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-rw-r--r--libdimension-png/png.c53
1 files changed, 46 insertions, 7 deletions
diff --git a/libdimension-png/png.c b/libdimension-png/png.c
index b639900..7e9036f 100644
--- a/libdimension-png/png.c
+++ b/libdimension-png/png.c
@@ -24,6 +24,8 @@
#include <arpa/inet.h>
#include <stdlib.h>
+/* Write a canvas to a png file, using libpng. Return 0 on success, nonzero on
+ failure. */
int
dmnsn_png_write_canvas(const dmnsn_canvas *canvas, FILE *file)
{
@@ -35,54 +37,65 @@ dmnsn_png_write_canvas(const dmnsn_canvas *canvas, FILE *file)
dmnsn_color color;
dmnsn_sRGB sRGB;
+ if (!file) return 1; /* file was NULL */
+
width = canvas->x;
height = canvas->y;
- if (!file) return 1;
-
png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
- if (!png_ptr) return 1;
+ if (!png_ptr) return 1; /* Couldn't create libpng write struct */
info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr) {
+ /* Couldn't create libpng info struct */
png_destroy_write_struct(&png_ptr, NULL);
return 1;
}
+ /* libpng will longjmp here if it encounters an error from here on */
if (setjmp(png_jmpbuf(png_ptr))) {
+ /* libpng error */
if (row) free(row);
png_destroy_write_struct(&png_ptr, &info_ptr);
return 1;
}
+ /* Associate file with the libpng write struct */
png_init_io(png_ptr, file);
+ /* Set header correctly for 16-bit sRGB image */
png_set_IHDR(png_ptr, info_ptr, width, height, 16,
PNG_COLOR_TYPE_RGB_ALPHA, PNG_INTERLACE_NONE,
PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
-
png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr, PNG_sRGB_INTENT_ABSOLUTE);
+ /* We think of transparency in the opposite way that PNG does */
png_set_invert_alpha(png_ptr);
+
+ /* Write the info struct */
png_write_info(png_ptr, info_ptr);
if (htonl(1) != 1) {
- /* We are little-endian */
+ /* We are little-endian; swap the byte order of the pixels */
png_set_swap(png_ptr);
}
+ /* Allocate the temporary row of RGBA values */
row = malloc(4*sizeof(uint16_t)*width);
if (!row) {
png_destroy_write_struct(&png_ptr, &info_ptr);
return 1;
}
+ /* Write the pixels */
for (y = 0; y < height; ++y) {
for (x = 0; x < width; ++x) {
/* Invert the rows. PNG coordinates are fourth quadrant. */
color = dmnsn_get_pixel(canvas, x, height - y - 1);
sRGB = dmnsn_sRGB_from_color(color);
+ /* Saturate R, G, and B to [0, UINT16_MAX] */
+
if (sRGB.R <= 0.0) {
row[4*x] = 0;
} else if (sRGB.R >= 1.0) {
@@ -107,11 +120,15 @@ dmnsn_png_write_canvas(const dmnsn_canvas *canvas, FILE *file)
row[4*x + 2] = sRGB.B*UINT16_MAX;
}
+ /* color.filter + color.trans is in [0.0, 1.0] by definition */
row[4*x + 3] = (color.filter + color.trans)*UINT16_MAX;
}
+
+ /* Write the row */
png_write_row(png_ptr, (png_bytep)row);
}
+ /* Finish the PNG file */
png_write_end(png_ptr, info_ptr);
free(row);
@@ -119,6 +136,7 @@ dmnsn_png_write_canvas(const dmnsn_canvas *canvas, FILE *file)
return 0;
}
+/* Read a canvas from the PNG file `file'. Return NULL on error. */
dmnsn_canvas *
dmnsn_png_read_canvas(FILE *file)
{
@@ -135,32 +153,40 @@ dmnsn_png_read_canvas(FILE *file)
dmnsn_sRGB sRGB;
png_bytep png_pixel;
- if (!file) return NULL;
+ if (!file) return NULL; /* file was NULL */
fread(header, 1, 8, file);
- if (png_sig_cmp(header, 0, 8)) return NULL;
+ if (png_sig_cmp(header, 0, 8)) return NULL; /* file is not a PNG file */
+ /* Create the libpng read struct */
png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
if (!png_ptr) return NULL;
+ /* Create the libpng info struct */
info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr) {
png_destroy_read_struct(&png_ptr, NULL, NULL);
return NULL;
}
+ /* libpng will longjmp here if it encounters an error from here on */
if (setjmp(png_jmpbuf(png_ptr))) {
+ /* libpng error */
if (row_pointers) free(row_pointers);
if (image) free(image);
png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
return NULL;
}
+ /* Associate the read struct with the file, and tell it we've already checked
+ 8 bytes of signature */
png_init_io(png_ptr, file);
png_set_sig_bytes(png_ptr, 8);
+ /* Read the PNG header into info struct */
png_read_info(png_ptr, info_ptr);
+ /* Get useful information from the info struct */
png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth, &color_type,
&interlace_type, &compression_type, &filter_method);
@@ -182,15 +208,21 @@ dmnsn_png_read_canvas(FILE *file)
}
png_set_invert_alpha(png_ptr);
+ /* Update the info struct */
png_read_update_info(png_ptr, info_ptr);
+
+ /* Get bytes/image row */
rowbytes = png_get_rowbytes(png_ptr, info_ptr);
+ /* Allocate the temporary image buffer */
image = malloc(rowbytes*height);
if (!image) {
png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
return NULL;
}
+ /* Allocate and set an array of pointers to rows in image */
+
row_pointers = malloc(sizeof(png_bytep)*height);
if (!row_pointers) {
free(image);
@@ -202,8 +234,11 @@ dmnsn_png_read_canvas(FILE *file)
row_pointers[y] = image + y*rowbytes;
}
+ /* Read the image to image all at once. At the expense of greater memory use,
+ this handles interlacing for us. */
png_read_image(png_ptr, row_pointers);
+ /* Allocate the canvas */
canvas = dmnsn_new_canvas(width, height);
if (!canvas) {
free(row_pointers);
@@ -213,6 +248,10 @@ dmnsn_png_read_canvas(FILE *file)
return NULL;
}
+ /* Now we convert the image to our canvas format. This depends on the image
+ bit depth (which has been scaled up to at least 8 or 16), and the presence
+ of an alpha channel. For performance reasons, the tests are outside the
+ loops, although that doesn't really matter for a decent compiler. */
if (bit_depth == 16) {
if (color_type & PNG_COLOR_MASK_ALPHA) {
for (x = 0; x < width; ++x) {