png.c

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/*
 * 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.
 *
 * You can also choose to distribute this program under the terms of
 * the Unmodified Binary Distribution Licence (as given in the file
 * COPYING.UBDL), provided that you have satisfied its requirements.
 */

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
FILE_SECBOOT ( PERMITTED );

#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 ) {
        const 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 */
        ihdr = ( image->data + png->offset );
        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 ) {
        const struct png_palette_entry *palette;
        unsigned int i;

        /* Populate palette */
        palette = ( image->data + png->offset );
        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 */
                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 */
                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 ) {
        int rc;

        /* Deflate this chunk */
        if ( ( rc = deflate_inflate ( &png->deflate,
                                      ( image->data + png->offset ),
                                      len, &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 pixel_len = png_pixel_len ( png );
        size_t scanline_len = png_scanline_len ( png, interlace );
        uint8_t *data = ( png->raw.data + png->raw.offset );
        struct png_filter *filter;
        unsigned int scanline;
        unsigned int byte;
        unsigned int filter_type;
        unsigned int left;
        unsigned int above;
        unsigned int above_left;

        /* 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 */
                filter_type = *(data++);
                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 )
                                left = *( data - pixel_len );
                        if ( scanline > 0 )
                                above = *( data - scanline_len );
                        if ( ( scanline > 0 ) && ( byte >= pixel_len ) ) {
                                above_left = *( data - scanline_len -
                                                pixel_len );
                        }

                        /* Unfilter current byte */
                        *data = filter->unfilter ( *data, left, above,
                                                   above_left );
                        data++;
                }
        }

        /* Update offset */
        png->raw.offset = ( ( ( void * ) data ) - png->raw.data );

        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 ) {
        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 );
        const uint8_t *data = ( png->raw.data + png->raw.offset );
        size_t data_stride;
        unsigned int pixbuf_y_index;
        unsigned int pixbuf_index;
        unsigned int pixbuf_x_stride;
        unsigned int pixbuf_y_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;
        data_stride = ( ( depth + 7 ) / 8 );
        if ( depth > 8 )
                depth = 8;
        max = ( ( 1 << depth ) - 1 );

        /* Calculate pixel buffer offset and strides */
        pixbuf_y_index = ( ( ( interlace->y_indent * png->pixbuf->width ) +
                              interlace->x_indent ) );
        pixbuf_x_stride = interlace->x_stride;
        pixbuf_y_stride = ( interlace->y_stride * png->pixbuf->width );
        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 */
                data++;

                /* Iterate over each pixel in turn */
                bits = depth;
                pixbuf_index = pixbuf_y_index;
                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 ) {
                                        current = *data;
                                        data += data_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 */
                        png->pixbuf->data[pixbuf_index] = pixel;
                        pixbuf_index += pixbuf_x_stride;
                }

                /* Move to next output row */
                pixbuf_y_index += pixbuf_y_stride;
        }

        /* Update offset */
        png->raw.offset = ( ( ( const void * ) data ) - png->raw.data );
}

/**
 * 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;
        const struct png_chunk_header *header;
        const 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 ) + sizeof ( *footer ) ) ){
                        DBGC ( image, "PNG %s truncated chunk header/footer "
                               "at offset %zd\n", image->name, png->offset );
                        rc = -EINVAL;
                        goto err_truncated;
                }
                header = ( image->data + png->offset );
                png->offset += sizeof ( *header );

                /* Validate chunk length */
                chunk_len = ntohl ( header->len );
                if ( chunk_len > ( remaining - sizeof ( *header ) -
                                   sizeof ( *footer ) ) ) {
                        DBGC ( image, "PNG %s truncated chunk data 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 ) {
        const 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 */
        signature = image->data;
        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,
};