION_UNKNOWN ) {
+		DBGC ( image, "PNG %s unknown compression method %d\n",
+		       image->name, ihdr.compression );
+		return -ENOTSUP;
+	}
+	if ( ihdr.filter >= PNG_FILTER_UNKNOWN ) {
+		DBGC ( image, "PNG %s unknown filter method %d\n",
+		       image->name, ihdr.filter );
+		return -ENOTSUP;
+	}
+	if ( ihdr.interlace >= PNG_INTERLACE_UNKNOWN ) {
+		DBGC ( image, "PNG %s unknown interlace method %d\n",
+		       image->name, ihdr.interlace );
+		return -ENOTSUP;
+	}
+
+	/* Allocate pixel buffer */
+	png->pixbuf = alloc_pixbuf ( ntohl ( ihdr.width ),
+				     ntohl ( ihdr.height ) );
+	if ( ! png->pixbuf ) {
+		DBGC ( image, "PNG %s could not allocate pixel buffer\n",
+		       image->name );
+		return -ENOMEM;
+	}
+
+	/* Extract bit depth */
+	png->depth = ihdr.depth;
+	if ( ( png->depth == 0 ) ||
+	     ( ( png->depth & ( png->depth - 1 ) ) != 0 ) ) {
+		DBGC ( image, "PNG %s invalid depth %d\n",
+		       image->name, png->depth );
+		return -EINVAL;
+	}
+
+	/* Calculate number of channels */
+	png->colour_type = ihdr.colour_type;
+	png->channels = 1;
+	if ( ! ( ihdr.colour_type & PNG_COLOUR_TYPE_PALETTE ) ) {
+		if ( ihdr.colour_type & PNG_COLOUR_TYPE_RGB )
+			png->channels += 2;
+		if ( ihdr.colour_type & PNG_COLOUR_TYPE_ALPHA )
+			png->channels += 1;
+	}
+
+	/* Calculate number of interlace passes */
+	png->passes = png_interlace_passes[ihdr.interlace];
+
+	/* Calculate length of raw data buffer */
+	for ( pass = 0 ; pass < png->passes ; pass++ ) {
+		png_interlace ( png, pass, &interlace );
+		if ( interlace.width == 0 )
+			continue;
+		png->raw.len += ( interlace.height *
+				  png_scanline_len ( png, &interlace ) );
+	}
+
+	/* Allocate raw data buffer */
+	png->raw.data = umalloc ( png->raw.len );
+	if ( ! png->raw.data ) {
+		DBGC ( image, "PNG %s could not allocate data buffer\n",
+		       image->name );
+		return -ENOMEM;
+	}
+
+	return 0;
+}
+
+/**
+ * Handle PNG palette chunk
+ *
+ * @v image		PNG image
+ * @v png		PNG context
+ * @v len		Chunk length
+ * @ret rc		Return status code
+ */
+static int png_palette ( struct image *image, struct png_context *png,
+			 size_t len ) {
+	size_t offset = png->offset;
+	struct png_palette_entry palette;
+	unsigned int i;
+
+	/* Populate palette */
+	for ( i = 0 ; i < ( sizeof ( png->palette ) /
+			    sizeof ( png->palette[0] ) ) ; i++ ) {
+
+		/* Stop when we run out of palette data */
+		if ( len < sizeof ( palette ) )
+			break;
+
+		/* Extract palette entry */
+		copy_from_user ( &palette, image->data, offset,
+				 sizeof ( palette ) );
+		png->palette[i] = ( ( palette.red << 16 ) |
+				    ( palette.green << 8 ) |
+				    ( palette.blue << 0 ) );
+		DBGC2 ( image, "PNG %s palette entry %d is %#06x\n",
+			image->name, i, png->palette[i] );
+
+		/* Move to next entry */
+		offset += sizeof ( palette );
+		len -= sizeof ( palette );
+	}
+
+	return 0;
+}
+
+/**
+ * Handle PNG image data chunk
+ *
+ * @v image		PNG image
+ * @v png		PNG context
+ * @v len		Chunk length
+ * @ret rc		Return status code
+ */
+static int png_image_data ( struct image *image, struct png_context *png,
+			    size_t len ) {
+	struct deflate_chunk in;
+	int rc;
+
+	/* Deflate this chunk */
+	deflate_chunk_init ( &in, image->data, png->offset,
+			     ( png->offset + len ) );
+	if ( ( rc = deflate_inflate ( &png->deflate, &in, &png->raw ) ) != 0 ) {
+		DBGC ( image, "PNG %s could not decompress: %s\n",
+		       image->name, strerror ( rc ) );
+		return rc;
+	}
+
+	return 0;
+}
+
+/**
+ * Unfilter byte using the "None" filter
+ *
+ * @v current		Filtered current byte
+ * @v left		Unfiltered left byte
+ * @v above		Unfiltered above byte
+ * @v above_left	Unfiltered above-left byte
+ * @ret current		Unfiltered current byte
+ */
