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SVG in HTML: How to use a multicolored SVG as a mask

I know how to use SVG masks to completely "cut out" the mask in another shape, if the mask is monochrome.
How can I use a multicolored SVG definition X as the mask so that the outer shape of X defines the "hole" to be cut out?
Here are three images that illustrate what I am trying to achieve:
svg #1 to be used as mask
svg #2 on which the outer shape of #1 should be used as a cut-out
result
Creating a white-filled version of the shape as #enxaneta proposed is not applicable to my problem, as I have many "complicated" external SVG definitions, and I don't want to change every single one of them.
Is there another, simpler way to achieve what I want?

You need to define your paths with no fill. Then you use your paths for the mask and fill them with white. To draw the image you fill those paths with the colors of your choice.
svg{border:1px solid; width:49vw}
svg:nth-child(2){background:red;}
mask use{fill:white;}
<svg viewBox="0 0 100 50">
<defs>
<polygon id="a" points="30,5 70,20 75,40 20,20" />
<circle id="b" cx="50" cy="25" r="15" />
<circle id="c" cx="60" cy="35" r="10" />
<mask id="m">
<use xlink:href="#a"/>
<use xlink:href="#b"/>
<use xlink:href="#c"/>
</mask>
</defs>
<g id="complexShape">
<use xlink:href="#a" fill="lightblue" />
<use xlink:href="#b" fill="gold"/>
<use xlink:href="#c" fill="red"/>
</g>
</svg>
<svg viewBox="0 0 100 50">
<rect width="100" height="50" style="mask: url(#m)" />
</svg>

The colour of a mask determines the final opacity of the masked object at that point. The R, G, B, and A components of the mask colour are combined in a formula to determine a luminance value that is used to set the final transparency that the mask will be a that point. So, for example, if the mask is red, the final masked result will be semi transparent.
There is no way to make a coloured object be a solid (not translucent) mask. Only full white will do that.
Update
Assuming you have an external SVG image that looks like the following:
<svg viewBox="0 0 100 50">
<polygon id="a" points="30,5 70,20 75,40 20,20" fill="lightblue"/>
<circle id="b" cx="50" cy="25" r="15" fill="gold"/>
<circle id="c" cx="60" cy="35" r="10" fill="red" stroke="blue" stroke-width="4"/>
</svg>
You can turn this into a "mask" version by adding three lines to the start of your SVG.
<svg viewBox="0 0 100 50">
<filter id="blacken"><feFlood flood-color="black"/><feComposite operator="in" in2="SourceGraphic"/></filter>
<style>svg :not(#maskbg) { filter: url(#blacken); }</style>
<rect id="maskbg" x="-100%" y="-100%" width="300%" height="300%" fill="white"/>
<polygon id="a" points="30,5 70,20 75,40 20,20" fill="lightblue"/>
<circle id="b" cx="50" cy="25" r="15" fill="gold"/>
<circle id="c" cx="60" cy="35" r="10" fill="red" stroke="blue" stroke-width="4"/>
</svg>
This is something that could easily be scripted. This method should work for almost all SVGs.
Once you have all the mask variants built, you can apply them using mask-image.
https://developer.mozilla.org/en-US/docs/Web/CSS/mask-image

Center elements inside SVG relative to each other-

Couldn't find it anywhere else to I might ask here :
I have svg rectangle which inside has other rectangles.
What I want to achieve is to center each of these rectangles in the centre of it's parent and in relation to each other.
If I drag rectangle 1 down I want the other one to move up to keep both of them centered - and same thing happening if I drag the other element down (should push upper one up).
Problem here is that I might have different width/heights and there would be 2 or more elements. Is there any mathematical equationfor that? Or a name that I can look for?

I would put the 3 rects inside a group and use the group as in the following example
svg{
border:1px solid;
width: 30vh;
}
<svg viewBox="0 0 100 280">
<defs>
<g id="rects">
<rect width="80" height="80" />
<rect width="60" height="25" x="10" y="10" />
<rect width="60" height="25" x="10" y="45" />
</g>
</defs>
<use xlink:href="#rects" x="10" y="10" stroke="black" fill="none" />
<use xlink:href="#rects" x="10" y="100" stroke="black" fill="none" />
<use xlink:href="#rects" x="10" y="190" stroke="black" fill="none" />
</svg>

Can I add a mask to an svg-element without using an id?

