1 |
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2 | /**
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3 | * Associates a canvas to a given image, containing a number of renderings
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4 | * of the image at various sizes.
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5 | *
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6 | * This technique is known as 'mipmapping'.
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7 | *
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8 | * NOTE: Images can also be of type 'data:svg+xml`. This code also works
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9 | * for svg, but the mipmapping may not be necessary.
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10 | *
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11 | * @param {Image} image
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12 | */
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13 | class CachedImage {
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14 | /**
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15 | * @ignore
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16 | */
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17 | constructor() { // eslint-disable-line no-unused-vars
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18 | this.NUM_ITERATIONS = 4; // Number of items in the coordinates array
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19 |
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20 | this.image = new Image();
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21 | this.canvas = document.createElement('canvas');
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22 | }
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23 |
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24 |
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25 | /**
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26 | * Called when the image has been successfully loaded.
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27 | */
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28 | init() {
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29 | if (this.initialized()) return;
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30 |
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31 | this.src = this.image.src; // For same interface with Image
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32 | var w = this.image.width;
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33 | var h = this.image.height;
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34 |
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35 | // Ease external access
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36 | this.width = w;
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37 | this.height = h;
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38 |
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39 | var h2 = Math.floor(h/2);
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40 | var h4 = Math.floor(h/4);
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41 | var h8 = Math.floor(h/8);
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42 | var h16 = Math.floor(h/16);
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43 |
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44 | var w2 = Math.floor(w/2);
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45 | var w4 = Math.floor(w/4);
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46 | var w8 = Math.floor(w/8);
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47 | var w16 = Math.floor(w/16);
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48 |
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49 | // Make canvas as small as possible
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50 | this.canvas.width = 3*w4;
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51 | this.canvas.height = h2;
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52 |
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53 | // Coordinates and sizes of images contained in the canvas
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54 | // Values per row: [top x, left y, width, height]
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55 |
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56 | this.coordinates = [
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57 | [ 0 , 0 , w2 , h2],
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58 | [ w2 , 0 , w4 , h4],
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59 | [ w2 , h4, w8 , h8],
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60 | [ 5*w8, h4, w16, h16]
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61 | ];
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62 |
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63 | this._fillMipMap();
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64 | }
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65 |
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66 |
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67 | /**
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68 | * @return {Boolean} true if init() has been called, false otherwise.
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69 | */
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70 | initialized() {
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71 | return (this.coordinates !== undefined);
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72 | }
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73 |
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74 |
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75 | /**
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76 | * Redraw main image in various sizes to the context.
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77 | *
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78 | * The rationale behind this is to reduce artefacts due to interpolation
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79 | * at differing zoom levels.
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80 | *
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81 | * Source: http://stackoverflow.com/q/18761404/1223531
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82 | *
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83 | * This methods takes the resizing out of the drawing loop, in order to
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84 | * reduce performance overhead.
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85 | *
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86 | * TODO: The code assumes that a 2D context can always be gotten. This is
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87 | * not necessarily true! OTOH, if not true then usage of this class
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88 | * is senseless.
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89 | *
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90 | * @private
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91 | */
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92 | _fillMipMap() {
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93 | var ctx = this.canvas.getContext('2d');
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94 |
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95 | // First zoom-level comes from the image
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96 | var to = this.coordinates[0];
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97 | ctx.drawImage(this.image, to[0], to[1], to[2], to[3]);
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98 |
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99 | // The rest are copy actions internal to the canvas/context
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100 | for (let iterations = 1; iterations < this.NUM_ITERATIONS; iterations++) {
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101 | let from = this.coordinates[iterations - 1];
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102 | let to = this.coordinates[iterations];
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103 |
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104 | ctx.drawImage(this.canvas,
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105 | from[0], from[1], from[2], from[3],
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106 | to[0], to[1], to[2], to[3]
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107 | );
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108 | }
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109 | }
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110 |
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111 |
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112 | /**
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113 | * Draw the image, using the mipmap if necessary.
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114 | *
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115 | * MipMap is only used if param factor > 2; otherwise, original bitmap
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116 | * is resized. This is also used to skip mipmap usage, e.g. by setting factor = 1
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117 | *
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118 | * Credits to 'Alex de Mulder' for original implementation.
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119 | *
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120 | * @param {CanvasRenderingContext2D} ctx context on which to draw zoomed image
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121 | * @param {Float} factor scale factor at which to draw
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122 | * @param {number} left
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123 | * @param {number} top
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124 | * @param {number} width
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125 | * @param {number} height
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126 | */
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127 | drawImageAtPosition(ctx, factor, left, top, width, height) {
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128 |
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129 | if(!this.initialized())
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130 | return; //can't draw image yet not intialized
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131 |
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132 | if (factor > 2) {
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133 | // Determine which zoomed image to use
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134 | factor *= 0.5;
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135 | let iterations = 0;
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136 | while (factor > 2 && iterations < this.NUM_ITERATIONS) {
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137 | factor *= 0.5;
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138 | iterations += 1;
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139 | }
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140 |
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141 | if (iterations >= this.NUM_ITERATIONS) {
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142 | iterations = this.NUM_ITERATIONS - 1;
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143 | }
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144 | //console.log("iterations: " + iterations);
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145 |
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146 | let from = this.coordinates[iterations];
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147 | ctx.drawImage(this.canvas,
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148 | from[0], from[1], from[2], from[3],
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149 | left, top, width, height
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150 | );
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151 | } else {
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152 | // Draw image directly
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153 | ctx.drawImage(this.image, left, top, width, height);
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154 | }
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155 | }
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156 |
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157 | }
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158 |
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159 |
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160 | export default CachedImage;
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