Resizing MTKView scales old content before redraw

Question

I'm using a MTKView to draw Metal content. It's configured as follows:

    mtkView = MTKView(frame: self.view.frame, device: device)
    mtkView.colorPixelFormat = .bgra8Unorm
    mtkView.delegate=self
    mtkView.sampleCount=4
    mtkView.isPaused=true
    mtkView.enableSetNeedsDisplay=true

setFrameSize is overriden to trigger a redisplay.

Whenever the view resizes it scales its old content before it redraws everything. This gives a jittering feeling.

I tried setting the contentGravity property of the MTKView's layer to a non-resizing value, but that totally messes up the scale and position of the content. It seems MTKView doesn't want me to fiddle with that parameter.

How can I make sure that during a resize the content is always properly redrawn?

Answer 1

In my usage of Metal and MTKView, I tried various combinations of presentsWithTransaction and waitUntilScheduled without success. I still experienced occasional frames of stretched content in between frames of properly rendered content during live resize.

Finally, I dropped MTKView altogether and made my own NSView subclass that uses CAMetalLayer and resize looks good now (without any use of presentsWithTransaction or waitUntilScheduled). One key bit is that I needed to set the layer's autoresizingMask to get the displayLayer method to be called every frame during window resize.

Here's the header file:

#import <Cocoa/Cocoa.h>
    
@interface MyMTLView : NSView<CALayerDelegate>    
@end

Here's the implementation:

#import <QuartzCore/CAMetalLayer.h>
#import <Metal/Metal.h>

@implementation MyMTLView

- (id)initWithFrame:(NSRect)frame
{
    if (!(self = [super initWithFrame:frame])) {
        return self;
    }

    // We want to be backed by a CAMetalLayer.
    self.wantsLayer = YES;

    // We want to redraw the layer during live window resize.
    self.layerContentsRedrawPolicy = NSViewLayerContentsRedrawDuringViewResize;

    // Not strictly necessary, but in case something goes wrong with live window
    // resize, this layer placement makes it more obvious what's going wrong.
    self.layerContentsPlacement = NSViewLayerContentsPlacementTopLeft;

    return self;
}

- (CALayer*)makeBackingLayer
{
    CAMetalLayer* metalLayer = [CAMetalLayer layer];
    metalLayer.device = MTLCreateSystemDefaultDevice();
    metalLayer.delegate = self;

    // *Both* of these properties are crucial to getting displayLayer to be
    // called during live window resize.
    metalLayer.autoresizingMask = kCALayerHeightSizable | kCALayerWidthSizable;
    metalLayer.needsDisplayOnBoundsChange = YES;

    return metalLayer;
}

- (CAMetalLayer*)metalLayer
{
    return (CAMetalLayer*)self.layer;
}

- (void)setFrameSize:(NSSize)newSize
{
    [super setFrameSize:newSize];

    self.metalLayer.drawableSize = newSize;
}

- (void)displayLayer:(CALayer*)layer
{
    // Do drawing with Metal.
}

@end

For reference, I do all my Metal drawing in MTKView's drawRect method.

Answer 2

I have the same problem with glitches on view resizing. You can even reproduce it in the HelloTriangle example from the Apple's developer site. However the effect is minimized because the triangle is drawn near the middle of the screen, and it's the content closest to the edge of the window, opposite the corner that drags, that is effected worst. The developer notes regarding use of presentsWithTransaction and waitUntilScheduled do not work for me either.

My solution was to add a Metal layer beneath the window.contentView.layer, and to make that layer large enough that it rarely needs to be resized. The reason this works is that, unlike the window.contentView.layer, which sizes itself automatically to the view (in turn maintaining the window size), you have explicit control of the sublayer size. This eliminates the flickering.