I want to assign a svg-mask to a svg-image. I can make this work using an id on the mask like this:
<svg id="svg1" width="5cm" height="5cm" viewBox="0 0 200 200"
xmlns="http://www.w3.org/2000/svg" version="1.1">
<defs>
<mask id="mask">
<circle cx="100" cy="100" r="100" fill="white"></circle>
</mask>
</defs>
<rect x="0" y="0" width="200" height="200" fill="red" mask="url(#mask)"></rect>
</svg>
However I want to load this svg multiple times, with a different id in the svg-tag. Therefore I will generate duplicates of the '#mask'-id. Using multiple id's is invalid code. So I want to use a class to refer to the appropriate mask. That means I cannot use the mask=url()-technique.
<svg id="svg2" width="5cm" height="5cm" viewBox="0 0 200 200"
xmlns="http://www.w3.org/2000/svg" version="1.1">
<defs>
<mask class="mask">
<circle cx="100" cy="100" r="100" fill="white"></circle>
</mask>
</defs>
<rect x="0" y="0" width="200" height="200" fill="red" mask="url(can't use this)"></rect>
</svg>
Is there a way I can apply a mask to the rect element if the mask has a class instead of id? Maybe using javaScript or some other way I didn't think of.
The full story/context:
I am actually making an svg image slider-module for Joomla with php. This php generates a module containing javascript, css and an svg. I use the javascript to animate the mask.
I do actually have it working with unique id's. I was just wondering if there is a way to assign a mask to an element without referring to id's. I may want to do this because my code is getting a bit more confusing to read, because I have to use some php in my javascript/svg and css for each unique id.

No. You can only reference masks via an id. You cannot reference SVG masks any other way.

According to your description I understand you have a identical grafical entity you want to mask with different forms, multiple times. Write that down DRY:
<!-- start with an invisible svg that only contains mask definitions -->
<svg width="0" height="0"
xmlns="http://www.w3.org/2000/svg">
<defs>
<!-- first, you have a circular mask -->
<mask id="circle-mask">
<circle cx="100" cy="100" r="80" fill="white" />
</mask>
<!-- then, you have a different mask, lets say a diamond -->
<mask id="diamond-mask">
<polygon points="100,20 180,100 100,180 20,100" fill="white" />
</mask>
</defs>
</svg>
<!-- further into your document, you want to mask a rectangle -->
<svg id="svg1" width="5cm" height="5cm" viewBox="0 0 200 200"
xmlns="http://www.w3.org/2000/svg">
<!-- reference the circle mask -->
<rect x="0" y="0" width="200" height="200" fill="red" mask="url(#circle-mask)" />
</svg>
<!-- with the circle again, as often as you want, nothing changes -->
<svg id="svg2" width="5cm" height="5cm" viewBox="0 0 200 200"
xmlns="http://www.w3.org/2000/svg">
<!-- the mask is the same, so no difference to above -->
<rect x="0" y="0" width="200" height="200" fill="red" mask="url(#circle-mask)" />
</svg>
<!-- and now with the diamond; that one is different -->
<svg id="svg3" width="5cm" height="5cm" viewBox="0 0 200 200"
xmlns="http://www.w3.org/2000/svg">
<!-- if the mask changes, you need to change the reference -->
<rect x="0" y="0" width="200" height="200" fill="red" mask="url(#diamond-mask)" />
</svg>
You could also reference the masks in a stylesheet and give your referencing elements a class according to the mask shape:
.masked.circular rect {
mask: url(#circle-mask);
}
.masked.diamond rect {
mask: url(#diamond-mask);
}
<svg width="0" height="0"
xmlns="http://www.w3.org/2000/svg">
<defs>
<mask id="circle-mask">
<circle cx="100" cy="100" r="80" fill="white" />
</mask>
<mask id="diamond-mask">
<polygon points="100,20 180,100 100,180 20,100" fill="white" />
</mask>
</defs>
</svg>
<svg id="svg1" class="masked circular" width="5cm" height="5cm" viewBox="0 0 200 200"
xmlns="http://www.w3.org/2000/svg">
<rect x="0" y="0" width="200" height="200" fill="red" />
</svg>
<svg id="svg2" class="masked circular" width="5cm" height="5cm" viewBox="0 0 200 200"
xmlns="http://www.w3.org/2000/svg">
<rect x="0" y="0" width="200" height="200" fill="red" />
</svg>
<svg id="svg1" class="masked diamond" width="5cm" height="5cm" viewBox="0 0 200 200"
xmlns="http://www.w3.org/2000/svg">
<rect x="0" y="0" width="200" height="200" fill="red" />
</svg>

