PHP extension development notes (10) Customize the IO function in the libpng library to write pictures into memory

When developing this QR code extension dcode, the generated QR code png image needs to be returned to the caller in the form of a string instead of directly generating a file. This is more convenient because there is no need to operate the file. The operation of files is completely left to the user.

The libpng library is used to generate images. For documentation about libpng, you can go to png documentation here. When I used this library to compile my extension on Ubuntu14.04, I still had a small problem: png_create_write_struct in Unknown on line 0 on ubuntu 14. After searching on the Internet, it is still very common.

The code is simply listed below:

<code><span>/** {{{ dcode_png_writer()
 * function is custom png_write callback function
 * Return void */</span><span>static</span><span>void</span> dcode_png_writer(png_structp png_ptr, png_bytep data, png_size_t length)
{
    png_mem_encode* p = (png_mem_encode*) png_get_io_ptr(png_ptr);
    size_t nsize = p->size + length;

    <span>if</span> (p->buffer)
        p->buffer = erealloc(p->buffer, nsize);
    <span>else</span>
        p->buffer = emalloc(nsize);

    <span>if</span> (!p->buffer)
    {
        png_error(png_ptr, <span>"PNG allocate memory error"</span>);
        <span>exit</span>(FAILURE);
    }

    <span>memcpy</span>(p->buffer + p->size, data, length);
    p->size += length;
}
<span>/* }}} */</span></code>

<code><span>/** {{{ dcode_write_to_png()
 * write qrcode struct to memory
 * Return char* */</span><span>static</span><span>char</span>* dcode_write_to_png(QRcode *qrcode, <span>int</span> size, <span>int</span> margin, <span>int</span> *pp_len)
{

    png_structp png_ptr;
    png_infop info_ptr;

    <span>unsigned</span><span>char</span> *row, *p, *q;
    <span>int</span> x, y, xx, yy, bit;
    <span>int</span> realwidth;

    realwidth = (qrcode->width + margin * <span>2</span>) * size;
    <span>int</span> row_fill_len = (realwidth + <span>7</span>) / <span>8</span>;

    png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    <span>if</span> (png_ptr == NULL)
    {
        php_error(E_ERROR, <span>"Failed to initialize PNG writer"</span>);
        <span>return</span> NULL;
    }

    info_ptr = png_create_info_struct(png_ptr);
    <span>if</span> (info_ptr == NULL)
    {
        php_error(E_ERROR, <span>"Failed to initialize PNG info"</span>);
        <span>return</span> NULL;
    }

    <span>if</span> (setjmp(png_jmpbuf(png_ptr)))
    {
        png_destroy_write_struct(&png_ptr, &info_ptr);
        php_error(E_ERROR, <span>"Failed to set PNG jmpbuf"</span>);
        <span>return</span> NULL;
    }

    row = (<span>unsigned</span><span>char</span> *) emalloc(row_fill_len);
    <span>if</span> (row == NULL)
    {
        png_destroy_write_struct(&png_ptr, &info_ptr);
        php_error(E_ERROR, <span>"Failed to allocate memory"</span>);
        <span>return</span> NULL;
    }

    png_mem_encode state = {NULL, <span>0</span>};
    png_set_write_fn(png_ptr, &state, &dcode_png_writer, NULL);

    png_set_IHDR(png_ptr,
                info_ptr,
                realwidth,
                realwidth,
                <span>1</span>,
                PNG_COLOR_TYPE_GRAY,
                PNG_INTERLACE_NONE,
                PNG_COMPRESSION_TYPE_DEFAULT,
                PNG_FILTER_TYPE_DEFAULT);

    png_write_info(png_ptr, info_ptr);
    <span>memset</span>(row, <span>0xff</span>, (realwidth + <span>7</span>) / <span>8</span>);
    <span>for</span>(y = <span>0</span>; y data;
    <span>for</span>(y = <span>0</span>; y width; y ++) {
        bit = <span>7</span>;
        <span>memset</span>(row, <span>0xff</span>, (realwidth + <span>7</span>) / <span>8</span>);
        q = row;
        q += margin * size / <span>8</span>;
        bit = <span>7</span> - (margin * size % <span>8</span>);
        <span>for</span>(x = <span>0</span>; x width; x ++) {
            <span>for</span>(xx = <span>0</span>; xx <size xx>1) if(bit 0) {
                    q++;
                    bit = <span>7</span>;
                }
            }
            p++;
        }
        <span>for</span>(yy = <span>0</span>; yy memset(row, <span>0xff</span>, (realwidth + <span>7</span>) / <span>8</span>);
    <span>for</span>(y = <span>0</span>; y char *bin_data = NULL;
    <span>if</span> (state.buffer) {
        bin_data = estrndup(state.buffer, state.size);
        *pp_len = state.size;
        efree(state.buffer);
    }

    <span>return</span> bin_data;
}
<span>/** }}} */</span></size></code>

1. The first function dcode_png_writer is a custom callback function for writing png data.
2. The second functiondcode_write_to_png is to write QRcode data to png

You can mainly look at this part

<code>png_set_write_fn(png_ptr, &state, &dcode_png_writer, NULL);</code>

This is where the custom write function dcode_png_writer is called to write the data to state In the structure, the state structure is as follows. The

<code><span>typedef</span><span>struct</span> _png_mem_encode {
    <span>char</span> *buffer;
    size_t size;
} png_mem_encode ;</code>

png_set_write_fn function sets a custom write function, and uses dcode_png_writer to write data like state and dynamically allocate memory.

For the definition of png_set_write_fn, you can refer to the PNG document mentioned above. The custom function can also customize functions such as error handling, so that it can take over the error handler according to the actual situation instead of letting it exit internally. For more related codes, please see DCode extension

The speed of generating QRCode is still very fast. If you use for ($i = 0; $i as a parameter, 10,000 can be generated in 3 seconds. <code>

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