No subimage when creating an image from scratch

[alevesely]

I'm unable to create an image from computed values.

For example (skipping error checks):

	spng_ctx *ctx = spng_ctx_new(SPNG_CTX_ENCODER);
	FILE *fp = fopen("my.png", "w");
	spng_set_png_file(ctx, fp);

	struct spng_ihdr ihdr = {0}; /* zero-initialize to set valid defaults */
	ihdr.width = 400;
	ihdr.height = 256;
	ihdr.color_type = SPNG_COLOR_TYPE_TRUECOLOR;
	ihdr.bit_depth = 8;
	spng_set_ihdr(ctx, &ihdr); // this does not calculate subimage

	size_t size = 4*400;
	unsigned char *linebuf = malloc(size);
	for (int i = 0; i < 256; ++i)
	{
                calculate_line_colors(linebuf);
		ret = spng_encode_scanline(ctx, linebuf, size);

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

This prints "spng_encode_scanline() error: invalid buffer size" at line 4689 in spng.c as it checks len < (ctx->subimage[ctx->row_info.pass].scanline_width -1); scanline_width is 0, and subtracting 1 overflows.

If instead I call spng_encode_row() it prints "internal error", from line 4574 in spng.c, after it finds scanline_width to be 0.

[randy408]

For progressive image encoding you have to initialize with spng_encode_image(ctx, NULL, 0, SPNG_FMT_PNG, SPNG_ENCODE_PROGRESSIVE); as described in the Progressive image encoding section.

I should probably do some extra error checking for this and return SPNG_EOPSTATE ("invalid operation for state") or something.