我们如何才能使ExtractMpegFramesTest中的saveFrame（）方法更有效？ [英] How can we make the saveFrame() method in ExtractMpegFramesTest more efficient?

问题描述

[edit]在fadden @ suggestion之后重新格式化为问答格式。

[edit] Reformatting into question and answer format following fadden@ suggestion.

在 ExtractMpegFramesTest_egl14.java.txt ，方法saveFrame（），有一个循环用于将RGBA重新排序为ARGB以进行Bitmap png压缩（参见下面的文件引用），这怎么可能是否优化？

In ExtractMpegFramesTest_egl14.java.txt, method saveFrame(), there is a loop for reordering RGBA into ARGB for Bitmap png compression (see below quotes from that file), how can this be optimised?

// glReadPixels gives us a ByteBuffer filled with what is essentially big-endian RGBA
// data (i.e. a byte of red, followed by a byte of green...).  We need an int[] filled
// with little-endian ARGB data to feed to Bitmap.
//

...

// So... we set the ByteBuffer to little-endian, which should turn the bulk IntBuffer
// get() into a straight memcpy on most Android devices.  Our ints will hold ABGR data.
// Swapping B and R gives us ARGB.  We need about 30ms for the bulk get(), and another
// 270ms for the color swap.

...

for (int i = 0; i < pixelCount; i++) {
    int c = colors[i];
    colors[i] = (c & 0xff00ff00) | ((c & 0x00ff0000) >> 16) | ((c & 0x000000ff) << 16);
}

推荐答案

事实证明这是一个平等的更快的方法。

It turns out there's an even faster approach.

使用@ elmiguelao答案中的建议，我修改了片段着色器来进行像素交换。这允许我从saveFrame（）中删除交换代码。由于我不再需要内存中像素的临时副本，因此我完全取消了 int [] 缓冲区，从此切换：

Using the suggestion in @elmiguelao's answer, I modified the fragment shader to do the pixel swap. This allowed me to remove the swap code from saveFrame(). Since I no longer needed a temporary copy of the pixels in memory, I eliminated the int[] buffer entirely, switching from this:

int[] colors = [... copy from mPixelBuf, swap ...]
Bitmap.createBitmap(colors, mWidth, mHeight, Bitmap.Config.ARGB_8888);

到此：

Bitmap bmp = Bitmap.createBitmap(mWidth, mHeight, Bitmap.Config.ARGB_8888);
bmp.copyPixelsFromBuffer(mPixelBuf);

一旦我这样做，我的所有颜色都错了。

As soon as I did that, all of my colors were wrong.

事实证明 Bitmap＃copyPixelsFromBuffer（）想要RGBA顺序的像素，不 ARGB顺序。来自 glReadPixels（）的值已经采用正确的格式。因此，通过这种方式，我避免交换，避免不必要的副本，并且根本不需要调整片段着色器。

It turns out that Bitmap#copyPixelsFromBuffer() wants the pixels in RGBA order, not ARGB order. The values coming out of glReadPixels() are already in the right format. So by doing it this way I avoid the swap, avoid an unnecessary copy, and don't need to tweak the fragment shader at all.

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