+ * working PNG reader/writer, see pngtest.c, included in this distribution;
+ * see also the programs in the contrib directory.
+ */
+
+/* The simple, but restricted, approach to reading a PNG file or data stream
+ * just requires two function calls, as in the following complete program.
+ * Writing a file just needs one function call, so long as the data has an
+ * appropriate layout.
+ *
+ * The following code reads PNG image data from a file and writes it, in a
+ * potentially new format, to a new file. While this code will compile there is
+ * minimal (insufficient) error checking; for a more realistic version look at
+ * contrib/examples/pngtopng.c
+ */
+#include <stddef.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+#include <png.h>
+#include <zlib.h>
+
+int main(int argc, const char **argv)
+{
+ if (argc == 3)
+ {
+ png_image image; /* The control structure used by libpng */
+
+ /* Initialize the 'png_image' structure. */
+ memset(&image, 0, (sizeof image));
+ image.version = PNG_IMAGE_VERSION;
+
+ /* The first argument is the file to read: */
+ if (png_image_begin_read_from_file(&image, argv[1]))
+ {
+ png_bytep buffer;
+
+ /* Set the format in which to read the PNG file; this code chooses a
+ * simple sRGB format with a non-associated alpha channel, adequate to
+ * store most images.
+ */
+ image.format = PNG_FORMAT_RGBA;
+
+ /* Now allocate enough memory to hold the image in this format; the
+ * PNG_IMAGE_SIZE macro uses the information about the image (width,
+ * height and format) stored in 'image'.
+ */
+ buffer = malloc(PNG_IMAGE_SIZE(image));
+
+ /* If enough memory was available read the image in the desired format
+ * then write the result out to the new file. 'background' is not
+ * necessary when reading the image because the alpha channel is
+ * preserved; if it were to be removed, for example if we requested
+ * PNG_FORMAT_RGB, then either a solid background color would have to
+ * be supplied or the output buffer would have to be initialized to the
+ * actual background of the image.
+ *
+ * The fourth argument to png_image_finish_read is the 'row_stride' -
+ * this is the number of components allocated for the image in each
+ * row. It has to be at least as big as the value returned by
+ * PNG_IMAGE_ROW_STRIDE, but if you just allocate space for the
+ * default, minimum, size using PNG_IMAGE_SIZE as above you can pass
+ * zero.
+ *
+ * The final argument is a pointer to a buffer for the colormap;
+ * colormaps have exactly the same format as a row of image pixels (so
+ * you choose what format to make the colormap by setting
+ * image.format). A colormap is only returned if
+ * PNG_FORMAT_FLAG_COLORMAP is also set in image.format, so in this
+ * case NULL is passed as the final argument. If you do want to force
+ * all images into an index/color-mapped format then you can use:
+ *
+ * PNG_IMAGE_COLORMAP_SIZE(image)
+ *
+ * to find the maximum size of the colormap in bytes.
+ */
+ if (buffer != NULL &&
+ png_image_finish_read(&image, NULL/*background*/, buffer,
+ 0/*row_stride*/, NULL/*colormap*/))
+ {
+ /* Now write the image out to the second argument. In the write
+ * call 'convert_to_8bit' allows 16-bit data to be squashed down to
+ * 8 bits; this isn't necessary here because the original read was
+ * to the 8-bit format.
+ */
+ if (png_image_write_to_file(&image, argv[2], 0/*convert_to_8bit*/,
+ buffer, 0/*row_stride*/, NULL/*colormap*/))
+ {
+ /* The image has been written successfully. */
+ exit(0);
+ }
+ }
+
+ else
+ {
+ /* Calling png_free_image is optional unless the simplified API was
+ * not run to completion. In this case if there wasn't enough
+ * memory for 'buffer' we didn't complete the read, so we must free
+ * the image:
+ */
+ if (buffer == NULL)
+ png_free_image(&image);
+
+ else
+ free(buffer);
+ }
+
+ /* Something went wrong reading or writing the image. libpng stores a
+ * textual message in the 'png_image' structure:
+ */
+ fprintf(stderr, "pngtopng: error: %s\n", image.message);
+ exit (1);
+ }
+
+ fprintf(stderr, "pngtopng: usage: pngtopng input-file output-file\n");
+ exit(1);
+}
+
+/* That's it ;-) Of course you probably want to do more with PNG files than
+ * just converting them all to 32-bit RGBA PNG files; you can do that between
+ * the call to png_image_finish_read and png_image_write_to_file. You can also
+ * ask for the image data to be presented in a number of different formats. You
+ * do this by simply changing the 'format' parameter set before allocating the
+ * buffer.
