Question

有没有办法在Google Web Toolkit中进行图像颜色过滤？像使用Action Script 3的Color Matrix Filter之类的东西。这是动作脚本示例： AS3:How to change a colored Bitmap's BitmapData to black and white?

Answer 1

对于HTML（以及GWT）中的图像处理，您需要使用canvas元素。据我所知，canvas不会为您提供应用过滤器的任何快捷方式，您需要手动进入并修改像素。我们以黑白为例。在调用此方法之前，您需要确保图像已经加载：

public void toBlackAndWhite(Image image)
{
    // Create an off-screen canvas element to do the work.
    Canvas canvas = Canvas.createIfSupported();

    // Do nothing if canvas is not supported.
    if (canvas == null)
        return;

    // Size the canvas to fit the image.
    canvas.setCoordinateSpaceHeight(image.getHeight());
    canvas.setCoordinateSpaceWidth(image.getWidth());

    // Pull the image's underlying DOM element
    ImageElement img = ImageElement.as(image.getElement());

    // The 2D context does all the drawing work on the canvas
    Context2d context = canvas.getContext2d();

    context.drawImage(img, 0, 0); // Now the canvas contains the image.

    // ImageData represents the canvas rgba data as an array of bytes.
    ImageData imageData = context.getImageData(0, 0,
                              image.getWidth(), image.getHeight());

    // Now the real work:
    for (int x = 0; x < imageData.getWidth(); ++x) {
        for (int y = 0; y < imageData.getHeight(); ++y) {
             // RGB values are 0-255
             int average = (imageData.getRedAt(x,y) +
                            imageData.getGreenAt(x,y) +
                            imageData.getBlueAt(x,y)) / 3;

             imageData.setRedAt(average, x,y);
             imageData.setGreenAt(average, x,y);
             imageData.setBlueAt(average, x,y);
         }
    }

    // ImageData is a copy of the data on the canvas, so
    // we need to write it back.
    context.putImageData(imageData,0,0);

    // Now the canvas contains a black and white version
    // of the image. Canvas is a Widget so you could attach
    // it to the page directly if you want. Here we're going
    // to replace the contents of the original image element
    // with a url-encoded version of the canvas contents.
    image.setUrl(canvas.toDataUrl());
}

不像动作一样优雅，但它会完成这项工作。您可以想象，如果您需要更好的性能，可能会在较大的图像上占用大量的处理器时间gwtgl。

GWT中的颜色过滤

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