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Basic usage

libspng can decode images to an explicit output format (e.g. 8/16-bit RGBA) from any PNG format. With SPNG_FMT_PNG the pixel format will identical to the PNG’s format in host-endian, or big-endian with SPNG_FMT_RAW, the latter does not support any transforms e.g. gamma correction.

#include <spng.h>

/* Create a context */
spng_ctx *ctx = spng_ctx_new(0);

/* Set an input buffer */
spng_set_png_buffer(ctx, buf, buf_size);

/* Calculate output image size */
spng_decoded_image_size(ctx, SPNG_FMT_RGBA8, &out_size);

/* Get an 8-bit RGBA image regardless of PNG format */
spng_decode_image(ctx, SPNG_FMT_RGBA8, out, out_size, 0);

/* Free context memory */
spng_ctx_free(ctx);

For a complete example see example.c.

Decoding untrusted files

To decode untrusted files safely it is required to at least:

  • Set an upper image width and height limit with spng_set_image_limits().

  • Use spng_decoded_image_size() to calculate the output image size and check it against a constant limit.

  • Set a chunk size and chunk cache limit with spng_set_chunks_limits() to avoid running out of memory. Note that exceeding either limit is handled as an out-of-memory error since v0.6.0.