Processing: AlphaBlend

Date
2007-04-05 (Thu)
Category
Processing

Processing を久々にやっているので、ピクセル処理でいつも忘れるのでメモ。

image() っていつも思うが、なぜキャンバスに直接しかかけないのだろうか。 PImage 同士の合成ができてもいいじゃん！という事で書いたのがこれです。ポイントは AlphaBlend でしょうか…

void compositeImage (PImage top, int x, int y, PImage base) {
  int curX = 0;
  int curY = 0;
  for (int i = 0; i < top.pixels.length; i++) {
    curX = x + iToX(i, top.width);
    curY = y + iToY(i, top.width);
    if ((curX < base.width) && (curY < base.height)) {
      base.pixels[xyToI(curX, curY, base.width)] = blendByPixel(top.pixels[i], base.pixels[xyToI(curX, curY, base.width)]);
    }
  }
}

int blendByPixel (int top, int bottom) {
  int result = 0;
  int tA = (0xFF000000 & top) >>> 24;

  if (tA == 255) {
    result = top;
  } else if (tA != 0) {
    result = alphaBlend(top, bottom, tA);
  } else {
    result = bottom;
  }
  return result;
}

int alphaBlend(int fg, int bg, int a) {

  int fR = (0x00FF0000 & fg) >>> 16;
  int fG = (0x0000FF00 & fg) >>> 8;
  int fB =  0x000000FF & fg;

  int bR = (0x00FF0000 & bg) >>> 16;
  int bG = (0x0000FF00 & bg) >>> 8;
  int bB =  0x000000FF & bg;

  int rR = (((fR - bR) * a) >>> 8 ) + bR;
  int rG = (((fG - bG) * a) >>> 8 ) + bG;
  int rB = (((fB - bB) * a) >>> 8 ) + bB;

  return 0xFF000000 | (rR << 16) | (rG << 8) | rB;
}

int iToX (int i, int w) {
  return i % w;
}

int iToY (int i, int w) {
  return i / w;
}

int xyToI (int x, int y, int w) {
  return (w * y) + x;
}

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