how to render CIImage to offscreen MTLTexture? - metal

Question

I don't want to draw the CIImage to the currentDrawable.texture of the MTKView, I want to get the texture, and then pass it to the fragment shader to do other things. But why the obtained texture is completely transparent?
CIContext *ciContext = [CIContext contextWithMTLCommandQueue:commandQueue options:#{ kCIContextWorkingFormat: [NSNumber numberWithInt:kCIFormatRGBAf], kCIContextCacheIntermediates: #NO, kCIContextName: #"Image Processor" }];
NSBundle *bundle = [NSBundle bundleForClass:[CGMetalLookup class]];
NSString *path = [bundle pathForResource:#"CGMetal.bundle/Graphic" ofType:#"png"];
CIImage *cimage = [[CIImage alloc] initWithContentsOfURL:[NSURL fileURLWithPath:path]];
id<MTLCommandBuffer> commandBuffer = [commandQueue commandBuffer];
CIFilter *sepiaFilter = [CIFilter filterWithName:#"CISepiaTone"];
[sepiaFilter setValue:cimage forKey:kCIInputImageKey];
[sepiaFilter setValue:#(5) forKey:kCIInputIntensityKey];
CIImage *outimage = sepiaFilter.outputImage;
// Initialize MTLTextures
MTLTextureDescriptor* descriptor = [MTLTextureDescriptor texture2DDescriptorWithPixelFormat:MTLPixelFormatRGBA8Unorm width:cimage.extent.size.width height:cimage.extent.size.height mipmapped:NO];
descriptor.usage = MTLTextureUsageShaderWrite | MTLTextureUsageShaderRead;
id<MTLTexture> _ciTexture = [[CGMetalDevice sharedDevice].device newTextureWithDescriptor:descriptor];
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGRect rect = outimage.extent;
[ciContext render:outimage toMTLTexture:_ciTexture commandBuffer:commandBuffer bounds:rect colorSpace:colorSpaceRef];
CIImage *tImage = [[CIImage alloc] initWithMTLTexture:_ciTexture options:nil];
CGImageRef cgImage1 = [ciContext createCGImage:tImage fromRect:tImage.extent];
this documention https://developer.apple.com/library/archive/documentation/GraphicsImaging/Conceptual/CoreImaging/ci_tasks/ci_tasks.html#//apple_ref/doc/uid/TP30001185-CH3-DontLinkElementID_12
This example shows only the minimal code needed to render with Core Image using Metal. In a real application, you’d likely perform additional rendering passes before or after the one managed by Core Image, or render Core Image output into a secondary texture and use that texture in another rendering pass. For more information on drawing with Metal, see Metal Programming Guide.
it should support rendering to custom textures, how should I do it?
how to understand this sentence
NOTE: Rendering to a texture initialized with a commandBuffer requires encoding all the commands to render an image into the specified buffer.
/* Render 'bounds' of 'image' to a Metal texture, optionally specifying what command buffer to use.
* Texture type must be MTLTexture2D.
* NOTE: Rendering to a texture initialized with a commandBuffer requires encoding all the commands to render an image into the specified buffer.
* This may impact system responsiveness and may result in higher memory usage if the image requires many passes to render.
* To avoid this impact, it is recommended to create a context using [CIContext contextWithMTLCommandQueue:].
*/
- (void)render:(CIImage *)image
toMTLTexture:(id<MTLTexture>)texture
commandBuffer:(nullable id<MTLCommandBuffer>)commandBuffer
bounds:(CGRect)bounds
colorSpace:(CGColorSpaceRef)colorSpace NS_AVAILABLE(10_11,9_0);