+static unsigned int png_unfilter_none ( unsigned int current,
+					unsigned int left __unused,
+					unsigned int above __unused,
+					unsigned int above_left __unused ) {
+
+	return current;
+}
+
+/**
+ * Unfilter byte using the "Sub" filter
+ *
+ * @v current		Filtered current byte
+ * @v left		Unfiltered left byte
+ * @v above		Unfiltered above byte
+ * @v above_left	Unfiltered above-left byte
+ * @ret current		Unfiltered current byte
+ */
+static unsigned int png_unfilter_sub ( unsigned int current,
+				       unsigned int left,
+				       unsigned int above __unused,
+				       unsigned int above_left __unused ) {
+
+	return ( current + left );
+}
+
+/**
+ * Unfilter byte using the "Up" filter
+ *
+ * @v current		Filtered current byte
+ * @v left		Unfiltered left byte
+ * @v above		Unfiltered above byte
+ * @v above_left	Unfiltered above-left byte
+ * @ret current		Unfiltered current byte
+ */
+static unsigned int png_unfilter_up ( unsigned int current,
+				      unsigned int left __unused,
+				      unsigned int above,
+				      unsigned int above_left __unused ) {
+
+	return ( current + above );
+}
+
+/**
+ * Unfilter byte using the "Average" filter
+ *
+ * @v current		Filtered current byte
+ * @v left		Unfiltered left byte
+ * @v above		Unfiltered above byte
+ * @v above_left	Unfiltered above-left byte
+ * @ret current		Unfiltered current byte
+ */
+static unsigned int png_unfilter_average ( unsigned int current,
+					   unsigned int left,
+					   unsigned int above,
+					   unsigned int above_left __unused ) {
+
+	return ( current + ( ( above + left ) >> 1 ) );
+}
+
+/**
+ * Paeth predictor function (defined in RFC 2083)
+ *
+ * @v a			Pixel A
+ * @v b			Pixel B
+ * @v c			Pixel C
+ * @ret predictor	Predictor pixel
+ */
+static unsigned int png_paeth_predictor ( unsigned int a, unsigned int b,
+					  unsigned int c ) {
+	unsigned int p;
+	unsigned int pa;
+	unsigned int pb;
+	unsigned int pc;
+
+	/* Algorithm as defined in RFC 2083 section 6.6 */
+	p = ( a + b - c );
+	pa = abs ( p - a );
+	pb = abs ( p - b );
+	pc = abs ( p - c );
+	if ( ( pa <= pb ) && ( pa <= pc ) ) {
+		return a;
+	} else if ( pb <= pc ) {
+		return b;
+	} else {
+		return c;
+	}
+}
+
+/**
+ * Unfilter byte using the "Paeth" filter
+ *
+ * @v current		Filtered current byte
+ * @v above_left	Unfiltered above-left byte
+ * @v above		Unfiltered above byte
+ * @v left		Unfiltered left byte
+ * @ret current		Unfiltered current byte
+ */
+static unsigned int png_unfilter_paeth ( unsigned int current,
+					 unsigned int left,
+					 unsigned int above,
+					 unsigned int above_left ) {
+
+	return ( current + png_paeth_predictor ( left, above, above_left ) );
+}
+
+/** A PNG filter */
+struct png_filter {
+	/**
+	 * Unfilter byte
+	 *
+	 * @v current		Filtered current byte
+	 * @v left		Unfiltered left byte
+	 * @v above		Unfiltered above byte
+	 * @v above_left	Unfiltered above-left byte
+	 * @ret current		Unfiltered current byte
+	 */
+	unsigned int ( * unfilter ) ( unsigned int current,
+				      unsigned int left,
+				      unsigned int above,
+				      unsigned int above_left );
+};
+
+/** PNG filter types */
+static struct png_filter png_filters[] = {
+	[PNG_FILTER_BASIC_NONE] = { png_unfilter_none },
+	[PNG_FILTER_BASIC_SUB] = { png_unfilter_sub },
+	[PNG_FILTER_BASIC_UP] = { png_unfilter_up },
+	[PNG_FILTER_BASIC_AVERAGE] = { png_unfilter_average },
+	[PNG_FILTER_BASIC_PAETH] = { png_unfilter_paeth },
+};
+
+/**
+ * Unfilter one interlace pass of PNG raw data
+ *
+ * @v image		PNG image
+ * @v png		PNG context
+ * @v interlace		Interlace pass
+ * @ret rc		Return status code
+ *
+ * This routine may assume that it is impossible to overrun the raw
+ * data buffer, since the size is determined by the image dimensions.