+ *
+ * The format parameter consists of five flags that define various aspects of
+ * the image, you can simply add these together to get the format or you can use
+ * one of the predefined macros from png.h (as above):
+ *
+ * PNG_FORMAT_FLAG_COLOR: if set the image will have three color components per
+ * pixel (red, green and blue), if not set the image will just have one
+ * luminance (grayscale) component.
+ *
+ * PNG_FORMAT_FLAG_ALPHA: if set each pixel in the image will have an additional
+ * alpha value; a linear value that describes the degree the image pixel
+ * covers (overwrites) the contents of the existing pixel on the display.
+ *
+ * PNG_FORMAT_FLAG_LINEAR: if set the components of each pixel will be returned
+ * as a series of 16-bit linear values, if not set the components will be
+ * returned as a series of 8-bit values encoded according to the 'sRGB'
+ * standard. The 8-bit format is the normal format for images intended for
+ * direct display, because almost all display devices do the inverse of the
+ * sRGB transformation to the data they receive. The 16-bit format is more
+ * common for scientific data and image data that must be further processed;
+ * because it is linear simple math can be done on the component values.
+ * Regardless of the setting of this flag the alpha channel is always linear,
+ * although it will be 8 bits or 16 bits wide as specified by the flag.
+ *
+ * PNG_FORMAT_FLAG_BGR: if set the components of a color pixel will be returned
+ * in the order blue, then green, then red. If not set the pixel components
+ * are in the order red, then green, then blue.
+ *
+ * PNG_FORMAT_FLAG_AFIRST: if set the alpha channel (if present) precedes the
+ * color or grayscale components. If not set the alpha channel follows the
+ * components.
+ *
+ * You do not have to read directly from a file. You can read from memory or,
+ * on systems that support it, from a <stdio.h> FILE*. This is controlled by
+ * the particular png_image_read_from_ function you call at the start. Likewise
+ * on write you can write to a FILE* if your system supports it. Check the
+ * macro PNG_STDIO_SUPPORTED to see if stdio support has been included in your
+ * libpng build.
+ *
+ * If you read 16-bit (PNG_FORMAT_FLAG_LINEAR) data you may need to write it in
+ * the 8-bit format for display. You do this by setting the convert_to_8bit
+ * flag to 'true'.
+ *
+ * Don't repeatedly convert between the 8-bit and 16-bit forms. There is
+ * significant data loss when 16-bit data is converted to the 8-bit encoding and
+ * the current libpng implementation of convertion to 16-bit is also
+ * significantly lossy. The latter will be fixed in the future, but the former
+ * is unavoidable - the 8-bit format just doesn't have enough resolution.
+ */
+
+/* If your program needs more information from the PNG data it reads, or if you
+ * need to do more complex transformations, or minimise transformations, on the
+ * data you read, then you must use one of the several lower level libpng
+ * interfaces.
+ *
+ * All these interfaces require that you do your own error handling - your
+ * program must be able to arrange for control to return to your own code any
+ * time libpng encounters a problem. There are several ways to do this, but the
+ * standard way is to use the ANSI-C (C90) <setjmp.h> interface to establish a
+ * return point within your own code. You must do this if you do not use the
+ * simplified interface (above).
+ *
+ * The first step is to include the header files you need, including the libpng
+ * header file. Include any standard headers and feature test macros your
+ * program requires before including png.h:
projects / wxWidgets.git / blobdiff