MC3D - Platform Agnostic 3D Foundation

Sorry for the lack of posts recently. Things have been, well… you know. Same old story. Super busy. Which is good, but it's murder on blog post frequency.

I've recently had to port some OpenGL ES work I did from iOS to Android. It used to be that doing so would have been insanely painful (as opposed to just painful). I would have had to convert the Objective-C code to Java, and then maintain completely distinct sets of code that do the same exact thing. Fortunately, the Android NDK (Native Development Kit) allows you to write code for Android in C/C++. The version of the NDK supported on 2.2 still requires part of the Activity (Android's counterpart to an iOS view controller) to be written in Java, but does allow you to call C/C++ code using JNI. In 2.3 and 3.0, you can do entire activities in C or C++.

This is a huge step forward for Android for those of us who do performance-critical work on multiple platforms, but it's not without some pain. Debugging across the JNI bridge is… less than easy. But, being able to share code across platforms is a huge win, and being able to get native speeds in the process is teh awseome.

During these projects, I've been taking a lot of my 3D-related code and creating a new set of platform-agnostic C functions and types. I've been cleaning up and making names consistent, and placing appropriate pre-compiler macros to make sure the code compiles correctly everywhere. On iOS, the library will take advantage of the Accelerate Framework in places, but doesn't require Accelerate to function.

I've chosen C because I don't like mixing C++ and Objective-C. The object models are too different for my tastes. But I've also made sure to include proper ifdef'd extern statements so that you can import the MC3D header files from C++ without hassle.

I've dubbed this set of functions MC3D, and I'm making it open source under a simplified version of the simplified BSD license (simplified simplified BSD license?). I've taken out the attribution requirement, so the only requirement is that if you re-distribute the source code, you have to leave the copyright and license text intact. That's it. Otherwise, you can use it for free in any project, commercial or otherwise, without paying anything, without attributing, and without asking (no really, you don't need to ask).

MC3D is still very much a work in progress, and I'm only adding code to the repository that I feel is ready for public consumption. Much of what's in MC3D has been posted here before, sometimes with different names or in slightly different form.

I have other code that I plan to add in the future, including higher-level functionality like model loading, scene management, and skeletal animation, but I won't add anything until its both solid and platform agnostic.

Currently, documentation is very sparse, and I currently can't offer any support or help with using it, so caveat emptor! I will gladly accept contributions, bug fixes, and new functionality back into the MC3D codeline.

MC3D on GitHub.

Link fixed, sorry about that

Drawing Part of a UIImage

Happy Thanksgiving, and sorry for the relative lack of posts here lately. Things are crazier than ever and it's been a challenge finding time to shower, let alone blog.

I do have something to share, today, though. No, it's not the next chapter of OpenGL ES 2.0 for iOS. It's a category that some of you may find useful: a method that allows you to draw only part of a UIImage rather than the entire thing.

On the Mac, NSImage has a handy instance method called drawInRect:fromRect:operation:fraction: that lets you specify exactly which part of an image to draw. On UIImage, we've only got the ability to draw the entire image either unless we drop down to Core Graphics calls. We don't have a nice, easy, convenient way using just UIImage to draw a portion of the image it represents.

I needed this ability in an application I'm working on, so I hacked out the following category. At first glance, this may look to be inefficient, since we're making a copy of the instance's backing CGImage in order to create the sub-image, however I believe that CGImageCreateWithImageInRect() references the original image's bitmap data. I haven't confirmed that it doesn't make a copy of the bitmap data, but the documentation certainly seems to imply it. Anyone know?

Anyway, here is the category; I've even commented the code more pedantically than is normal for me in case anyone might be confused about what's going on. Improvements and bug fixes are, as always, welcome.

@implementation UIImage(MCDrawSubImage)
- (void)drawInRect:(CGRect)drawRect fromRect:(CGRect)fromRect blendMode:(CGBlendMode)blendMode alpha:(CGFloat)alpha
{
    CGImageRef drawImage = CGImageCreateWithImageInRect(self.CGImage, fromRect); 
    if (drawImage != NULL)
    {
        CGContextRef context = UIGraphicsGetCurrentContext();
        
        // Push current graphics state so we can restore later
        CGContextSaveGState(context);   
        
        // Set the alpha and blend based on passed in settings
        CGContextSetBlendMode(context, blendMode);
        CGContextSetAlpha(context, alpha);
        
        // Take care of Y-axis inversion problem by translating the context on the y axis
        CGContextTranslateCTM(context, 0, drawRect.origin.y + fromRect.size.height);  
        
        // Scaling -1.0 on y-axis to flip
        CGContextScaleCTM(context, 1.0, -1.0);
        
        // Then accommodate the translate by adjusting the draw rect
        drawRect.origin.y = 0.0f;
        
        // Draw the image
        CGContextDrawImage(context, drawRect, drawImage);
        
        // Clean up memory and restore previous state
        CGImageRelease(drawImage);
        
        // Restore previous graphics state to what it was before we tweaked it
        CGContextRestoreGState(context); 
    }
}
@end