問題描述
更改 BufferedImage 的 alpha 值？ (Change the alpha value of a BufferedImage?)
How do I change the global alpha value of a BufferedImage in Java? (I.E. make every pixel in the image that has a alpha value of 100 have a alpha value of 80)
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參考解法
方法 1:
<p>@Neil Coffey:
Thanks, I've been looking for this too; however, Your code didn't work very well for me (white background became black).</p>
I coded something like this and it works perfectly:
public void setAlpha(byte alpha) {
alpha %= 0xff;
for (int cx=0;cx<obj_img.getWidth();cx++) {
for (int cy=0;cy<obj_img.getHeight();cy++) {
int color = obj_img.getRGB(cx, cy);
int mc = (alpha << 24) | 0x00ffffff;
int newcolor = color & mc;
obj_img.setRGB(cx, cy, newcolor);
}
}
}
Where obj_img is BufferedImage.TYPE_INT_ARGB.
I change alpha with setAlpha((byte)125); alpha range is now 0‑255.
Hope someone finds this useful.
方法 2:
I don't believe there's a single simple command to do this. A few options:
- copy into another image with an AlphaComposite specified (downside: not converted in place)
- directly manipulate the raster (downside: can lead to unmanaged images)
- use a filter or BufferedImageOp
The first is the simplest to implement, IMO.
方法 3:
This is an old question, so I'm not answering for the sake of the OP, but for those like me who find this question later.
AlphaComposite
As @Michael's excellent outline mentioned, an AlphaComposite operation can modify the alpha channel. But only in certain ways, which to me are somewhat difficult to understand:
is the formula for how the "over" operation affects the alpha channel. Moreover, this affects the RGB channels too, so if you have color data that needs to be unchanged, AlphaComposite is not the answer.
BufferedImageOps
LookupOp
There are several varieties of BufferedImageOp (see 4.10.6 here). In the more general case, the OP's task could be met by a LookupOp, which requires building lookup arrays. To modify only the alpha channel, supply an identity array (an array where table[i] = i) for the RGB channels, and a separate array for the alpha channel. Populate the latter array with table[i] = f(i), where f() is the function by which you want to map from old alpha value to new. E.g. if you want to "make every pixel in the image that has a alpha value of 100 have a alpha value of 80", set table[100] = 80. (The full range is 0 to 255.) See how to increase opacity in gaussian blur for a code sample.
RescaleOp
But for a subset of these cases, there is a simpler way to do it, that doesn't require setting up a lookup table. If f() is a simple, linear function, use a RescaleOp. For example, if you want to set newAlpha = oldAlpha ‑ 20, use a RescaleOp with a scaleFactor of 1 and an offset of ‑20. If you want to set newAlpha = oldAlpha * 0.8, use a scaleFactor of 0.8 and an offset of 0. In either case, you again have to provide dummy scaleFactors and offsets for the RGB channels:
new RescaleOp({1.0f, 1.0f, 1.0f, /* alpha scaleFactor */ 0.8f},
{0f, 0f, 0f, /* alpha offset */ ‑20f}, null)
Again see 4.10.6 here for some examples that illustrate the principles well, but are not specific to the alpha channel.
Both RescaleOp and LookupOp allow modifying a BufferedImage in‑place.
方法 4:
for a nicer looking alpha change effect, you can use relative alpha change per pixel (rather than static set, or clipping linear)
public static void modAlpha(BufferedImage modMe, double modAmount) {
//
for (int x = 0; x < modMe.getWidth(); x++) {
for (int y = 0; y < modMe.getHeight(); y++) {
//
int argb = modMe.getRGB(x, y); //always returns TYPE_INT_ARGB
int alpha = (argb >> 24) & 0xff; //isolate alpha
alpha *= modAmount; //similar distortion to tape saturation (has scrunching effect, eliminates clipping)
alpha &= 0xff; //keeps alpha in 0‑255 range
argb &= 0x00ffffff; //remove old alpha info
argb |= (alpha << 24); //add new alpha info
modMe.setRGB(x, y, argb);
}
}
}
方法 5:
I'm 99% sure the methods that claim to deal with an "RGB" value packed into an int actually deal with ARGB. So you ought to be able to do something like:
for (all x,y values of image) {
int argb = img.getRGB(x, y);
int oldAlpha = (argb >>> 24);
if (oldAlpha == 100) {
argb = (80 << 24) | (argb & 0xffffff);
img.setRGB(x, y, argb);
}
}
For speed, you could maybe use the methods to retrieve blocks of pixel values.
(by William、Felix、Michael Brewer‑Davis、LarsH、Thumbz、Neil Coffey)