Default to 800x600 transparent image output

src/fart/color.d

@@ -40,4 +40,6 @@ struct Color
     {
         return cast(ubyte)(0xFF * v);
     }
+
+    public static immutable Color TRANSPARENT = Color(0, 0, 0, 0);
 }

src/fart/main.d

@@ -1,7 +1,27 @@
-import fart.png;
+module fart.main;
 
+import fart.png;
+import fart.color;
+
+/**
+ * Main program entry point.
+ *
+ * @param args
+ *   Program arguments.
+ */
 int main(string[] args)
 {
-    png_test();
+    size_t width = 800;
+    size_t height = 600;
+    string output_filename = "out.png";
+    Color[] pixels = [];
+    for (size_t y = 0; y < height; y++)
+    {
+        for (size_t x = 0; x < width; x++)
+        {
+            pixels ~= Color.TRANSPARENT;
+        }
+    }
+    write_png(output_filename, width, height, pixels);
     return 0;
 }

src/fart/png.d

@@ -1,7 +1,6 @@
 module fart.png;
 
 import std.zlib;
-import std.math;
 import std.digest;
 
 import fart.bfile;
@@ -9,8 +8,14 @@ import fart.crc32;
 import fart.hton;
 import fart.color;
 
+/**
+ * PNG file header.
+ */
 private immutable ubyte[] HEADER = [0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A];
 
+/**
+ * PNG IHDR chunk.
+ */
 struct IHDR
 {
     uint width;
@@ -22,10 +27,10 @@ struct IHDR
     ubyte interlace;
     ubyte[0] end;
 
-    this(uint width, uint height)
+    this(size_t width, size_t height)
     {
-        this.width = htonl(width);
-        this.height = htonl(height);
+        this.width = htonl(cast(uint)width);
+        this.height = htonl(cast(uint)height);
         this.bit_depth = 8;
         this.color_type = 6;
         this.compression = 0;
@@ -44,6 +49,9 @@ struct IHDR
     }
 }
 
+/**
+ * PNG IDAT chunk.
+ */
 struct IDAT
 {
     private const(ubyte)[] m_data;
@@ -64,6 +72,9 @@ struct IDAT
     }
 }
 
+/**
+ * PNG IEND chunk.
+ */
 struct IEND
 {
     public const(void) * data()
@@ -77,6 +88,16 @@ struct IEND
     }
 }
 
+/**
+ * Write a PNG chunk to the output file.
+ *
+ * @param file
+ *   Output file
+ * @param chunk_type
+ *   Chunk type.
+ * @param chunk
+ *   Chunk to write.
+ */
 private void write_chunk(Chunk)(BFile file, string chunk_type, Chunk chunk)
 {
     size_t chunk_data_length = chunk.data_length();
@@ -89,11 +110,24 @@ private void write_chunk(Chunk)(BFile file, string chunk_type, Chunk chunk)
     file.writeObject(crc_be32);
 }
 
-public void write_png(string filename, uint width, uint height, const(Color)[] data)
+/**
+ * Write a PNG image from the given metadata and pixel data.
+ *
+ * @param filename
+ *   Name of file to write.
+ * @param width
+ *   Width of image.
+ * @param height
+ *   Height of image.
+ * @param data
+ *   Pixel color data.
+ */
+public void write_png(string filename, size_t width, size_t height, const(Color)[] data)
 {
     if (data.length != width * height)
         return;
 
+    /* Convert Color values to 32-bit RGBA values. */
     ubyte[] pixel_data;
     size_t data_index;
     for (size_t y = 0; y < height; y++)
@@ -105,30 +139,20 @@ public void write_png(string filename, uint width, uint height, const(Color)[] d
             pixel_data ~= data[data_index++].rgba32();
         }
     }
+
+    /* Open output file. */
     BFile file = BFile(filename);
+    /* Write PNG header. */
     file.write(HEADER);
+    /* Write IHDR chunk. */
     IHDR ihdr = IHDR(width, height);
     write_chunk(file, "IHDR", ihdr);
+    /* Write IDAT chunk. */
     IDAT idat = IDAT(pixel_data);
     write_chunk(file, "IDAT", idat);
+    /* Write IEND chunk. */
     IEND iend;
     write_chunk(file, "IEND", iend);
+    /* Close output file. */
     file.close();
 }
-
-void png_test()
-{
-    Color[] pixel_data = [];
-    for (size_t y = 0; y < 500; y++)
-    {
-        for (size_t x = 0; x < 500; x++)
-        {
-            ubyte r = cast(ubyte)(x / 2);
-            ubyte g = 0x80;
-            ubyte b = cast(ubyte)((500 - y) / 2);
-            ubyte a = cast(ubyte)((abs(cast(int)x - 250) + abs(cast(int)y - 250)) / 2);
-            pixel_data ~= Color(r / 255.0, g / 255.0, b / 255.0, a / 255.0);
-        }
-    }
-    write_png("out.png", 500, 500, pixel_data);
-}