SVG masks with transformation matrices

Well, basically I have a SVG file which coordinates are calculated taking into account the screen resolution. Nonetheless, I have now a process that sends the SVG to another computer and adjusts each object to the resolution of the receiving computer. For that, I use the transformation matrix as you can see in object "OjeB0J0NzcPolyline1507". Everything went well, until now that I have to deal with masks.
Basically, the mask crops the SVG like if the mask was applied before the transformation and since the mask has a specific size, everything that is outside the mask is cropped. Nonetheless, I have already tried to make the svgjsRubberRect bigger and anything seems to work.
Is there any way to adjust the mask so the whole object is shown with exception to what is covered by the element"OjeB0J0NzcPath1519"
EDIT:
Original SVG:
<g id="canvas">
<polyline fill="none" stroke-width="185" stroke="#000000" stroke-opacity="1" stroke-linecap="round" stroke-linejoin="round" class="svg-selectable svg-editable svg-replaceable" mask="url('#zxjzF2hIO8Mask1276')" points="38,64 38,64 37,64 40,62 49,61 61,59 78,57 98,55 122,51 167,43 203,40 245,34 286,34 343,34 399,34 535,35 606,44 678,54 747,65 821,75 895,82 965,97 1036,103 1101,103 1157,103 1206,103 1252,103 1287,103 1322,95 1363,47 1265,27 1207,23 1159,16 1108,16 1062,14 1016,14 969,14 921,14 883,14 850,14 821,9 803,6 792,4 769,1 775,1 788,1 808,1 833,1 868,1 912,1 955,1 1005,6 1062,6 1296,32 1327,114 1250,123 1170,131 1070,134 960,134 844,134 712,134 580,134 432,134 286,134 152,134 12,134 0,134 0,147 0,158 0,168 0,172 8,172 27,172 51,172 81,172 122,172 157,172 260,172 325,172 398,172 487,172 578,172 665,172 846,179 918,189 1064,208 1125,216 1198,228 1213,233 1220,237 1215,245 1196,249 1154,256 1093,266 1016,277 923,284 707,284 592,284 462,284 330,284 190,284 58,284 0,284 0,286 0,277 0,276 0,275 0,276 3,278 9,282 17,283 47,286 74,286 112,286 161,286 220,286 301,286 482,286 590,286 703,286 946,286 1086,286 1337,298 1265,369 1144,369 1016,363 883,347 756,340 523,324 421,308 345,306 253,306 243,306 269,306 323,306 391,306 480,306 574,306 674,306 777,308 895,308 1104,312 1190,314 1263,314 1320,314 1343,314 1286,314 1217,307 1148,296 1085,288 995,275 981,270 974,266 973,266 973,264 977,261 996,256 1152,236 1217,232 1362,221 1335,214 1282,213 1266,213 1254,213 1242,215 1234,220 1232,225 1232,229 1232,233 1307,244 1364,226 1302,226 1226,226 857,226 654,227 557,232 150,276 68,287 11,306 0,331 0,336 0,337 1,337 3,339 10,339 14,341 19,343 21,343 24,343 26,343 28,343 30,342 33,342 53,342 87,339 219,348 288,354 362,365 428,375 500,386 688,412 733,420 788,429 804,437 804,441 768,456 713,466 547,477 72,477 0,477 0,461 0,454 19,454 71,454 128,454 203,454 295,454 406,452 519,448 1038,499 819,552 719,561 511,565 397,565 279,565 176,565 6,565 0,565 14,568 91,568 142,568 215,568 401,568 604,568 703,568 872,575 935,591" id="2HCqPWHVecPolyline1238"></polyline>
</g>
<defs id="zxjzF2hIO8Defs1001">
<rect id="SvgjsRubberRect" width="800" height="1317" fill="#ffffff" stroke="none" class="svg-reserved svg-rubber svg-replaceable"></rect>
<path id="2HCqPWHVecPath1253" d="M212 177L212 177L217 177L235 179L243 179L268 179L280 179L319 163L362 155L386 155L398 155L496 177L507 182L528 187L560 196L611 211L667 221L677 222L721 228L730 228L762 228L794 229 " fill="none" stroke="black" stroke-linejoin="round" stroke-linecap="round" class="svg-reserved svg-rubber svg-replaceable"></path>