+ */
+static int png_unfilter_pass ( struct image *image, struct png_context *png,
+			       struct png_interlace *interlace ) {
+	size_t offset = png->raw.offset;
+	size_t pixel_len = png_pixel_len ( png );
+	size_t scanline_len = png_scanline_len ( png, interlace );
+	struct png_filter *filter;
+	unsigned int scanline;
+	unsigned int byte;
+	uint8_t filter_type;
+	uint8_t left;
+	uint8_t above;
+	uint8_t above_left;
+	uint8_t current;
+
+	/* On the first scanline of a pass, above bytes are assumed to
+	 * be zero.
+	 */
+	above = 0;
+
+	/* Iterate over each scanline in turn */
+	for ( scanline = 0 ; scanline < interlace->height ; scanline++ ) {
+
+		/* Extract filter byte and determine filter type */
+		copy_from_user ( &filter_type, png->raw.data, offset++,
+				 sizeof ( filter_type ) );
+		if ( filter_type >= ( sizeof ( png_filters ) /
+				      sizeof ( png_filters[0] ) ) ) {
+			DBGC ( image, "PNG %s unknown filter type %d\n",
+			       image->name, filter_type );
+			return -ENOTSUP;
+		}
+		filter = &png_filters[filter_type];
+		assert ( filter->unfilter != NULL );
+		DBGC2 ( image, "PNG %s pass %d scanline %d filter type %d\n",
+			image->name, interlace->pass, scanline, filter_type );
+
+		/* At the start of a line, both above-left and left
+		 * bytes are taken to be zero.
+		 */
+		left = 0;
+		above_left = 0;
+
+		/* Iterate over each byte (not pixel) in turn */
+		for ( byte = 0 ; byte < ( scanline_len - 1 ) ; byte++ ) {
+
+			/* Extract predictor bytes, if applicable */
+			if ( byte >= pixel_len ) {
+				copy_from_user ( &left, png->raw.data,
+						 ( offset - pixel_len ),
+						 sizeof ( left ) );
+			}
+			if ( scanline > 0 ) {
+				copy_from_user ( &above, png->raw.data,
+						 ( offset - scanline_len ),
+						 sizeof ( above ) );
+			}
+			if ( ( scanline > 0 ) && ( byte >= pixel_len ) ) {
+				copy_from_user ( &above_left, png->raw.data,
+						 ( offset - scanline_len -
+						   pixel_len ),
+						 sizeof ( above_left ) );
+			}
+
+			/* Unfilter current byte */
+			copy_from_user ( &current, png->raw.data,
+					 offset, sizeof ( current ) );
+			current = filter->unfilter ( current, left, above,
+						     above_left );
+			copy_to_user ( png->raw.data, offset++,
+				       &current, sizeof ( current ) );
+		}
+	}
+
+	/* Update offset */
+	png->raw.offset = offset;
+
+	return 0;
+}
+
+/**
+ * Unfilter PNG raw data
+ *
+ * @v image		PNG image
+ * @v png		PNG context
+ * @ret rc		Return status code
+ *
+ * This routine may assume that it is impossible to overrun the raw
+ * data buffer, since the size is determined by the image dimensions.
+ */
+static int png_unfilter ( struct image *image, struct png_context *png ) {
+	struct png_interlace interlace;
+	unsigned int pass;
+	int rc;
+
+	/* Process each interlace pass */
+	png->raw.offset = 0;
+	for ( pass = 0 ; pass < png->passes ; pass++ ) {
+
+		/* Calculate interlace pass parameters */
+		png_interlace ( png, pass, &interlace );
+
+		/* Skip zero-width rows (which have no filter bytes) */
+		if ( interlace.width == 0 )
+			continue;
+
+		/* Unfilter this pass */
+		if ( ( rc = png_unfilter_pass ( image, png,
+						&interlace ) ) != 0 )
+			return rc;
+	}
+	assert ( png->raw.offset == png->raw.len );
+
+	return 0;
+}
+
+/**
+ * Calculate PNG pixel component value
+ *
+ * @v raw		Raw component value
+ * @v alpha		Alpha value
+ * @v max		Maximum raw/alpha value
+ * @ret value		Component value in range 0-255
+ */
+static inline unsigned int png_pixel ( unsigned int raw, unsigned int alpha,
+				       unsigned int max ) {
+
+	/* The basic calculation is 255*(raw/max)*(value/max).  We use
+	 * fixed-point arithmetic (scaling up to the maximum range for
+	 * a 32-bit integer), in order to get the same results for
+	 * alpha blending as the test cases (produced using
+	 * ImageMagick).
