improved rendering of invisible glyphs

An invisible glyph could be an ASCII Space (0x20) or Null (0x0) character, or any other empty glyph loaded into the VGA's font memory.

Invisible glyphs (which are not rare) are currently drawn to the canvas exactly like regular glyphs, this patch handles that more efficiently.

- added array with per-glyph visibility attribute
- added invisible glyph detection in render_font_bitmap()
- instead of drawing invisible glyphs in render_changed_rows(), now erase whole row buffer once per row and just skip invisible glyphs
- removed one nested loop from render_font_bitmap()

Characters hidden on screem caused by their blink attribute (rare) are also treated as invisible.

src/browser/screen.js

@@ -74,6 +74,7 @@ function ScreenAdapter(options, screen_fill_buffer)
         // fonts
         font_context,
         font_image_data,
+        font_is_visible = new Int8Array(8 * 256),
         font_height,
         font_width,
         font_width_9px,
@@ -133,16 +134,18 @@ function ScreenAdapter(options, screen_fill_buffer)
         }
 
         const font_bitmap = font_image_data.data;
-        let i_dst = 0;
+        let i_dst = 0, is_visible;
         const put_bit = font_width_dbl ?
             function(value)
             {
+                is_visible = is_visible || value;
                 font_bitmap[i_dst + 3] = value;
                 font_bitmap[i_dst + 7] = value;
                 i_dst += 8;
             } :
             function(value)
             {
+                is_visible = is_visible || value;
                 font_bitmap[i_dst + 3] = value;
                 i_dst += 4;
             };
@@ -156,23 +159,27 @@ function ScreenAdapter(options, screen_fill_buffer)
         // move i_dst from end of a glyph's scanline to start of its next scanline
         const dst_inc_line = font_width * 255 * 4;
 
-        for(let i_font_page = 0, i_vga = 0; i_font_page < 8; ++i_font_page, i_dst += dst_inc_row)
+        for(let i_chr_all = 0, i_vga = 0; i_chr_all < 2048; ++i_chr_all, i_vga += vga_inc_chr, i_dst += dst_inc_col)
         {
-            for(let i_chr = 0; i_chr < 256; ++i_chr, i_vga += vga_inc_chr, i_dst += dst_inc_col)
+            const i_chr = i_chr_all % 256;
+            if(i_chr_all && !i_chr)
             {
-                for(let i_line = 0; i_line < font_height; ++i_line, ++i_vga, i_dst += dst_inc_line)
+                i_dst += dst_inc_row;
+            }
+            is_visible = false;
+            for(let i_line = 0; i_line < font_height; ++i_line, ++i_vga, i_dst += dst_inc_line)
+            {
+                const line_bits = vga_bitmap[i_vga];
+                for(let i_bit = 0x80; i_bit > 0; i_bit >>= 1)
                 {
-                    const line_bits = vga_bitmap[i_vga];
-                    for(let i_bit = 0x80; i_bit > 0; i_bit >>= 1)
-                    {
-                        put_bit(line_bits & i_bit ? 255 : 0);
-                    }
-                    if(font_width_9px)
-                    {
-                        put_bit(font_copy_8th_col && i_chr >= 0xC0 && i_chr <= 0xDF && line_bits & 1 ? 255 : 0);
-                    }
+                    put_bit(line_bits & i_bit ? 255 : 0);
+                }
+                if(font_width_9px)
+                {
+                    put_bit(font_copy_8th_col && i_chr >= 0xC0 && i_chr <= 0xDF && line_bits & 1 ? 255 : 0);
                 }
             }
+            font_is_visible[i_chr_all] = is_visible ? 1 : 0;
         }
 
         font_context.putImageData(font_image_data, 0, 0);
@@ -197,15 +204,18 @@ function ScreenAdapter(options, screen_fill_buffer)
             }
             ++n_rows_rendered;
 
+            // clear extra row 2
+            offscreen_extra_context.clearRect(0, row_extra_2_y, gfx_width, font_height);
+
             let fg_rgba, fg_x, bg_rgba, bg_x;
             for(let col_x = 0; col_x < gfx_width; col_x += font_width, txt_i += TEXT_BUF_COMPONENT_SIZE)
             {
                 const chr = text_mode_data[txt_i + CHARACTER_INDEX];
                 const chr_flags = text_mode_data[txt_i + FLAGS_INDEX];
                 const chr_bg_rgba = text_mode_data[txt_i + BG_COLOR_INDEX];
                 const chr_fg_rgba = text_mode_data[txt_i + FG_COLOR_INDEX];
-                const chr_blinking = chr_flags & FLAG_BLINKING;
                 const chr_font_page = chr_flags & FLAG_FONT_PAGE_B ? font_page_b : font_page_a;
+                const chr_visible = (!(chr_flags & FLAG_BLINKING) || blink_visible) && font_is_visible[(chr_font_page << 8) + chr];
 
                 if(bg_rgba !== chr_bg_rgba)
                 {
@@ -231,18 +241,13 @@ function ScreenAdapter(options, screen_fill_buffer)
                     fg_x = col_x;
                 }
 
-                if(!chr_blinking || blink_visible)
+                if(chr_visible)
                 {
                     // copy transparent glyphs into extra row 2
                     offscreen_extra_context.drawImage(font_canvas,
                         chr * font_width, chr_font_page * font_height, font_width, font_height,
                         col_x, row_extra_2_y, font_width, font_height);
                 }
-                else
-                {
-                    // erase hidden glyphs in extra row 2
-                    offscreen_extra_context.clearRect(col_x, row_extra_2_y, font_width, font_height);
-                }
             }
 
             // draw rightmost block of foreground color into extra row 1
@@ -281,6 +286,7 @@ function ScreenAdapter(options, screen_fill_buffer)
             }
             changed_rows.fill(0);
         }
+
         return n_rows_rendered;
     }