<mask id="zxjzF2hIO8Mask1276" maskContentUnits="userSpaceOnUse" maskUnits="userSpaceOnUse" class="svg-replaceable">
<use id="zxjzF2hIO8Use1277" xlink:href="#SvgjsRubberRect"></use>
<use id="zxjzF2hIO8Use1278" xlink:href="#2HCqPWHVecPath1253" stroke="#000000" stroke-width="10" > </use>
</mask>
</defs>
</svg>
Scaled SVG:
<svg id="zxjzF2hIO8Svg1000" xmlns="http://www.w3.org/2000/svg" version="1.1" width="100%" height="100%" xmlns:xlink="http://www.w3.org/1999/xlink">
<g id="canvas">
<polyline transform="matrix(0.312592 0 0 0.462912 0 0)" fill="none" stroke-width="185" stroke="#000000" stroke-opacity="1" stroke-linecap="round" stroke-linejoin="round" class="svg-selectable svg-editable svg-replaceable" mask="url('#zxjzF2hIO8Mask1276')" points="38,64 38,64 37,64 40,62 49,61 61,59 78,57 98,55 122,51 167,43 203,40 245,34 286,34 343,34 399,34 535,35 606,44 678,54 747,65 821,75 895,82 965,97 1036,103 1101,103 1157,103 1206,103 1252,103 1287,103 1322,95 1363,47 1265,27 1207,23 1159,16 1108,16 1062,14 1016,14 969,14 921,14 883,14 850,14 821,9 803,6 792,4 769,1 775,1 788,1 808,1 833,1 868,1 912,1 955,1 1005,6 1062,6 1296,32 1327,114 1250,123 1170,131 1070,134 960,134 844,134 712,134 580,134 432,134 286,134 152,134 12,134 0,134 0,147 0,158 0,168 0,172 8,172 27,172 51,172 81,172 122,172 157,172 260,172 325,172 398,172 487,172 578,172 665,172 846,179 918,189 1064,208 1125,216 1198,228 1213,233 1220,237 1215,245 1196,249 1154,256 1093,266 1016,277 923,284 707,284 592,284 462,284 330,284 190,284 58,284 0,284 0,286 0,277 0,276 0,275 0,276 3,278 9,282 17,283 47,286 74,286 112,286 161,286 220,286 301,286 482,286 590,286 703,286 946,286 1086,286 1337,298 1265,369 1144,369 1016,363 883,347 756,340 523,324 421,308 345,306 253,306 243,306 269,306 323,306 391,306 480,306 574,306 674,306 777,308 895,308 1104,312 1190,314 1263,314 1320,314 1343,314 1286,314 1217,307 1148,296 1085,288 995,275 981,270 974,266 973,266 973,264 977,261 996,256 1152,236 1217,232 1362,221 1335,214 1282,213 1266,213 1254,213 1242,215 1234,220 1232,225 1232,229 1232,233 1307,244 1364,226 1302,226 1226,226 857,226 654,227 557,232 150,276 68,287 11,306 0,331 0,336 0,337 1,337 3,339 10,339 14,341 19,343 21,343 24,343 26,343 28,343 30,342 33,342 53,342 87,339 219,348 288,354 362,365 428,375 500,386 688,412 733,420 788,429 804,437 804,441 768,456 713,466 547,477 72,477 0,477 0,461 0,454 19,454 71,454 128,454 203,454 295,454 406,452 519,448 1038,499 819,552 719,561 511,565 397,565 279,565 176,565 6,565 0,565 14,568 91,568 142,568 215,568 401,568 604,568 703,568 872,575 935,591" id="2HCqPWHVecPolyline1238"></polyline>
</g>
<defs id="zxjzF2hIO8Defs1001">
<rect id="SvgjsRubberRect" width="800" height="1317" fill="#ffffff" stroke="none" class="svg-reserved svg-rubber svg-replaceable"></rect>
<path id="2HCqPWHVecPath1253" d="M212 177L212 177L217 177L235 179L243 179L268 179L280 179L319 163L362 155L386 155L398 155L496 177L507 182L528 187L560 196L611 211L667 221L677 222L721 228L730 228L762 228L794 229 " fill="none" stroke="black" stroke-linejoin="round" stroke-linecap="round" class="svg-reserved svg-rubber svg-replaceable" transform="matrix(0.312592 0 0 0.462912 0 0)"></path>
<mask id="zxjzF2hIO8Mask1276" maskContentUnits="userSpaceOnUse" maskUnits="userSpaceOnUse" class="svg-replaceable">
<use id="zxjzF2hIO8Use1277" xlink:href="#SvgjsRubberRect"></use>
<use id="zxjzF2hIO8Use1278" xlink:href="#2HCqPWHVecPath1253" stroke="#000000" stroke-width="10" transform="matrix(3.19906 0 0 2.16024 0 0)"> </use>
</mask>
</defs>
</svg>