+	 */
+	return ( ( ( ( ( 0xff00 * raw * alpha ) / max ) / max ) + 0x80 ) >> 8 );
+}
+
+/**
+ * Fill one interlace pass of PNG pixels
+ *
+ * @v image		PNG image
+ * @v png		PNG context
+ * @v interlace		Interlace pass
+ *
+ * This routine may assume that it is impossible to overrun either the
+ * raw data buffer or the pixel buffer, since the sizes of both are
+ * determined by the image dimensions.
+ */
+static void png_pixels_pass ( struct image *image,
+			      struct png_context *png,
+			      struct png_interlace *interlace ) {
+	size_t raw_offset = png->raw.offset;
+	uint8_t channel[png->channels];
+	int is_indexed = ( png->colour_type & PNG_COLOUR_TYPE_PALETTE );
+	int is_rgb = ( png->colour_type & PNG_COLOUR_TYPE_RGB );
+	int has_alpha = ( png->colour_type & PNG_COLOUR_TYPE_ALPHA );
+	size_t pixbuf_y_offset;
+	size_t pixbuf_offset;
+	size_t pixbuf_x_stride;
+	size_t pixbuf_y_stride;
+	size_t raw_stride;
+	unsigned int y;
+	unsigned int x;
+	unsigned int c;
+	unsigned int bits;
+	unsigned int depth;
+	unsigned int max;
+	unsigned int alpha;
+	unsigned int raw;
+	unsigned int value;
+	uint8_t current = 0;
+	uint32_t pixel;
+
+	/* We only ever use the top byte of 16-bit pixels.  Model this
+	 * as a bit depth of 8 with a stride of more than one.
+	 */
+	depth = png->depth;
+	raw_stride = ( ( depth + 7 ) / 8 );
+	if ( depth > 8 )
+		depth = 8;
+	max = ( ( 1 << depth ) - 1 );
+
+	/* Calculate pixel buffer offset and strides */
+	pixbuf_y_offset = ( ( ( interlace->y_indent * png->pixbuf->width ) +
+			      interlace->x_indent ) * sizeof ( pixel ) );
+	pixbuf_x_stride = ( interlace->x_stride * sizeof ( pixel ) );
+	pixbuf_y_stride = ( interlace->y_stride * png->pixbuf->width *
+			    sizeof ( pixel ) );
+	DBGC2 ( image, "PNG %s pass %d %dx%d at (%d,%d) stride (%d,%d)\n",
+		image->name, interlace->pass, interlace->width,
+		interlace->height, interlace->x_indent, interlace->y_indent,
+		interlace->x_stride, interlace->y_stride );
+
+	/* Iterate over each scanline in turn */
+	for ( y = 0 ; y < interlace->height ; y++ ) {
+
+		/* Skip filter byte */
+		raw_offset++;
+
+		/* Iterate over each pixel in turn */
+		bits = depth;
+		pixbuf_offset = pixbuf_y_offset;
+		for ( x = 0 ; x < interlace->width ; x++ ) {
+
+			/* Extract sample value */
+			for ( c = 0 ; c < png->channels ; c++ ) {
+
+				/* Get sample value into high bits of current */
+				current <<= depth;
+				bits -= depth;
+				if ( ! bits ) {
+					copy_from_user ( &current,
+							 png->raw.data,
+							 raw_offset,
+							 sizeof ( current ) );
+					raw_offset += raw_stride;
+					bits = 8;
+				}
+
+				/* Extract sample value */
+				channel[c] = ( current >> ( 8 - depth ) );
+			}
+
+			/* Convert to native pixel format */
+			if ( is_indexed ) {
+
+				/* Indexed */
+				pixel = png->palette[channel[0]];
+
+			} else {
+
+				/* Determine alpha value */
+				alpha = ( has_alpha ?
