使用CILinearGradient创建线性渐变无显示，求问题排查

问题根源 #

1. 渐变无明确渲染范围
   CILinearGradient生成的是无限延伸的渐变图像，Core Image无法确定需要渲染的区域大小，直接使用会导致输出图像没有有效尺寸，UIImageView无法识别显示。

2. 未关联原始图像尺寸
   你的代码仅合成了两个渐变，但未将其与目标图像img的尺寸绑定，最终输出的渐变图像脱离了视图可识别的尺寸范围。

3. CIImage转UIImage步骤缺失
   直接将CIImage赋值给UIImageView时，若不指定正确的缩放比例和图像方向，系统可能无法正确解析图像的显示属性，导致无内容展示。

修复后的完整代码 #

// 假设img是你要处理的原始UIImage
CIImage *originalCIImage = [CIImage imageWithCGImage:img.CGImage];
CGRect imageExtent = originalCIImage.extent;

// 创建第一个渐变并裁剪到原始图像范围
CIFilter *gradientfilter = [CIFilter filterWithName:@"CILinearGradient"];
[gradientfilter setValue:[CIVector vectorWithX:CGRectGetMidX(imageExtent) Y:CGRectGetMaxY(imageExtent)*0.75] forKey:@"inputPoint0"];
[gradientfilter setValue:[CIColor colorWithRed:0 green:1 blue:0 alpha:1] forKey:@"inputColor0"];
[gradientfilter setValue:[CIVector vectorWithX:CGRectGetMidX(imageExtent) Y:CGRectGetMaxY(imageExtent)*0.5] forKey:@"inputPoint1"];
[gradientfilter setValue:[CIColor colorWithRed:0 green:1 blue:0 alpha:0] forKey:@"inputColor1"];

CIFilter *cropFilter1 = [CIFilter filterWithName:@"CICrop"];
[cropFilter1 setValue:gradientfilter.outputImage forKey:kCIInputImageKey];
[cropFilter1 setValue:[CIVector vectorWithCGRect:imageExtent] forKey:@"inputRectangle"];
CIImage *croppedGradient1 = cropFilter1.outputImage;

// 创建第二个渐变并裁剪到原始图像范围
CIFilter *gradientfilter2 = [CIFilter filterWithName:@"CILinearGradient"];
[gradientfilter2 setValue:[CIVector vectorWithX:CGRectGetMidX(imageExtent) Y:CGRectGetMaxY(imageExtent)*0.25] forKey:@"inputPoint0"];
[gradientfilter2 setValue:[CIColor colorWithRed:0 green:1 blue:0 alpha:1] forKey:@"inputColor0"];
[gradientfilter2 setValue:[CIVector vectorWithX:CGRectGetMidX(imageExtent) Y:CGRectGetMaxY(imageExtent)*0.5] forKey:@"inputPoint1"];
[gradientfilter2 setValue:[CIColor colorWithRed:0 green:1 blue:0 alpha:0] forKey:@"inputColor1"];

CIFilter *cropFilter2 = [CIFilter filterWithName:@"CICrop"];
[cropFilter2 setValue:gradientfilter2.outputImage forKey:kCIInputImageKey];
[cropFilter2 setValue:[CIVector vectorWithCGRect:imageExtent] forKey:@"inputRectangle"];
CIImage *croppedGradient2 = cropFilter2.outputImage;

// 合成两个渐变
CIFilter *compositingFilter = [CIFilter filterWithName:@"CIAdditionCompositing"];
[compositingFilter setValue:croppedGradient1 forKey:kCIInputImageKey];
[compositingFilter setValue:croppedGradient2 forKey:kCIInputBackgroundImageKey];
CIImage *combinedGradient = compositingFilter.outputImage;

// 将渐变与原始图像叠加（若不需要保留原始图像可跳过此步骤）
CIFilter *finalCompositing = [CIFilter filterWithName:@"CISourceOverCompositing"];
[finalCompositing setValue:combinedGradient forKey:kCIInputImageKey];
[finalCompositing setValue:originalCIImage forKey:kCIInputBackgroundImageKey];
CIImage *outImage = finalCompositing.outputImage;

// 转换为UIImage并设置到imageView
UIImage *resultImage = [UIImage imageWithCIImage:outImage scale:[UIScreen mainScreen].scale orientation:img.imageOrientation];
self.imageView.image = resultImage;

关键修复点说明 #

• 用CICrop滤镜将渐变裁剪到原始图像的extent范围，确保图像有明确的可识别尺寸。
• 若仅需要展示渐变效果，可跳过与原始图像叠加的步骤，直接将裁剪后的渐变转换为UIImage。
• 转换UIImage时指定屏幕缩放比例和原始图像方向，保证图像显示清晰且方向正确。

内容的提问来源于stack exchange，提问作者GeraldTheGerm