使用glTexImage2D的奇怪结果 [英] Odd results using glTexImage2D

问题描述

我一直在试图弄清楚 glTexImage2D 是如何工作的，并从一些漂亮的代码看到一些奇怪的结果。我的代码只是绘制一个粗糙的圆形到一个256 * 256长度的无符号数组，然后发送出来的数据成为一个纹理。然而，显示的纹理会变成红色和橙色的变化，无论我在图像创建循环中选择什么组合：

I've been trying to figure out how glTexImage2D works and an seeing some odd results from some pretty clear-cut code. My code simply draws a rough circle into a 256*256 length unsigned array and then sends that data out to become a texture. The texture displayed however is turning out as variations of red and orange no matter what combinations I select inside my image creation loop:

unsigned* data = new unsigned[256*256];
for (int y = 0; y < 256; ++y)
    for (int x = 0; x < 256; ++x)
        if ((x - 100)*(x - 100) + (y - 156)*(y - 156) < 75*75)
            data[256*y + x] = ((156 << 24) | (256 << 16) | (156 << 8) | (200 << 0));
        else
            data[256*y + x] = 0;  // I'd expect this to be transparent and the above to be slightly transparent and green, but it's red somehow.

glBindTexture(GL_TEXTURE_2D, texid);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 256, 256, 0, GL_RGBA, GL_UNSIGNED_BYTE, (GLvoid*)data);

OpenGL选项：

    glEnable(GL_TEXTURE_2D);
glShadeModel(GL_SMOOTH);
glClearColor(0.0f, 0.0f, 0.0f, 0.5f);
glClearDepth(1.0f);
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LEQUAL);
glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);
//glBlendFunc(GL_SRC_ALPHA, GL_ONE);
//glEnable(GL_BLEND);
//glDisable(GL_CULL_FACE);

glGenTextures(1, &leaf[0]);
    createLeaf(leaf[0]);  // createLeaf(GLuint& texid) is posted entirely above

在窗口中的单个四边形上的纹理。 （x64 win7）

The rest of the code does nothing but display the texture on a single quad in a window. (x64 win7)

编辑：我试过Rickard的解决方案，我还得到一个紫色的圆圈。

I tried Rickard's solution exactly and I'm still getting a purple circle.

推荐答案

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 256, 256, 0, GL_RGBA, GL_UNSIGNED_BYTE, (GLvoid*)data);

首先是正面的东西。您使用大小的内部格式（GL_RGBA8，而不是GL_RGBA）。这是非常好的;继续这样做。您清楚地了解内部格式（GL_RGBA8）和像素传输格式（GL_RGBA）之间的差异。这也很好。

First the positive things. You use a sized internal format (GL_RGBA8, rather than GL_RGBA). This is very good; keep doing that. You have a clear understanding of the difference between the internal format (GL_RGBA8) and the pixel transfer format (GL_RGBA). This is also good.

问题是这样的。你告诉OpenGL你的数据是一个无符号字节流。但它不是无符号字节流;它是一个无符号整数流。这是你如何声明 data ，这是你填写数据的方式。那么你为什么要说OpenGL？

The problem is this. You told OpenGL that your data was a stream of unsigned bytes. But it's not a stream of unsigned bytes; it's a stream of unsigned integers. That's how you declared data, that's how you filled data. So why are you lying to OpenGL?

问题是你的颜色。这是您的颜色值之一：

The problem is with your colors. This is one of your color values:

 ((156 << 24) | (256 << 16) | (156 << 8) | (200 << 0))

256不是有效的颜色。 256的十六进制是0x100，这是两个字节，而不是一个。

First, 256 is not a valid color. 256 in hex is 0x100, which is two bytes, not one.

从这里得到的无符号整数是：

The unsigned integer you would get from this is:

 0x9D009CC8

如果这些都是RGBA颜色的顺序，那么红色是0x9D，绿色是0x00，蓝色是0x9C，alpha是0xC8。

If these are intended to be RGBA colors in that order, then the red is 0x9D, green is 0x00, blue is 0x9C, and alpha is 0xC8.

现在， -endian计算机，这4个字节存储翻转，如下：

Now, because you're probably working on a little-endian computer, that 4 bytes is stored flipped, like this:

 0xC89C009D

当你告诉OpenGL假设这是一个字节数组（它不是），你就失去了小端序转换。所以OpenGL看到以0xC8开头的字节数组，所以这是红色值。等等。

When you tell OpenGL to pretend that this is a byte array (which it is not), you are losing the little-endian conversion. So OpenGL sees the byte array starting with 0xC8, so that is the red value. And so on.

你需要告诉OpenGL你实际上在做什么：你在一个无符号的32位整数中存储四个8位无符号值。为此，请使用以下命令：

You need to tell OpenGL what you're actually doing: you're storing four 8-bit unsigned values in a single unsigned 32-bit integer. To do this, use the following:

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 256, 256, 0, GL_RGBA, GL_UNSIGNED_INT_8_8_8_8, (GLvoid*)data);

GL_UNSIGNED_INT_8_8_8_8表示您正在向OpenGL提供一个无符号32位整数数组）。 32位整数的前8位是红色，第二个是绿色，第三个是蓝色，第四个是阿尔法。

The GL_UNSIGNED_INT_8_8_8_8 says that you're feeding OpenGL an array of unsigned 32-bit integers (which you are). The first 8-bits of the 32-bit integer is red, the second is green, the third is blue, and the fourth is alpha.

您的代码，您需要这样：

So, to completely fix your code, you need this:

GLuint* data = new GLuint[256*256]; //Use OpenGL's types
for (int y = 0; y < 256; ++y)
    for (int x = 0; x < 256; ++x)
        if ((x - 100)*(x - 100) + (y - 156)*(y - 156) < 75*75)
            data[256*y + x] = ((0x9C << 24) | (0xFF << 16) | (0x9C << 8) | (0xC8 << 0));
        else
            data[256*y + x] = 0;  // I'd expect this to be transparent and the above to be slightly transparent and green, but it's red somehow.

glBindTexture(GL_TEXTURE_2D, texid);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_BASE_LEVEL, 0);  //Always set the base and max mipmap levels of a texture.
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAX_LEVEL, 0);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 256, 256, 0, GL_RGBA, GL_UNSIGNED_INT_8_8_8_8, (GLvoid*)data);

---

---

// I'd expect this to be transparent and the above to be slightly transparent and green, but it's red somehow.

Alpha并不意味着这是透明的， 透明;它意味着没有什么，除非你给它一个意义。 Alpha只表示透明度，如果你使用混合和设置一个混合模式，导致低alpha使事情透明。否则，它根本就没有。

Alpha doesn't mean transparent; it means nothing at all unless you give it a meaning. Alpha only represents transparency if you use blending and set up a blending mode that causes a low alpha to make things transparent. Otherwise, it means nothing at all.

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