Fill complete image in svg container

I am new on stackoverflow.
I face a problem in svg code. I want to draw a container with a background image but when I set an image it breaks into 4 parts and gives a white space in mid of container.
This is my SVG code:
<svg id="do-as-cards" xmlns="http://www.w3.org/2000/svg" version="1.1" xmlns:xlink="http://www.w3.org/1999/xlink" viewbox="0,0,320,340" preserveAspectRatio="xMidYMin">
<defs>
<pattern id="imgDo" preserveAspectRatio="true" patternUnits="userSpaceOnUse" y="0" x="0" width="240" height="120" >
<image xlink:href="http://s22.postimg.org/ekd89tb8x/image.png" x="0" y="0" width="407px" height="220px" />
</pattern>
<pattern id="imgAs" preserveAspectRatio="true" patternUnits="userSpaceOnUse" y="0" x="0" width="240" height="120" >
<image xlink:href="http://s22.postimg.org/72zfguwc1/image.png" x="0" y="0" width="407px" height="220px" />
</pattern>
</defs>
<g transform="translate(160,86)">
<g id="doCard" class="animatedCard" transform="matrix(1 0 0 1 0 0)" onclick="spin()">
<path class="cardOutline" d="m -150,-60 c 0,-10 10,-20 20,-20 l260,0 c 10,0 20,10 20,20 l 0,120 c 0,10 -10,20 -20,20 l -260,0 c -10,0 -20,-10 -20,-20 l 0,-120 z" />
<foreignObject id="do" class="cardFace" x="-120" y="-60" width="240" height="120"></foreignObject>
</g>
</g>
<g transform="translate(160,253)">
<g id="asCard" class="animatedCard" transform="matrix(1 0 0 1 0 0)" onclick="spin()">
<path class="cardOutline" id="as_path" d="m -150,-60 c 0,-10 10,-20 20,-20 l260,0 c 10,0 20,10 20,20 l 0,120 c 0,10 -10,20 -20,20 l -260,0 c -10,0 -20,-10 -20,-20 l 0,-120 z"/>
<foreignObject id="as" class="cardFace" x="-120" y="-60" width="240" height="120"></foreignObject>
</g>
</g>
</svg>
You can see this code in running stage by using this url
I have already tried the following:
How to set a SVG rect element's background image?
Fill SVG path element with a background-image

Using a <pattern> may not be the best way to do what you want. It can be done though.
If you are using a pattern, stick to the default patternUnits (objectBoundingBox), and set width and height to 1. Then set the width and height of your image to the max width or height of the region you are trying to fill. In the case of your example shapes, that appears to be 300. Then adjust the x and y of the <image> so it is centred in your shape.
<pattern id="imgDo" y="0" x="0" width="1" height="1" >
<image xlink:href="http://s22.postimg.org/ekd89tb8x/image.png" x="0" y="-75" width="300" height="300" />
</pattern>
Demo: http://jsfiddle.net/TRLa7/1/
Personally, I would use a <clipPath> for this situation. Use your path shape as the clipPath for the image. You will need to add an additional copy of the <path> to apply your stroke effects etc. You can define your card(s) in the <defs> section and then use a <use> to instantiate each card.
Demo: http://jsfiddle.net/TRLa7/2/