清除 NSImage 的 Alpha 通道

发布于 2024-08-07 14:25:51 字数 742 浏览 10 评论 0 原文

可以通过分配一个每像素 32 位的临时位图来完成然后用 for 循环清除 alpha 分量最后再次将其转回 NSImage。

我怀疑可以使用聪明的方法以更简单的方式完成 NSColor 和 NSCompositingOperation。或者也许是图像需要使用drawAtPoint与其自身合成。

我的代码看起来像这样。

NSImage* img = some image with RGB and Alpha;
NSRect rect  = some rect inside the image;
[img lockFocus];
[[NSColor clearColor] set];
NSRectFillUsingOperation(rect, NSCompositeXOR);
[img unlockFocus];

注意：将 alpha 通道设置为 1 可以通过以下方式完成使用带有 NSCompositePlusLighter 的 blackColor。

清除阿尔法通道的秘诀是什么？

原文

It can be done by mallocing a temporary bitmap with 32bits per pixel
and then clearing the alpha component with a for loop and
and finally turn it back into a NSImage again.

I suspect is can be done in a simpler way using a clever
combination of NSColor and NSCompositingOperation. Or perhaps the image
needs to be composited with itself using drawAtPoint.

My code looks like this.

NSImage* img = some image with RGB and Alpha;
NSRect rect  = some rect inside the image;
[img lockFocus];
[[NSColor clearColor] set];
NSRectFillUsingOperation(rect, NSCompositeXOR);
[img unlockFocus];

NOTE: Setting the alpha channel to 1 can be done by
using a blackColor with NSCompositePlusLighter.

What is the secret in clearing the alpha channel?

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陪你搞怪i 2024-08-14 14:25:51

它不会很快，但也可以：

NSImage *newImage = [[NSImage alloc] initWithSize:[srcImage size]];
[newImage lockFocus];
[[NSColor whiteColor] set];
NSRectFill(NSMakeRect(0,0,[newImage size].width, [newImage size].height));
[srcImage compositeToPoint:NSZeroPoint operation:NSCompositeCopy];
[newImage unlockFocus];

It won't be fast but this will work as well:

NSImage *newImage = [[NSImage alloc] initWithSize:[srcImage size]];
[newImage lockFocus];
[[NSColor whiteColor] set];
NSRectFill(NSMakeRect(0,0,[newImage size].width, [newImage size].height));
[srcImage compositeToPoint:NSZeroPoint operation:NSCompositeCopy];
[newImage unlockFocus];

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