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 | |
| -alpha | on, activate, off, deactivate, set, opaque, copy", transparent, extract, background, or shape the alpha channel |
| -authenticate value | decrypt image with this password |
| -blend geometry | blend images |
| -blue-primary point | chromaticity blue primary point |
| -border geometry | surround image with a border of color |
| -bordercolor color | border color |
| -channel type | apply option to select image channels |
| -colors value | preferred number of colors in the image |
| -colorspace type | set image colorspace |
| -comment string | annotate image with comment |
| -compose operator | set image composite operator |
| -compress type | image compression type |
| -debug events | display copious debugging information |
| -decipher filename | convert cipher pixels to plain |
| -define format:option | define one or more image format options |
| -density geometry | horizontal and vertical density of the image |
| -depth value | image depth |
| -displace geometry | shift image pixels defined by a displacement map |
| -dissolve value | dissolve the two images a given percent |
| -dither method | apply error diffusion to image |
| -encipher filename | convert plain pixels to cipher pixels |
| -encoding type | text encoding type |
| -endian type | endianness (MSB or LSB) of the image |
| -extract geometry | extract area from image |
| -filter type | use this filter when resizing an image |
| -font name | render text with this font |
| -geometry geometry | preferred size or location of the image |
| -gravity type | horizontal and vertical text placement |
| -green-primary point | chromaticity green primary point |
| -help | print program options |
| -identify | identify the format and characteristics of the image |
| -interlace type | type of image interlacing scheme |
| -interpolate method | pixel color interpolation method |
| -label string | assign a label to an image |
| -level value | adjust the level of image contrast |
| -limit type value | pixel cache resource limit |
| -log format | format of debugging information |
| -monitor | monitor progress |
| -monochrome | transform image to black and white |
| -negate | replace each pixel with its complementary color |
| -page geometry | size and location of an image canvas (setting) |
| -pointsize value | font point size |
| -profile filename | add, delete, or apply an image profile |
| -quality value | JPEG/MIFF/PNG compression level |
| -quantize colorspace | reduce image colors in this colorspace |
| -quiet | suppress all warning messages |
| -red-primary point | chromaticity red primary point |
| -regard-warnings | pay attention to warning messages. |
| -respect-parentheses | settings remain in effect until parenthesis boundary. |
| -rotate degrees | apply Paeth rotation to the image |
| -sampling-factorgeometry | horizontal and vertical sampling factor |
| -scene value | image scene number |
| -seed value | seed a new sequence of pseudo-random numbers |
| -set attribute value | set an image attribute |
| -sharpen geometry | sharpen the image |
| -shave geometry | shave pixels from the image edges |
| -size geometry | width and height of image |
| -stegano offset | hide watermark within an image |
| -stereo geometry | combine two image to create a stereo anaglyph |
| -strip | strip image of all profiles and comments |
| -swap indexes | swap two images in the image sequence |
| -synchronize | synchronize image to storage device |
| -taint | mark the image as modified |
| -thumbnail geometry | create a thumbnail of the image |
| -tile | repeat composite operation across and down image |
| -transform | affine transform image |
| -transparent-colorcolor | transparent color |
| -treedepth value | color tree depth |
| -type type | image type |
| -units type | the units of image resolution |
| -unsharp geometry | sharpen the image |
| -verbose | print detailed information about the image |
| -version | print version information |
| -virtual-pixel method | access method for pixels outside the boundaries of the image |
| -watermarkgeometry | percent brightness and saturation of a watermark |
| -white-point point | chromaticity white point |
| -white-thresholdvalue | force all pixels above the threshold into white |
| -write filename | write images to this file |