Draw something to CAMetalLayer cause it shows all red

Question

Helloworld21Centry OP

Created Mar ’23

Replies 1

Boosts 0

Participants 2

I am writing something about video decoding. Now I have met some problems on darwin.

First init the layer and bind it to a view

CAMetalLayer *layer = [[CAMetalLayer alloc] init];
layer.device = MTLCreateSystemDefaultDevice();
layer.pixelFormat = MTLPixelFormatBGRA8Unorm;
layer.drawableSize = self.view.frame.size;
layer.contentsGravity = kCAGravityCenter;
self.view.wantsLayer = YES;
self.view.layer = layer;

next when the video decoded frame coming,

id &lt;CAMetalDrawable> drawable = [metal_layer_ nextDrawable];
if (!drawable) {
  return;
}
id &lt;MTLTexture> texture = [drawable texture];
IOSurfaceRef surface = [texture iosurface];
void *base_address = IOSurfaceGetBaseAddress(surface);
int pitch[] = {(int) (IOSurfaceGetBytesPerRow(surface)), 0};
uint8_t *pixels[] = {static_cast&lt;uint8_t *>(base_address), nullptr};
fn(pixels, pitch); &#x2F;&#x2F; Which is ffmpeg swscale.
[drawable present];

This does not work at all, the whole screen is red and would never change while new frame is coming. And I tried another way to present the video frame. It is still red.

Is there any thing wrong or missed.

Boost

Answer 1

Ceylo OP

Mar ’23

Why are you using a CAMetalLayer if you don't use any feature from Metal? Your example looks like a very uncommon usage to me:

in my experience you get red output when the framebuffer isn't even cleared because there is no render pipeline (MTLRenderCommandEncoder) scheduled to render into that drawable
CAMetalLayer provides drawable that cannot be written to by default, only rendered into, at least from Metal pipeline perspective.
Is your texture storage even shared? I would expect the texture created by CAMetalLayer to have private storage, so you can only write to it through Metal commands.

0