+					  channel[ png->channels - 1 ] : max );
+
+				/* Convert to RGB value */
+				pixel = 0;
+				for ( c = 0 ; c < 3 ; c++ ) {
+					raw = channel[ is_rgb ? c : 0 ];
+					value = png_pixel ( raw, alpha, max );
+					assert ( value <= 255 );
+					pixel = ( ( pixel << 8 ) | value );
+				}
+			}
+
+			/* Store pixel */
+			copy_to_user ( png->pixbuf->data, pixbuf_offset,
+				       &pixel, sizeof ( pixel ) );
+			pixbuf_offset += pixbuf_x_stride;
+		}
+
+		/* Move to next output row */
+		pixbuf_y_offset += pixbuf_y_stride;
+	}
+
+	/* Update offset */
+	png->raw.offset = raw_offset;
+}
+
+/**
+ * Fill PNG pixels
+ *
+ * @v image		PNG image
+ * @v png		PNG context
+ *
+ * This routine may assume that it is impossible to overrun either the
+ * raw data buffer or the pixel buffer, since the sizes of both are
+ * determined by the image dimensions.
+ */
+static void png_pixels ( struct image *image, struct png_context *png ) {
+	struct png_interlace interlace;
+	unsigned int pass;
+
+	/* Process each interlace pass */
+	png->raw.offset = 0;
+	for ( pass = 0 ; pass < png->passes ; pass++ ) {
+
+		/* Calculate interlace pass parameters */
+		png_interlace ( png, pass, &interlace );
+
+		/* Skip zero-width rows (which have no filter bytes) */
+		if ( interlace.width == 0 )
+			continue;
+
+		/* Unfilter this pass */
+		png_pixels_pass ( image, png, &interlace );
+	}
+	assert ( png->raw.offset == png->raw.len );
+}
+
+/**
+ * Handle PNG image end chunk
+ *
+ * @v image		PNG image
+ * @v png		PNG context
+ * @v len		Chunk length
+ * @ret rc		Return status code
+ */
+static int png_image_end ( struct image *image, struct png_context *png,
+			   size_t len ) {
+	int rc;
+
+	/* Sanity checks */
+	if ( len != 0 ) {
+		DBGC ( image, "PNG %s invalid IEND length %zd\n",
+		       image->name, len );
+		return -EINVAL;
+	}
+	if ( ! png->pixbuf ) {
+		DBGC ( image, "PNG %s missing pixel buffer (no IHDR?)\n",
+		       image->name );
+		return -EINVAL;
+	}
+	if ( ! deflate_finished ( &png->deflate ) ) {
+		DBGC ( image, "PNG %s decompression not complete\n",
+		       image->name );
+		return -EINVAL;
+	}
+	if ( png->raw.offset != png->raw.len ) {
+		DBGC ( image, "PNG %s incorrect decompressed length (expected "
+		       "%zd, got %zd)\n", image->name, png->raw.len,
+		       png->raw.offset );
+		return -EINVAL;
+	}
+
+	/* Unfilter raw data */
+	if ( ( rc = png_unfilter ( image, png ) ) != 0 )
+		return rc;
+
+	/* Fill pixel buffer */
+	png_pixels ( image, png );
+
+	return 0;
+}
+
+/** A PNG chunk handler */
+struct png_chunk_handler {
+	/** Chunk type */
+	uint32_t type;
+	/**
+	 * Handle chunk
+	 *
+	 * @v image		PNG image
+	 * @v png		PNG context
+	 * @v len		Chunk length
+	 * @ret rc		Return status code
+	 */
+	int ( * handle ) ( struct image *image, struct png_context *png,
+			   size_t len );
+};
+
+/** PNG chunk handlers */
+static struct png_chunk_handler png_chunk_handlers[] = {
+	{ htonl ( PNG_TYPE_IHDR ), png_image_header },
+	{ htonl ( PNG_TYPE_PLTE ), png_palette },
+	{ htonl ( PNG_TYPE_IDAT ), png_image_data },
+	{ htonl ( PNG_TYPE_IEND ), png_image_end },
+};
+
+/**
+ * Handle PNG chunk
+ *
+ * @v image		PNG image
+ * @v png		PNG context
+ * @v type		Chunk type
+ * @v len		Chunk length
+ * @ret rc		Return status code
+ */
+static int png_chunk ( struct image *image, struct png_context *png,
+		       uint32_t type, size_t len ) {
+	struct png_chunk_handler *handler;
+	unsigned int i;
+
+	DBGC ( image, "PNG %s chunk type %s offset %zd length %zd\n",
