Added comments and improved naming in some image functions

win32/main.c

@@ -136,20 +136,32 @@ void drawalpha(int bm, int x, int y, int width, int height, uint32_t color)
         }
     };
 
-    uint8_t *p = bitmap[bm], *end = p + width * height;
+    // create pointer to begining and end of the alpha-channel-only bitmap
+    uint8_t *alpha_pixel = bitmap[bm], *end = alpha_pixel + width * height;
 
 
-    uint32_t *np;
-    HBITMAP temp = CreateDIBSection(hdcMem, &bmi, DIB_RGB_COLORS, (void**)&np, NULL, 0);
+    // create temporary bitmap we'll combine the alpha and colors on
+    uint32_t *out_pixel;
+    HBITMAP temp = CreateDIBSection(hdcMem, &bmi, DIB_RGB_COLORS, (void**)&out_pixel, NULL, 0);
     SelectObject(hdcMem, temp);
 
-    while(p != end) {
-        uint8_t v = *p++;
-        *np++ = (((color & 0xFF) * v / 255) << 16) | ((((color >> 8) & 0xFF) * v / 255) << 8) | ((((color >> 16) & 0xFF) * v / 255) << 0) | (v << 24);
+    // create pixels for the drawable bitmap based on the alpha value of
+    // each pixel in the alpha bitmap and the color given by 'color',
+    // the Win32 API requires we pre-apply our alpha channel as well by
+    // doing (color * alpha / 255) for each color channel
+    // NOTE: Input color is in the format 0BGR, output pixel is in the format ARGB
+    while(alpha_pixel != end) {
+        uint8_t alpha = *alpha_pixel++;
+        *out_pixel++ = (((color & 0xFF) * alpha / 255) << 16) // red
+                     | ((((color >> 8) & 0xFF) * alpha / 255) << 8)  // green
+                     | ((((color >> 16) & 0xFF) * alpha / 255) << 0) // blue
+                     | (alpha << 24); // alpha
     }
 
+    // draw temporary bitmap on screen
     AlphaBlend(hdc, x, y, width, height, hdcMem, 0, 0, width, height, blend_function);
 
+    // clean up
     DeleteObject(temp);
 }
 
@@ -845,14 +857,14 @@ UTOX_NATIVE_IMAGE *png_to_image(const UTOX_PNG_IMAGE data, size_t size, uint16_t
     uint8_t *out;
     HBITMAP bitmap = CreateDIBSection(hdcMem, &bmi, DIB_RGB_COLORS, (void**)&out, NULL, 0);
 
-    // convert RGBA data to internal format, switching the first and
-    // third bytes, and pre-applying the alpha if we're keeping the
-    // alpha channel
+    // convert RGBA data to internal format
+    // pre-applying the alpha if we're keeping the alpha channel,
     // put the result in out
+    // NOTE: input pixels are in format RGBA, output is BGRA
     uint8_t *p, *end = rgba_data + width * height * 4;
-    uint8_t alpha;
     p = rgba_data;
     if (keep_alpha) {
+        uint8_t alpha;
         do {
             alpha = p[3];
             out[0] = p[2] * (alpha / 255.0); // pre-apply alpha

xlib/main.c

@@ -599,6 +599,8 @@ static void pastedata(void *data, Atom type, int len, _Bool select)
     }
 }
 
+// converts an XImage to a Picture usable by XRender, uses XRenderPictFormat given by
+// 'format', uses the default format if it is NULL
 static Picture ximage_to_picture(XImage *img, const XRenderPictFormat *format)
 {
     Pixmap pixmap = XCreatePixmap(display, window, img->width, img->height, img->depth);
@@ -620,6 +622,9 @@ void loadalpha(int bm, void *data, int width, int height)
 {
     XImage *img = XCreateImage(display, CopyFromParent, 8, ZPixmap, 0, data, width, height, 8, 0);
 
+    // create picture that only holds alpha values
+    // NOTE: the XImage made earlier should really be freed, but calling XDestroyImage on it will also
+    // automatically free the data it's pointing to(which we don't want), so there's no easy way to destroy them currently
     bitmap[bm] = ximage_to_picture(img, XRenderFindStandardFormat(display, PictStandardA8));
 }
 
@@ -629,12 +634,14 @@ void loadalpha(int bm, void *data, int width, int height)
  */
 static Picture generate_alpha_bitmask(const uint8_t *rgba_data, uint16_t width, uint16_t height, uint32_t rgba_size)
 {
+    // we don't need to free this, that's done by XDestroyImage()
     uint8_t *out = malloc(rgba_size / 4);
     uint32_t i, j;
     for (i = j = 0; i < rgba_size; i += 4, j++) {
         out[j] = (rgba_data+i)[3] & 0xFF; // take only alpha values
     }
 
+    // create 1-byte-per-pixel image and convert it to a Alpha-format Picture
     XImage *img = XCreateImage(display, CopyFromParent, 8, ZPixmap, 0, (char*)out, width, height, 8, width);
     Picture picture = ximage_to_picture(img, XRenderFindStandardFormat(display, PictStandardA8));
 
@@ -658,6 +665,7 @@ UTOX_NATIVE_IMAGE *png_to_image(const UTOX_PNG_IMAGE data, size_t size, uint16_t
     // we don't need to free this, that's done by XDestroyImage()
     uint8_t *out = malloc(rgba_size);
 
+    // colors are read into red, blue and green and written into the target pointer
     uint8_t red, blue, green;
     uint32_t *target;