+ png_debug(1, "in png_write_IDAT");
+
+ /* Optimize the CMF field in the zlib stream. */
+ /* This hack of the zlib stream is compliant to the stream specification. */
+ if (!(png_ptr->mode & PNG_HAVE_IDAT) &&
+ png_ptr->compression_type == PNG_COMPRESSION_TYPE_BASE)
+ {
+ unsigned int z_cmf = data[0]; /* zlib compression method and flags */
+ if ((z_cmf & 0x0f) == 8 && (z_cmf & 0xf0) <= 0x70)
+ {
+ /* Avoid memory underflows and multiplication overflows. */
+ /* The conditions below are practically always satisfied;
+ however, they still must be checked. */
+ if (length >= 2 &&
+ png_ptr->height < 16384 && png_ptr->width < 16384)
+ {
+ png_uint_32 uncompressed_idat_size = png_ptr->height *
+ ((png_ptr->width *
+ png_ptr->channels * png_ptr->bit_depth + 15) >> 3);
+ unsigned int z_cinfo = z_cmf >> 4;
+ unsigned int half_z_window_size = 1 << (z_cinfo + 7);
+ while (uncompressed_idat_size <= half_z_window_size &&
+ half_z_window_size >= 256)
+ {
+ z_cinfo--;
+ half_z_window_size >>= 1;
+ }
+ z_cmf = (z_cmf & 0x0f) | (z_cinfo << 4);
+ if (data[0] != (png_byte)z_cmf)
+ {
+ data[0] = (png_byte)z_cmf;
+ data[1] &= 0xe0;
+ data[1] += (png_byte)(0x1f - ((z_cmf << 8) + data[1]) % 0x1f);
+ }
+ }
+ }
+ else
+ png_error(png_ptr,
+ "Invalid zlib compression method or flags in IDAT");
+ }
+