Describe how the image is composed in the guide section, and add 8x16…

… object tile index failure case

README.md

@@ -23,16 +23,91 @@ formula to convert each 5-bit CGB palette component to 8-bit:
 
 ## Emulator Requirements
 A simple line based renderer is sufficient to generate the correct output. This
-is NOT a [PPU timing torture test requiring T-cycle accuracy](https://github.com/mattcurrie/mealybug-tearoom-tests), and does NOT perform register writes during the PPU's mode 3.
+is NOT a [PPU timing torture test requiring T-cycle accuracy](https://github.com/mattcurrie/mealybug-tearoom-tests),
+and does NOT perform register writes during the PPU's mode 3.
 
-The test uses LY=LYC coincidence interrupts to perform register writes on 
+The test uses LY=LYC coincidence interrupts to perform register writes on
 specific rows of the screen during mode 2 (OAM scan).
 
 Double speed mode and WRAM banking emulation are not required.
 
 ## Guide
-See the table below for examples of incorrect behaviour and the corresponding
-attribute/flag.
+
+### Hello World!
+The "Hello World" text is constructed from 10 objects, and the exclaimation
+mark is part of the background. There is also an additional solid white object
+where the exclaimation mark is. Due to the 10 object per line limit, the solid
+white object should not be drawn, allowing the background to show through.
+
+### Eyes
+The eyes consist of four background/window tiles using the same tile data that
+is flipped vertically and/or horizontally. The top left corner of the eye
+contains the unflipped tile. 
+
+The left eye is drawn using the background, and the right eye is drawn using
+the window (and also the right edge of the head beside the right eye). At the
+bottom of the eye, the WX register is set to an off-screen value, so the window
+is hidden until the WX register is set again for drawing the right side of the
+chin.
+
+The background/window tiles for the top left corner of both eyes use red for
+color 0. A green object is positioned overlapping it with OBJ-to-BG Priority
+(bit 7) set in the OAM flags, so the green object only shows through color 0 of
+the background/window tile and replaces the red color.
+
+The background/window tiles for the top right corner of both eye use red for
+color 0 with BG-to-OAM Priority (bit 7) set in the background map attributes. A
+green object is positioned overlapping it, so the green object only shows
+through color 0 of the background tile and replaces the red color.
+
+### Nose
+The nose consists of four objects using the same tile data that is flipped
+vertically and/or horizontally. The top left corner of the nose contains the
+unflipped tile. The tile data comes from VRAM bank 1.
+
+The background tiles behind the nose have BG-to-OAM Priority (bit 7) set in the
+background map attributes, however for this section of the screen the Master
+Priority (bit 0) of LCDC has been reset, so the objects are always on top
+regardless of the background map attribute setting.
+
+### Mole
+The mole is not visible in the reference image, but can be seen in the failure
+case image for Object Priority Mode (DMG/CGB).
+
+This part of the test consists of two overlapping objects: a blank yellow
+square, and a black dot/"mole". The blank yellow square object OAM entry is
+before the mole, so effectively covers the mole even though the mole object has
+a lower X coordinate.
+
+### Mouth
+The mouth consists of eight 8x16 objects. The left and right edges of the mouth
+contain unflipped tile data, and other objects use vertically flipped tile
+data.
+
+For the left side of the mouth, the objects specify tile index 6, and the right
+side of the mouth specify tile index 7. Because bit 0 of the tile index is
+ignored for 8x16 objects, the whole mouth effectively uses tile index 6.
+
+### Chin
+The right side of the chin is drawn using the window. After the right eye was
+drawn, the window was hidden by setting WX to an off-screen value. For the
+right side of the chin, the WX value is restored to an on-screen value. After
+the chin has been drawn, the window is disabled using bit 5 of the LCDC
+register so the window does not cover the footer text.
+
+For the right side of the chin, the window has been updated to use the tile map
+from the VRAM beginning at $9800.  Because 16 rows of window have already been
+drawn for the eye, the right side of the chin is rendered starting from address
+$9840.
+
+### Footer Text
+For the footer text, the background is set to use the $9c00-9fff region for
+the map data, and tile data is set to come from the $8800-97ff region.
+
+## Failure Examples
+See the table below for some examples of incorrect behaviour and the
+corresponding attribute/flag. This is not intended to be an exhaustive list of
+all possible failures.
 
 | Failure Example | Functionality Tested |
 | --------------- | ---------------------|
@@ -52,7 +127,8 @@ attribute/flag.
 | ![failure image](img/failures/win-enable.png) | Window Enable (bit 5) |
 | ![failure image](img/failures/win-map.png) | Window Tile Map (bit 6) |
 | ![failure image](img/failures/obj-priority-mode.png) | Object Priority Mode (DMG/CGB) |
-| ![failure image](img/failures/10-obj-limit.png) | 10 Object Limit |
+| ![failure image](img/failures/10-obj-limit.png) | 10 object per line limit |
+| ![failure image](img/failures/8x16-obj-tile-index-bit-0.png) | Bit 0 of tile index for 8x16 objects should be ignored |
 
 ## Credits
 Håkon Wium Lie and Ian Hickson for creation of the original