Objective-c – Any idea why glVertexPointer() wouldn’t draw anything when using GL_FLOAT

iphoneobjective copengl-es

Using GLfixed as my vertex number type the following code draws textures as expected:

GLfixed vertices[] = 
{
    (int)point.x,           (int)point.y + size.height,
    (int)point.x + size.width,  (int)point.y + size.height,
    (int)point.x,           (int)point.y,
    (int)point.x + size.width,  (int)point.y
};

glVertexPointer(2, GL_FIXED, 0, vertices);

I read in the OpenGL docs that GLfixed is the least efficient type and that I should be using GLfloat instead. But when I switched my code over to floats nothing gets drawn.

GLfloat vertices[] = 
{
    point.x,        point.y + size.height,
    point.x + size.width,   point.y + size.height,
    point.x,        point.y,
    point.x + size.width,   point.y
};

glVertexPointer(2, GL_FLOAT, 0, vertices);

Is there another flag I need to set in the OpenGL state machine to get this to behave as expected?

Best Answer

When I look at the documentation it looks like GL_FIXED is a fixed point 16:16 format. That means that the 16 upper bits of a GLfixed represents the integer part and the fraction is the lower 16 bits divided by 65536 (1 << 16).

To convert this to a float simply divide the number by 65536:

const float scale = 1.0f / 65536.0f;
GLfloat vertices[] = 
{
        point.x * scale               , (point.y + size.height) * scale,
        (point.x + size.width) * scale, (point.y + size.height) * scale,
        point.x * scale               , point.y * scale,
        (point.x + size.width) * scale, point.y * scale
};

glVertexPointer(2, GL_FLOAT, 0, vertices);

If you're vertex coordinates etc is also in GL_FIXEDformat you'll have to scale them as well.

Hope this helps.

Swift

Short answer

Use a NotificationCenter observer rather than viewWillAppear.

override func viewDidLoad() {
    super.viewDidLoad()

    // set observer for UIApplication.willEnterForegroundNotification
    NotificationCenter.default.addObserver(self, selector: #selector(willEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)

}

// my selector that was defined above
@objc func willEnterForeground() {
    // do stuff
}

Long answer

To find out when an app comes back from the background, use a NotificationCenter observer rather than viewWillAppear. Here is a sample project that shows which events happen when. (This is an adaptation of this Objective-C answer.)

import UIKit
class ViewController: UIViewController {

    // MARK: - Overrides

    override func viewDidLoad() {
        super.viewDidLoad()
        print("view did load")

        // add notification observers
        NotificationCenter.default.addObserver(self, selector: #selector(didBecomeActive), name: UIApplication.didBecomeActiveNotification, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(willEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)

    }

    override func viewWillAppear(_ animated: Bool) {
        print("view will appear")
    }

    override func viewDidAppear(_ animated: Bool) {
        print("view did appear")
    }

    // MARK: - Notification oberserver methods

    @objc func didBecomeActive() {
        print("did become active")
    }

    @objc func willEnterForeground() {
        print("will enter foreground")
    }

}

On first starting the app, the output order is:

view did load
view will appear
did become active
view did appear

After pushing the home button and then bringing the app back to the foreground, the output order is:

will enter foreground
did become active

So if you were originally trying to use viewWillAppear then UIApplication.willEnterForegroundNotification is probably what you want.

Note

As of iOS 9 and later, you don't need to remove the observer. The documentation states:

If your app targets iOS 9.0 and later or macOS 10.11 and later, you don't need to unregister an observer in its dealloc method.

Best Answer

Related Solutions

Iphone – optimizing iPhone OpenGL ES fill rate

Ios – Why does viewWillAppear not get called when an app comes back from the background

Swift

Note

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