+	       image->name, png_type_name ( type ), png->offset, len );
+
+	/* Handle according to chunk type */
+	for ( i = 0 ; i < ( sizeof ( png_chunk_handlers ) /
+			    sizeof ( png_chunk_handlers[0] ) ) ; i++ ) {
+		handler = &png_chunk_handlers[i];
+		if ( handler->type == type )
+			return handler->handle ( image, png, len );
+	}
+
+	/* Fail if unknown chunk type is critical */
+	if ( ! ( type & htonl ( PNG_CHUNK_ANCILLARY ) ) ) {
+		DBGC ( image, "PNG %s unknown critical chunk type %s\n",
+		       image->name, png_type_name ( type ) );
+		return -ENOTSUP;
+	}
+
+	/* Ignore non-critical unknown chunk types */
+	return 0;
+}
+
+/**
+ * Convert PNG image to pixel buffer
+ *
+ * @v image		PNG image
+ * @v pixbuf		Pixel buffer to fill in
+ * @ret rc		Return status code
+ */
+static int png_pixbuf ( struct image *image, struct pixel_buffer **pixbuf ) {
+	struct png_context *png;
+	struct png_chunk_header header;
+	struct png_chunk_footer footer;
+	size_t remaining;
+	size_t chunk_len;
+	int rc;
+
+	/* Allocate and initialise context */
+	png = zalloc ( sizeof ( *png ) );
+	if ( ! png ) {
+		rc = -ENOMEM;
+		goto err_alloc;
+	}
+	png->offset = sizeof ( struct png_signature );
+	deflate_init ( &png->deflate, DEFLATE_ZLIB );
+
+	/* Process chunks */
+	do {
+
+		/* Extract chunk header */
+		remaining = ( image->len - png->offset );
+		if ( remaining < sizeof ( header ) ) {
+			DBGC ( image, "PNG %s truncated chunk header at offset "
+			       "%zd\n", image->name, png->offset );
+			rc = -EINVAL;
+			goto err_truncated;
+		}
+		copy_from_user ( &header, image->data, png->offset,
+				 sizeof ( header ) );
+		png->offset += sizeof ( header );
+
+		/* Validate chunk length */
+		chunk_len = ntohl ( header.len );
+		if ( remaining < ( sizeof ( header ) + chunk_len +
+				   sizeof ( footer ) ) ) {
+			DBGC ( image, "PNG %s truncated chunk data/footer at "
+			       "offset %zd\n", image->name, png->offset );
+			rc = -EINVAL;
+			goto err_truncated;
+		}
+
+		/* Handle chunk */
+		if ( ( rc = png_chunk ( image, png, header.type,
+					chunk_len ) ) != 0 )
+			goto err_chunk;
+
+		/* Move to next chunk */
+		png->offset += ( chunk_len + sizeof ( footer ) );
+
+	} while ( png->offset < image->len );
+
+	/* Check that we finished with an IEND chunk */
+	if ( header.type != htonl ( PNG_TYPE_IEND ) ) {
+		DBGC ( image, "PNG %s did not finish with IEND\n",
+		       image->name );
+		rc = -EINVAL;
+		goto err_iend;
+	}
+
+	/* Return pixel buffer */
+	*pixbuf = pixbuf_get ( png->pixbuf );
+
+	/* Success */
+	rc = 0;
+
+ err_iend:
+ err_chunk:
+ err_truncated:
+	pixbuf_put ( png->pixbuf );
+	ufree ( png->raw.data );
+	free ( png );
+ err_alloc:
+	return rc;
+}
+
+/**
+ * Probe PNG image
+ *
+ * @v image		PNG image
+ * @ret rc		Return status code
+ */
+static int png_probe ( struct image *image ) {
+	struct png_signature signature;
+
+	/* Sanity check */
+	if ( image->len < sizeof ( signature ) ) {
+		DBGC ( image, "PNG %s is too short\n", image->name );
+		return -ENOEXEC;
+	}
+
+	/* Check signature */
+	copy_from_user ( &signature, image->data, 0, sizeof ( signature ) );
+	if ( memcmp ( &signature, &png_signature, sizeof ( signature ) ) != 0 ){
+		DBGC ( image, "PNG %s has invalid signature\n", image->name );
+		return -ENOEXEC;
+	}
+
+	return 0;
+}
+
+/** PNG image type */
+struct image_type png_image_type __image_type ( PROBE_NORMAL ) = {
+	.name = "PNG",
+	.probe = png_probe,
+	.pixbuf = png_pixbuf,
+};