An blog post I wrote about the subject, interested in your thoughts (would like you to leave comments on the blog though). http://www.underthecode.com/2009/04/20/source-code-game-engines-unity/

You've jumped to some assumptions in your article. It is entirely possible to make a Diablo RPG in the Unreal engine without source access. It's just that there arn't any internet tutorials telling you how to do this.
Having dabbled around with Unreal Scripting and the editor I have found it quite easy to modify the engine enough to create demos of an RTS a Point and Click adventure and also a 2.5D scrolling Shoot em up.

In fact if you can be bothered to dig around the unreal scripts provides pretty much all the features that the Unity scripts provide plus more.

Don't get me wrong I think that Unity 3D is a great engine and I am even considering using it for a future project but, your article make one or two sugestions that are just untrue and provides a couple of percentage figures that are just made up.

Well the percentage were just a rough number, mot meant for exact figures.

As for modding Unreal, I will admit that I probably don't know enough about the capabilities of modding Unreal, I probably should have used Torque as an example because I was specifically told by it's community I would need some to a lot of core modification depending on how far I want to go to say create a Mythos type game (which was the most like Diablo 3 of any game). Since you seem to know a lot more about Unreal modding, let me pick your brain if you don't mind. Let me know if you know if the following are possible when modding Unreal (from your current knowledge):

• Camera style of Mythos/Diablo 3


• Implement a skill/spell system for multiple classes including skill trees with skill points


• Implement a complete inventory system


• Implement an experience system for leveling


• Implement attribute system (like strength, int, agility, etc...) to calculate hit percentage, crit percentage, etc...


• Implement equipment system (so the character can wear head, feet, chest, etc... pieces)


• have durability for equipment


• Level zoning like when you go into a cave

[Edited by - 3dmodelerguy on April 21, 2009 6:49:24 AM]

http://gamearchitect.net/2008/09/19/good-middleware/

Quote: Good middleware gives you source. Despite the previous point, having access to the source for any middleware package is a must. Sometimes you’ll suspect that there’s a bug in the middleware. Sometimes you need to see how a particular input led to a particular result before you can understand why the input was wrong. Sometimes you’ll have to fix a bug no matter how good the middleware is. And frequently you’ll need to recompile to handle a new platform SDK release or to link with some esoteric build configuration.

http://www.satori.org/2009/03/the_engine_survey_technology_r.html

Quote:
A strong theme coming out of the technologists’ comments in particular is the difficulty of working with and modifying an unfamiliar code base. It is stated repeatedly in multiple sections of the survey – it is the most important thing next to genre appropriateness. Having access to engine source code helps out immensely, but being able to evaluate the engine to make sure that it integrates well with existing tools/tech is vital, as is the level of support. Commenters pointed out that they have spent more time debugging poorly-crafted middleware than they would have spent writing it from scratch themselves. And what if the middleware company were to go out of business, or be purchased? Clearly having source code helps immensely in these circumstances, but it’s a frightening prospect that should also be covered in any legal agreements.

Would you like more quotes on the topic?

3dmodelerguy
Yes all of those things are entirely possible with Unreal Scripting.
Most of them are fairly trivial to impliment. The biggest hurdle is geting used to working with the scripts and realizing that most of these things require inheriting from existing scripts and tweeking then here and there rather than writing huge scripts yourself.
Also some of the functionality is already there scripted for you the problem is working your way through the scripts to find out where it is implimented.

CmpDev
Whilst I'd agree with most of these two posts the second one does seem to echo a great deal of the NIH mentalaty and the over optomism that currently exists in a lot of games studios. There is a huge number of developers out there that will claim that they've spent more time debugging middleware than it would take them to write it from scratch but, this simply can't be true.

Buster2000: Ok, I did strikeout that comment about Unreal as I don't have a ton of experience with modding unreal. I replaced it with the comment TGEA with information I received directly from the TGEA community.