static int gif_image_write_header(uint8_t **bytestream, int width, int height, int loop_count, uint32_t *palette) { int i; unsigned int v; bytestream_put_buffer(bytestream, "GIF", 3); bytestream_put_buffer(bytestream, "89a", 3); bytestream_put_le16(bytestream, width); bytestream_put_le16(bytestream, height); bytestream_put_byte(bytestream, 0xf7); /* flags: global clut, 256 entries */ bytestream_put_byte(bytestream, 0x1f); /* background color index */ bytestream_put_byte(bytestream, 0); /* aspect ratio */ /* the global palette */ if (!palette) { bytestream_put_buffer(bytestream, (const unsigned char *)gif_clut, 216*3); for(i=0;i<((256-216)*3);i++) bytestream_put_byte(bytestream, 0); } else { for(i=0;i<256;i++) { v = palette[i]; bytestream_put_be24(bytestream, v); } } /* update: this is the 'NETSCAPE EXTENSION' that allows for looped animated gif see http://members.aol.com/royalef/gifabout.htm#net-extension byte 1 : 33 (hex 0x21) GIF Extension code byte 2 : 255 (hex 0xFF) Application Extension Label byte 3 : 11 (hex (0x0B) Length of Application Block (eleven bytes of data to follow) bytes 4 to 11 : "NETSCAPE" bytes 12 to 14 : "2.0" byte 15 : 3 (hex 0x03) Length of Data Sub-Block (three bytes of data to follow) byte 16 : 1 (hex 0x01) bytes 17 to 18 : 0 to 65535, an unsigned integer in lo-hi byte format. This indicate the number of iterations the loop should be executed. bytes 19 : 0 (hex 0x00) a Data Sub-block Terminator */ /* application extension header */ #ifdef GIF_ADD_APP_HEADER if (loop_count >= 0 && loop_count <= 65535) { bytestream_put_byte(bytestream, 0x21); bytestream_put_byte(bytestream, 0xff); bytestream_put_byte(bytestream, 0x0b); bytestream_put_buffer(bytestream, "NETSCAPE2.0", 11); // bytes 4 to 14 bytestream_put_byte(bytestream, 0x03); // byte 15 bytestream_put_byte(bytestream, 0x01); // byte 16 bytestream_put_le16(bytestream, (uint16_t)loop_count); bytestream_put_byte(bytestream, 0x00); // byte 19 } #endif return 0; }