example.c

#include <spng.h>

#include <inttypes.h>
#include <stdio.h>

int encode_image(void *image, size_t length, uint32_t width, uint32_t height, enum spng_color_type color_type, int bit_depth)
{
    int fmt;
    int ret = 0;
    spng_ctx *ctx = NULL;
    struct spng_ihdr ihdr = {0}; /* zero-initialize to set valid defaults */

    /* Creating an encoder context requires a flag */
    ctx = spng_ctx_new(SPNG_CTX_ENCODER);

    /* Encode to internal buffer managed by the library */
    spng_set_option(ctx, SPNG_ENCODE_TO_BUFFER, 1);

    /* Alternatively you can set an output FILE* or stream with spng_set_png_file() or spng_set_png_stream() */

    /* Set image properties, this determines the destination image format */
    ihdr.width = width;
    ihdr.height = height;
    ihdr.color_type = color_type;
    ihdr.bit_depth = bit_depth;
    /* Valid color type, bit depth combinations: https://www.w3.org/TR/2003/REC-PNG-20031110/#table111 */

    spng_set_ihdr(ctx, &ihdr);

    /* When encoding fmt is the source format */
    /* SPNG_FMT_PNG is a special value that matches the format in ihdr */
    fmt = SPNG_FMT_PNG;

    /* SPNG_ENCODE_FINALIZE will finalize the PNG with the end-of-file marker */
    ret = spng_encode_image(ctx, image, length, fmt, SPNG_ENCODE_FINALIZE);

    if(ret)
    {
        printf("spng_encode_image() error: %s\n", spng_strerror(ret));
        goto encode_error;
    }

    size_t png_size;
    void *png_buf = NULL;

    /* Get the internal buffer of the finished PNG */
    png_buf = spng_get_png_buffer(ctx, &png_size, &ret);

    if(png_buf == NULL)
    {
        printf("spng_get_png_buffer() error: %s\n", spng_strerror(ret));
    }

    /* User owns the buffer after a successful call */
    free(png_buf);

encode_error:

    spng_ctx_free(ctx);

    return ret;
}

const char *color_type_str(enum spng_color_type color_type)
{
    switch(color_type)
    {
        case SPNG_COLOR_TYPE_GRAYSCALE: return "grayscale";
        case SPNG_COLOR_TYPE_TRUECOLOR: return "truecolor";
        case SPNG_COLOR_TYPE_INDEXED: return "indexed color";
        case SPNG_COLOR_TYPE_GRAYSCALE_ALPHA: return "grayscale with alpha";
        case SPNG_COLOR_TYPE_TRUECOLOR_ALPHA: return "truecolor with alpha";
        default: return "(invalid)";
    }
}

int main(int argc, char **argv)
{
    FILE *png;
    int ret = 0;
    spng_ctx *ctx = NULL;
    unsigned char *image = NULL;

    if(argc < 2)
    {
        printf("no input file\n");
        goto error;
    }

    png = fopen(argv[1], "rb");

    if(png == NULL)
    {
        printf("error opening input file %s\n", argv[1]);
        goto error;
    }

    ctx = spng_ctx_new(0);

    if(ctx == NULL)
    {
        printf("spng_ctx_new() failed\n");
        goto error;
    }

    /* Ignore and don't calculate chunk CRC's */
    spng_set_crc_action(ctx, SPNG_CRC_USE, SPNG_CRC_USE);

    /* Set memory usage limits for storing standard and unknown chunks,
       this is important when reading untrusted files! */
    size_t limit = 1024 * 1024 * 64;
    spng_set_chunk_limits(ctx, limit, limit);

    /* Set source PNG */
    spng_set_png_file(ctx, png); /* or _buffer(), _stream() */

    struct spng_ihdr ihdr;
    ret = spng_get_ihdr(ctx, &ihdr);

    if(ret)
    {
        printf("spng_get_ihdr() error: %s\n", spng_strerror(ret));
        goto error;
    }

    const char *color_name = color_type_str(ihdr.color_type);

    printf("width: %u\n"
           "height: %u\n"
           "bit depth: %u\n"
           "color type: %u - %s\n",
           ihdr.width, ihdr.height, ihdr.bit_depth, ihdr.color_type, color_name);

    printf("compression method: %u\n"
           "filter method: %u\n"
           "interlace method: %u\n",
           ihdr.compression_method, ihdr.filter_method, ihdr.interlace_method);

    struct spng_plte plte = {0};
    ret = spng_get_plte(ctx, &plte);

    if(ret && ret != SPNG_ECHUNKAVAIL)
    {
        printf("spng_get_plte() error: %s\n", spng_strerror(ret));
        goto error;
    }

    if(!ret) printf("palette entries: %u\n", plte.n_entries);


    size_t image_size, image_width;

