static int encode_apng(AVCodecContext *avctx, AVPacket *pkt,
                       const AVFrame *pict, int *got_packet)
{
    PNGEncContext *s = avctx->priv_data;
    int ret;
    int enc_row_size;
    size_t max_packet_size;
    APNGFctlChunk fctl_chunk = {0};
    if (pict && avctx->codec_id == AV_CODEC_ID_APNG && s->color_type == PNG_COLOR_TYPE_PALETTE) {
        uint32_t checksum = ~av_crc(av_crc_get_table(AV_CRC_32_IEEE_LE), ~0U, pict->data[1], 256 * sizeof(uint32_t));
        if (avctx->frame_number == 0) {
            s->palette_checksum = checksum;
        } else if (checksum != s->palette_checksum) {
            av_log(avctx, AV_LOG_ERROR,
                   "Input contains more than one unique palette. APNG does not support multiple palettes.\n");
            return -1;
        }
    }
    enc_row_size    = deflateBound(&s->zstream, (avctx->width * s->bits_per_pixel + 7) >> 3);
    max_packet_size =
        AV_INPUT_BUFFER_MIN_SIZE + // headers
        avctx->height * (
            enc_row_size +
            (4 + 12) * (((int64_t)enc_row_size + IOBUF_SIZE - 1) / IOBUF_SIZE) // fdAT * ceil(enc_row_size / IOBUF_SIZE)
        );
    if (max_packet_size > INT_MAX)
        return AVERROR(ENOMEM);
    if (avctx->frame_number == 0) {
        s->bytestream = avctx->extradata = av_malloc(FF_MIN_BUFFER_SIZE);
        if (!avctx->extradata)
            return AVERROR(ENOMEM);
        ret = encode_headers(avctx, pict);
        if (ret < 0)
            return ret;
        avctx->extradata_size = s->bytestream - avctx->extradata;
        s->last_frame_packet = av_malloc(max_packet_size);
        if (!s->last_frame_packet)
            return AVERROR(ENOMEM);
    } else if (s->last_frame) {
        ret = ff_alloc_packet2(avctx, pkt, max_packet_size, 0);
        if (ret < 0)
            return ret;
        memcpy(pkt->data, s->last_frame_packet, s->last_frame_packet_size);
        pkt->size = s->last_frame_packet_size;
        pkt->pts = pkt->dts = s->last_frame->pts;
    }
    if (pict) {
        s->bytestream_start =
        s->bytestream       = s->last_frame_packet;
        s->bytestream_end   = s->bytestream + max_packet_size;
        // We're encoding the frame first, so we have to do a bit of shuffling around
        // to have the image data write to the correct place in the buffer
        fctl_chunk.sequence_number = s->sequence_number;
        ++s->sequence_number;
        s->bytestream += 26 + 12;
        ret = apng_encode_frame(avctx, pict, &fctl_chunk, &s->last_frame_fctl);
        if (ret < 0)
            return ret;
        fctl_chunk.delay_num = 0; // delay filled in during muxing
        fctl_chunk.delay_den = 0;
    } else {
        s->last_frame_fctl.dispose_op = APNG_DISPOSE_OP_NONE;
    }
    if (s->last_frame) {
        uint8_t* last_fctl_chunk_start = pkt->data;
        uint8_t buf[26];
        AV_WB32(buf + 0, s->last_frame_fctl.sequence_number);
        AV_WB32(buf + 4, s->last_frame_fctl.width);
        AV_WB32(buf + 8, s->last_frame_fctl.height);
        AV_WB32(buf + 12, s->last_frame_fctl.x_offset);
        AV_WB32(buf + 16, s->last_frame_fctl.y_offset);
        AV_WB16(buf + 20, s->last_frame_fctl.delay_num);
        AV_WB16(buf + 22, s->last_frame_fctl.delay_den);
        buf[24] = s->last_frame_fctl.dispose_op;
        buf[25] = s->last_frame_fctl.blend_op;
        png_write_chunk(&last_fctl_chunk_start, MKTAG('f', 'c', 'T', 'L'), buf, 26);
        *got_packet = 1;
    }
    if (pict) {
        if (!s->last_frame) {
            s->last_frame = av_frame_alloc();
            if (!s->last_frame)
                return AVERROR(ENOMEM);
        } else if (s->last_frame_fctl.dispose_op != APNG_DISPOSE_OP_PREVIOUS) {
            if (!s->prev_frame) {
                s->prev_frame = av_frame_alloc();
                if (!s->prev_frame)
                    return AVERROR(ENOMEM);
                s->prev_frame->format = pict->format;
                s->prev_frame->width = pict->width;
                s->prev_frame->height = pict->height;
                if ((ret = av_frame_get_buffer(s->prev_frame, 32)) < 0)
                    return ret;
            }
            // Do disposal, but not blending
            memcpy(s->prev_frame->data[0], s->last_frame->data[0],
                   s->last_frame->linesize[0] * s->last_frame->height);
            if (s->last_frame_fctl.dispose_op == APNG_DISPOSE_OP_BACKGROUND) {
                uint32_t y;
                uint8_t bpp = (s->bits_per_pixel + 7) >> 3;
                for (y = s->last_frame_fctl.y_offset; y < s->last_frame_fctl.y_offset + s->last_frame_fctl.height; ++y) {
                    size_t row_start = s->last_frame->linesize[0] * y + bpp * s->last_frame_fctl.x_offset;
                    memset(s->prev_frame->data[0] + row_start, 0, bpp * s->last_frame_fctl.width);
                }
            }
        }
        av_frame_unref(s->last_frame);
        ret = av_frame_ref(s->last_frame, (AVFrame*)pict);
        if (ret < 0)
            return ret;
        s->last_frame_fctl = fctl_chunk;
        s->last_frame_packet_size = s->bytestream - s->bytestream_start;
    } else {
        av_frame_free(&s->last_frame);
    }
    return 0;
}