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Adjusting Contrast

Last updated: 2022-11-09 15:27:33

    Feature Overview

    CI uses the imageMogr2 API to adjust the contrast of an image, which is the difference in luminance between the brightest and darkest points in the image (i.e., the contrast in grayscale).

    An image can be processed:

    • During download
    • During upload
    • In the cloud

    Restrictions

    • Format: Currently, JPG, BMP, GIF, PNG, and WebP images can be processed, and HEIF images can be decoded and processed.
    • Size: The input image cannot be larger than 32 MB, with its width and height not exceeding 30,000 pixels respectively, and the total number of pixels not exceeding 250 million. The width and height of the output image cannot exceed 9,999 pixels respectively. For an animated input image, the total number of pixels (width * height * number of frames) cannot exceed 250 million.
    • Frames (for animated images): For GIF images, the number of frames cannot exceed 300.

    API Sample

    1. Processing during download

    GET /<ObjectKey>?imageMogr2/contrast/<value> HTTP/1.1
    Host: <BucketName-APPID>.cos.<Region>.myqcloud.com
    Date: <GMT Date>
    Authorization: <Auth String>
    

    2. Processing during upload

    PUT /<ObjectKey> HTTP/1.1
    Host: <BucketName-APPID>.cos.<Region>.myqcloud.com
    Date: GMT Date
    Authorization: Auth String
    Pic-Operations: 
    {
    "is_pic_info": 1,
    "rules": [{
        "fileid": "exampleobject",
        "rule": "imageMogr2/contrast/<value>"
    }]
    }
    
    Note:

    Pic-Operations is a JSON string. Its parameters are as described in Persistent Image Processing.

    3. Processing in-cloud data

    POST /<ObjectKey>?image_process HTTP/1.1
    Host: <BucketName-APPID>.cos.<Region>.myqcloud.com
    Date: GMT Date
    Content-length: Size
    Authorization: Auth String
    Pic-Operations: 
    {
    "is_pic_info": 1,
    "rules": [{
        "fileid": "exampleobject",
        "rule": "imageMogr2/contrast/<value>"
    }]
    }
    
    Note:

    Parameters

    Operation name: contrast.

    Parameter Description
    ObjectKey Object name, such as folder/sample.jpg.
    /contrast/<value> Adjusts the contrast of an image. The value must be an integer in the range of [-100, 100].
  • value < 0: Reduces the contrast.
  • value = 0: Does not adjust the contrast.
  • value > 0: Increases the contrast.
  • /ignore-error/1 If this parameter is carried and the image fails to be processed because the image is too large or a parameter value exceeds the limit, the input image will be returned with no error reported.

    Samples

    Sample 1: Adjusting the contrast

    Note:

    Processing during download is used as an example here, which does not store the output image in a bucket. If you need to store the output image, see Persistent Image Processing and use the processing during upload or processing in-cloud data feature.

    This example reduces the contrast of an image by 50:

    http://examples-1251000004.cos.ap-shanghai.myqcloud.com/sample.jpeg?imageMogr2/contrast/-50
    

    Output image:

    Sample 2: Adjusting the contrast with a signature carried

    This example processes the image in the same way as in the example above, except that a signature is carried. The signature is concatenated with other processing parameters by an ampersand (&).

    http://examples-1251000004.cos.ap-shanghai.myqcloud.com/sample.jpeg?q-sign-algorithm=<signature>&imageMogr2/contrast/-50
    
    Note:

    You can get the value of <signature> as instructed in Request Signature.

    Notes

    To prevent unauthorized users from accessing or downloading the input image by using a URL that does not contain any processing parameter, you can add the processing parameters to the request signature, making the processing parameters the key of the parameter with the value left empty. The following is a simple sample for your reference (it might have expired or become inaccessible). For more information, see Upload via Pre-Signed URL.

    http://examples-1251000004.cos.ap-shanghai.myqcloud.com/sample.jpeg?q-sign-algorithm=sha1&q-ak=AKID********************&q-sign-time=1593342360;1593342720&q-key-time=1593342360;1593342720&q-header-list=&q-url-param-list=watermark%252f1%252fimage%252fahr0cdovl2v4yw1wbgvzlteyntewmdawmdqucgljc2gubxlxy2xvdwquy29tl3nodwl5aw4uanbn%252fgravity%252fsoutheast&q-signature=26a429871963375c88081ef60247c5746e834a98&watermark/1/image/aHR0cDovL2V4YW1wbGVzLTEyNTEwMDAwMDQucGljc2gubXlxY2xvdWQuY29tL3NodWl5aW4uanBn/gravity/southeast
    
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