--- title: Processing an Image Using Built-in Filters framework: coreimage role: article role_heading: Article path: coreimage/processing-an-image-using-built-in-filters --- # Processing an Image Using Built-in Filters Apply effects such as sepia tint, highlight strengthening, and scaling to images. ## Overview Overview You can add effects to images by applying Core Image filters to CIImage objects. Figure 1 shows three filters chained together to achieve a cumulative effect: Apply the sepia filter to tint an image with a reddish-brown hue. Add the bloom filter to accentuate highlights. Use the Lanczos scale filter to scale an image down. Create a Context CIImage processing occurs in a CIContext object. Creating a CIContext is expensive, so create one during your initial setup and reuse it throughout your app. CIContext* context = [CIContext context]; Load an Image to Process The next step is to load an image to process. This example loads an image from the project bundle. NSURL* imageURL = [[NSBundle mainBundle] URLForResource:@"YourImageName" withExtension:@"png"]; CIImage* originalCIImage = [CIImage imageWithContentsOfURL:imageURL]; self.imageView.image = [UIImage imageWithCIImage:originalCIImage]; The CIImage object isn’t itself a displayable image, but rather image data. To display it, you must convert it to another type, such as UIImage. Apply Built-In Core Image Filters A CIFilter represents a single operation or recipe for a particular effect. To process a CIImage object, pass it through CIFilter objects. You can subclass CIFilter or draw from the existing library of built-in filters. Tint Reddish-Brown with the Sepia Filter Although you can chain filters without separating them into functions, the following example shows how to configure a single CIFilter, the sepiaTone() filter. - (CIImage*) sepiaFilterImage: (CIImage*)inputImage withIntensity:(CGFloat)intensity { CIFilter* sepiaFilter = CIFilter.sepiaToneFilter; sepiaFilter.inputImage = inputImage; sepiaFilter.intensity = intensity; return sepiaFilter.outputImage; } To pass the image through the filter, call the sepia filter function. CIImage* sepiaCIImage = [self sepiaFilterImage:originalCIImage withIntensity:0.9]; You can check the intermediate result at any point in the filter chain by converting from CIImage to a UIImage. You can then assign this UIImage to a UIImageView for display. _imageView.image = [UIImage imageWithCIImage:sepiaCIImage]; Strengthen Highlights with the Bloom Filter The bloom filter accentuates the highlights of an image. You can apply it as part of a chain without factoring it into a separate function, but this example encapsulates its functionality into a function. - (CIImage*) bloomFilterImage: (CIImage*)inputImage withIntensity:(CGFloat)intensity radius:(CGFloat)radius { CIFilter* bloomFilter = CIFilter.bloomFilter; bloomFilter.inputImage = inputImage; bloomFilter.intensity = intensity; bloomFilter.radius = radius; return bloomFilter.outputImage; } Like the sepia filter, the intensity of the bloom filter’s effect ranges between 0 and 1, with 1 being the most intense effect. The bloom filter has an additional radius parameter to determine how much the glowing regions expand. Experiment with a range to values to fine tune the effect, or assign the input parameter to a control like a UISlider to allow your users to tweak its values. note: The gloom() filter performs the opposite effect. To display the output, convert the CIImage to a UIImage. CIImage* bloomCIImage = [self bloomFilterImage:sepiaCIImage withIntensity:1 radius:10]; _filteredImageView.image = [UIImage imageWithCIImage:bloomCIImage]; Scale Image Size with the Lanczos Scale Filter Apply the lanczosScaleTransform() to obtain a high-quality downsampling of the image, preserving the original image’s aspect ratio through the lanczosScaleTransform() filter’s parameter aspectRatio. For built-in Core Image filters, calculate the aspect ratio as the image’s width over height. CGFloat imageWidth = originalUIImage.size.width; CGFloat imageHeight = originalUIImage.size.height; CGFloat aspectRatio = imageHeight / imageWidth; CIImage* scaledCIImage = [self scaleFilterImage:bloomCIImage withAspectRatio:aspectRatio scale:0.05]; Like other built-in filters, the lanczosScaleTransform() filter also outputs its result as a CIImage. - (CIImage*) scaleFilterImage: (CIImage*)inputImage withAspectRatio:(CGFloat)aspectRatio scale:(CGFloat)scale { CIFilter* scaleFilter = CIFilter.lanczosScaleTransformFilter; scaleFilter.inputImage = inputImage; scaleFilter.scale = scale; scaleFilter.aspectRatio = aspectRatio; return scaleFilter.outputImage; } important: To optimize computation, Core Image doesn’t actually render any intermediate CIImage result until you force the CIImage to display its content onscreen, as you might do using UIImageView. _imageView.image = [UIImage imageWithCIImage:scaledCIImage]; note: Core Image optimizes filtering by reordering the three chained filters and concatenating them into a single image processing kernel, saving computation and rendering cycles. In addition to trying out the built-in filters for a fixed effect, you can combine filters in certain Filter Recipes to accomplish tasks such as Applying a Chroma Key Effect, Selectively Focusing on an Image, Customizing Image Transitions, and Simulating Scratchy Analog Film. ## See Also ### Essentials - [CIContext](coreimage/cicontext.md) - [CIImage](coreimage/ciimage.md)