Document the internal alpha channel format better, and fixes for 32bit alpha bitmaps.

For images, rows need to be even (this is not true for anti-aliased font files).
Fix stride and size calculation. This makes images that have odd pixel rows display properly and fixes buffer overflows.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@30966 a1c6a512-1295-4272-9138-f99709370657

apps/recorder/bmp.c

@@ -438,7 +438,7 @@ void output_row_8_native(uint32_t row, void * row_in,
                 if (ctx->bm->alpha_offset > 0)
                     bm_alpha = ctx->bm->data + ctx->bm->alpha_offset;
                 if (bm_alpha)
-                    bm_alpha += ctx->bm->width*row/2;
+                    bm_alpha += ALIGN_UP(ctx->bm->width, 2) * row/2;
 
                 for (col = 0; col < ctx->bm->width; col++) {
                     if (ctx->dither)
@@ -453,7 +453,7 @@ void output_row_8_native(uint32_t row, void * row_in,
                     dest += STRIDE_MAIN(1, ctx->bm->height);
                     if (bm_alpha) {
                         /* pack alpha channel for 2 pixels into 1 byte */
-                        unsigned alpha = 255-qp->alpha;
+                        unsigned alpha = qp->alpha;
                         if (col%2)
                             *bm_alpha++ |= alpha&0xf0;
                         else
@@ -612,6 +612,8 @@ int read_bmp_fd(int fd,
         rset.rowstop = -1;
     }
 
+    /* need even rows (see lcd-16bit-common.c for details) */
+    int alphasize = ALIGN_UP(bm->width, 2) * bm->height / 2;
     if (cformat)
         totalsize = cformat->get_size(bm);
     else {
@@ -620,7 +622,7 @@ int read_bmp_fd(int fd,
         if (!remote)
 #endif
             if (depth == 32 && read_alpha) /* account for possible 4bit alpha per pixel */
-                totalsize += bm->width * bm->height / 2;
+                totalsize += alphasize;
     }
 
     if(return_size)
@@ -718,7 +720,7 @@ int read_bmp_fd(int fd,
 
 #ifdef HAVE_LCD_COLOR
     if (read_alpha && depth == 32)
-        bm->alpha_offset = totalsize - (bm->width * bm->height / 2);
+        bm->alpha_offset = totalsize - alphasize;
     else
         bm->alpha_offset = 0;
 #endif
@@ -882,7 +884,7 @@ int read_bmp_fd(int fd,
     {   /* if this has an alpha channel, totalsize accounts for it as well
          * subtract if no actual alpha information was found */
         if (bm->alpha_offset > 0)
-            totalsize -= bm->width*bm->height/2;
+            totalsize -= alphasize;
         bm->alpha_offset = 0;
     }
 #endif

apps/recorder/resize.c

@@ -775,7 +775,7 @@ static void output_row_32_native(uint32_t row, void * row_in,
                 if (ctx->bm->alpha_offset > 0)
                     bm_alpha = ctx->bm->data + ctx->bm->alpha_offset;
                 if (bm_alpha)
-                    bm_alpha += ctx->bm->width*row/2;
+                    bm_alpha += ALIGN_UP(ctx->bm->width, 2)*row/2;
 
                 for (col = 0; col < ctx->bm->width; col++) {
                     if (ctx->dither)
@@ -791,7 +791,7 @@ static void output_row_32_native(uint32_t row, void * row_in,
                     dest += STRIDE_MAIN(1, ctx->bm->height);
                     if (bm_alpha) {
                         /* pack alpha channel for 2 pixels into 1 byte */
-                        unsigned alpha = 255-SC_OUT(q0.a, ctx);
+                        unsigned alpha = SC_OUT(q0.a, ctx);
                         if (col%2)
                             *bm_alpha++ |= alpha&0xf0;
                         else

firmware/drivers/lcd-16bit-common.c

@@ -229,6 +229,25 @@ void lcd_mono_bitmap(const unsigned char *src, int x, int y, int width, int heig
     lcd_mono_bitmap_part(src, 0, 0, width, x, y, width, height);
 }
 
+
+/* About Rockbox' internal alpha channel format (for ALPHA_COLOR_FONT_DEPTH == 2)
+ *
+ * For each pixel, 4bit  of alpha information is stored in a byte-stream,
+ * so two pixels are packed into one byte.
+ * The lower nibble is the first pixel, the upper one the second. The stride is
+ * horizontal. E.g row0: pixel0: byte0[0:3], pixel1: byte0[4:7], pixel2: byte1[0:3],...
+ * The format is independant of the internal display orientation, as to
+ * support the same font files on
+ * The values go linear from 0 (fully transparent) to 15 (fully opaque).
+ *
+ * This might suggest that rows need to have an even number of pixels.
+ * However this is generally not the case. lcd_alpha_bitmap_part_mix() can deal
+ * with uneven colums (i.e. two rows can share one byte). And font files do
+ * exploit this.
+ * However, this is difficult to do for image files, especially bottom-up bitmaps,
+ * so lcd_bmp() do expect even rows.
+ */
+
 #define ALPHA_COLOR_FONT_DEPTH 2
 #define ALPHA_COLOR_LOOKUP_SHIFT (1 << ALPHA_COLOR_FONT_DEPTH)
 #define ALPHA_COLOR_LOOKUP_SIZE ((1 << ALPHA_COLOR_LOOKUP_SHIFT) - 1)
@@ -357,17 +376,16 @@ static void ICODE_ATTR lcd_alpha_bitmap_part_mix(const fb_data* image,
         dmask = 0xffffffff;
         drmode &= DRMODE_SOLID; /* mask out inversevid */
     }
-    if (drmode == DRMODE_BG)
-    {
-        dmask = ~dmask;
-    }
     /* sourcing from an image ignore drawmode.
      * Set to DRMODE_BG as we use its code path in the switch below */
     if (image != NULL)
     {
-        dmask = 0;
         drmode = DRMODE_BG;
     }
+    if (drmode == DRMODE_BG)
+    {
+        dmask = ~dmask;
+    }
 
     dst_row = LCDADDR(x, y);
 
@@ -560,7 +578,7 @@ void ICODE_ATTR lcd_bmp_part(const struct bitmap* bm, int src_x, int src_y,
     else if (bm->alpha_offset > 0)
         lcd_alpha_bitmap_part_mix((fb_data*)bm->data, bm->data+bm->alpha_offset,
             src_x, src_y, x, y, width, height,
-            bitmap_stride, bm->width);
+            bitmap_stride, ALIGN_UP(bm->width, 2));
     else
         lcd_bitmap_transparent_part((fb_data*)bm->data,
             src_x, src_y, bitmap_stride, x, y, width, height);