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composite命令可以非常方便的合并两张图片
因此用来进行图像加水印、批量增加边框等常用的变换
最简单的用法为：
composite -gravity north src.jpg coverback.jpg des.jpg
其中src.jpg为前景图片
coverback.jpg为背景图片。
des.jpg为叠加后的结果
-gravity north 指叠加位置为垂直据顶部、水平居中（正北方向）
如果要求在正中间，参数为center
如果要求在右下角，参数为southeast
composite还提供更灵活的定位，可以使用 -geometry 配置
具体的composite参数表见下表
|
| affine transform matrix |
| on, activate, off, deactivate, set, opaque, copy", transparent, extract, background, or shape the alpha channel | |
| decrypt image with this password | |
| blend images | |
| chromaticity blue primary point | |
| surround image with a border of color | |
| border color | |
| apply option to select image channels | |
| preferred number of colors in the image | |
| set image colorspace | |
| annotate image with comment | |
| set image composite operator | |
| image compression type | |
| display copious debugging information | |
| convert cipher pixels to plain | |
| define one or more image format options | |
| horizontal and vertical density of the image | |
| image depth | |
| shift image pixels defined by a displacement map | |
| dissolve the two images a given percent | |
| apply error diffusion to image | |
| convert plain pixels to cipher pixels | |
| text encoding type | |
| endianness (MSB or LSB) of the image | |
| extract area from image | |
| use this filter when resizing an image | |
| render text with this font | |
| preferred size or location of the image | |
| horizontal and vertical text placement | |
| chromaticity green primary point | |
| print program options | |
| identify the format and characteristics of the image | |
| type of image interlacing scheme | |
| pixel color interpolation method | |
| assign a label to an image | |
| adjust the level of image contrast | |
| pixel cache resource limit | |
| format of debugging information | |
| monitor progress | |
| transform image to black and white | |
| replace each pixel with its complementary color | |
| size and location of an image canvas (setting) | |
| font point size | |
| add, delete, or apply an image profile | |
| JPEG/MIFF/PNG compression level | |
| reduce image colors in this colorspace | |
| suppress all warning messages | |
| chromaticity red primary point | |
| pay attention to warning messages. | |
| settings remain in effect until parenthesis boundary. | |
| apply Paeth rotation to the image | |
| horizontal and vertical sampling factor | |
| image scene number | |
| seed a new sequence of pseudo-random numbers | |
| set an image attribute | |
| sharpen the image | |
| shave pixels from the image edges | |
| width and height of image | |
| hide watermark within an image | |
| combine two image to create a stereo anaglyph | |
| strip image of all profiles and comments | |
| swap two images in the image sequence | |
| synchronize image to storage device | |
| mark the image as modified | |
| create a thumbnail of the image | |
| repeat composite operation across and down image | |
| affine transform image | |
| transparent color | |
| color tree depth | |
| image type | |
| the units of image resolution | |
| sharpen the image | |
| print detailed information about the image | |
| print version information | |
| access method for pixels outside the boundaries of the image | |
| percent brightness and saturation of a watermark | |
| chromaticity white point | |
| force all pixels above the threshold into white | |
| write images to this file |
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