Sonic the Hedgehog was the second of two Sonic games released for LeapFrog systems, following Sonic X, and like its predecessor contains educational aspects (in this case, spelling).
Unlike Sonic X, which was a reskin of another game, the Didj's Sonic couches the educational stuff in Genesis Sonic-style gameplay, bringing over numerous graphics, sounds, enemies, and items from those titles. One big difference is that the Special Stage rings take Sonic to the spelling challenges; the Chaos Emeralds (of which there are only three) are instead awarded for beating bosses, with the final boss giving up the Master Emerald. Unfortunately, the physics are far from perfect, resulting in some odd collision detection and making the final boss rather easy. (Oh, and Sonic has blue eyelids.)
To do:
There's unused levels, but we need a way to access and view them.
Sonic walking and running diagonally. In-game, his jumping/rolling sprites are used for diagonal movement instead.
While most of Sonic's sprites in this game are based on those from Sonic the Hedgehog 3, these are lifted from Sonic 2 despite equivalent sprites existing in Sonic 3.
Both walking animations have one frame where the sprites are dislodged from their intended maskings.
A version of Sonic's standing frame edited to make him look down. Amusingly, only his right eye was edited, making him look almost cross-eyed. One pixel of the mask is visible on the sprite.
Sonic hanging on a object, unused because there's nothing to hang on in this game. Notably, this also uses sprites from Sonic 2 despite equivalent Sonic 3 sprites existing.
Sonic punching the air in celebration when jumping after passing a signpost from the Sonic 1 prototype.
The drowning sprite from Sonic 3.
Debug Mode
A debug font that's used in Debug Mode.
Early HUD
A version of the HUD (taken from Sonic 1) without the line to track the number of Gadgets collected. It's possible the Gadgets idea (or at least the idea of tracking it in the HUD) came later in development.
Sonic's life counter icon without the text, suggesting that "SONIC" and the "×" symbol were separate sprites at one point. The final game stores it all as one sprite, likely for optimization.
Unused Menu Sprites
To do:
Find code for these sprites if there's any.
Find a way to implement these back into the game.
Graphics/Sprites
Notes
A graphic of the Didj, probably intended for a controls screen.
A graphic for a supposed rocket slider option that would've been in the Options menu.
A slider handle for the above rocket slider.
Unused light toggle sprites that would've let you enable...something.
Unused A- and B-button sprites, could have been used for switching the jump button. Interestingly, the button shape here differs from those on the actual Didj (see above).
A small arrow, likely for scrolling through more options.
Unused Backgrounds
Emerald Hill Zone's background from Sonic 2, meant for the title screen (not the first time it was swapped out later in development). The title screen instead uses part of Emerald Hill Act 2 as the background.
Notably, a version of this background is used in the game's preview icon.
A blue background, purpose unknown. Could be a placeholder for the menu background.
Unused Objects
To do: Check if there is any leftover unused code behaviors?
A blank monitor. Similar to the used version, but with the top being a bit taller, to be used alongside...
...these numbers! It's possible the game was planned to feature math-related Special Stages in addition to spelling ones.
Here's what it looks like assembled.
Unused Icons
Unused
Used
An unused version of the game's icon that's quite poorly resized and uses a brighter background.
Sprite
Notes
Green Hill Zone from Sonic 1.
Chemical Plant Zone from Sonic 2.
Scrap Brain Zone, also from Sonic 1. Oddly, the icon is a bit off-center.
Hydrocity Zone from Sonic 3, also a bit off-center.
Various icons for the Level Select.
Unused Audio
Unused
Used
An alternate version of the sound used when Sonic loses rings.