    /* Output format, does not depend on source PNG format except for
       SPNG_FMT_PNG, which is the PNG's format in host-endian or
       big-endian for SPNG_FMT_RAW.
       Note that for these two formats <8-bit images are left byte-packed */
    int fmt = SPNG_FMT_PNG;

    /* With SPNG_FMT_PNG indexed color images are output as palette indices,
       pick another format to expand them. */
    if(ihdr.color_type == SPNG_COLOR_TYPE_INDEXED) fmt = SPNG_FMT_RGB8;

    ret = spng_decoded_image_size(ctx, fmt, &image_size);

    if(ret) goto error;

    image = malloc(image_size);

    if(image == NULL) goto error;

    /* Decode the image in one go */
    /* ret = spng_decode_image(ctx, image, image_size, SPNG_FMT_RGBA8, 0);

    if(ret)
    {
        printf("spng_decode_image() error: %s\n", spng_strerror(ret));
        goto error;
    }*/

    /* Alternatively you can decode the image progressively,
       this requires an initialization step. */
    ret = spng_decode_image(ctx, NULL, 0, fmt, SPNG_DECODE_PROGRESSIVE);

    if(ret)
    {
        printf("progressive spng_decode_image() error: %s\n", spng_strerror(ret));
        goto error;
    }

    /* ihdr.height will always be non-zero if spng_get_ihdr() succeeds */
    image_width = image_size / ihdr.height;

    struct spng_row_info row_info = {0};

    do
    {
        ret = spng_get_row_info(ctx, &row_info);
        if(ret) break;

        ret = spng_decode_row(ctx, image + row_info.row_num * image_width, image_width);
    }
    while(!ret);

    if(ret != SPNG_EOI)
    {
        printf("progressive decode error: %s\n", spng_strerror(ret));

        if(ihdr.interlace_method)
            printf("last pass: %d, scanline: %u\n", row_info.pass, row_info.scanline_idx);
        else
            printf("last row: %u\n", row_info.row_num);
    }

    uint32_t n_text = 0;
    struct spng_text *text = NULL;

    ret = spng_get_text(ctx, NULL, &n_text);

    if(ret == SPNG_ECHUNKAVAIL) /* No text chunks in file */
    {
        ret = 0;
        goto no_text;
    }

    if(ret)
    {
        printf("spng_get_text() error: %s\n", spng_strerror(ret));
        goto error;
    }

    text = malloc(n_text * sizeof(struct spng_text));

    if(text == NULL) goto error;

    ret = spng_get_text(ctx, text, &n_text);

    if(ret)
    {
        printf("spng_get_text() error: %s\n", spng_strerror(ret));
        goto no_text;
    }

    uint32_t i;
    for(i=0; i < n_text; i++)
    {
        char *type_str = "tEXt";
        if(text[i].type == SPNG_ITXT) type_str = "iTXt";
        else if(text[i].type == SPNG_ZTXT) type_str = "zTXt";

        printf("\ntext type: %s\n", type_str);
        printf("keyword: %s\n", text[i].keyword);

        if(text[i].type == SPNG_ITXT)
        {
            printf("language tag: %s\n", text[i].language_tag);
            printf("translated keyword: %s\n", text[i].translated_keyword);
        }

        printf("text is %scompressed\n", text[i].compression_flag ? "" : "not ");
        printf("text length: %lu\n", (unsigned long int)text[i].length);
        printf("text: %s\n", text[i].text);
    }

no_text:
    free(text);

    /* This example assumes a non-paletted image */
    if(fmt == SPNG_FMT_PNG)
    {
        ret = encode_image(image, image_size, ihdr.width, ihdr.height, ihdr.color_type, ihdr.bit_depth);
    }

error:

    spng_ctx_free(ctx);
    free(image);

    return ret;
}