Opengl es 插值纹理坐标

我正在开发一个OpenGL ES，它使用纹理贴图来保存网格值。我需要通过模拟以下循环的网格进行迭代：

for (int r = 0; r < 128; r++)
   for (int c = 0; c < 128; c++)
      process grid element at (c,r)

顶点着色器仅通过未修改的对象传递位置和纹理坐标。为了了解生成的纹理坐标，我决定将纹理坐标捕捉到图像中。片段程序将其给定的2D纹理坐标指定给输出颜色的红色和绿色通道：

varying vec4 texCoord;
void main() {
  gl_FragColor = vec4(texCoord.st,0,1);
}

然后，我使用

glGetTexImage（）

将纹理图像读回客户机空间。 以下是图像中每个像素的生成纹理坐标（s，t）的示例：

(column, row) = [s, t] -> (128*s, 128*t)

(0,0) = [0.00392157, 0.00392157] -> (0,0)
(1,0) = [0.01176471, 0.00392157] -> (1,0)
(2,0) = [0.01960784, 0.00392157] -> (2,0)
(3,0) = [0.02745098, 0.00392157] -> (3,0)
(4,0) = [0.03529412, 0.00392157] -> (4,0)
(5,0) = [0.04313726, 0.00392157] -> (5,0)
(6,0) = [0.05098040, 0.00392157] -> (6,0)
(7,0) = [0.05882353, 0.00392157] -> (7,0)
(8,0) = [0.06666667, 0.00392157] -> (8,0)
(9,0) = [0.07450981, 0.00392157] -> (9,0)
(10,0) = [0.08235294, 0.00392157] -> (10,0)
<snip>
(125,0) = [0.98039222, 0.00392157] -> (125,0)
(126,0) = [0.98823535, 0.00392157] -> (126,0)
(127,0) = [0.99607849, 0.00392157] -> (127,0)
(0,1) = [0.00392157, 0.01176471] -> (0,1)
(1,1) = [0.01176471, 0.01176471] -> (1,1)
(2,1) = [0.01960784, 0.01176471] -> (2,1)
<snip>
(124,127) = [0.97254908, 0.99607849] -> (124,127)
(125,127) = [0.98039222, 0.99607849] -> (125,127)
(126,127) = [0.98823535, 0.99607849] -> (126,127)
(127,127) = [0.99607849, 0.99607849] -> (127,127)

（列，行）=[s，t]->（128*s，128*t）
(0,0) = [0.00392157, 0.00392157] -> (0,0)
(1,0) = [0.01176471, 0.00392157] -> (1,0)
(2,0) = [0.01960784, 0.00392157] -> (2,0)
(3,0) = [0.02745098, 0.00392157] -> (3,0)
(4,0) = [0.03529412, 0.00392157] -> (4,0)
(5,0) = [0.04313726, 0.00392157] -> (5,0)
(6,0) = [0.05098040, 0.00392157] -> (6,0)
(7,0) = [0.05882353, 0.00392157] -> (7,0)
(8,0) = [0.06666667, 0.00392157] -> (8,0)
(9,0) = [0.07450981, 0.00392157] -> (9,0)
(10,0) = [0.08235294, 0.00392157] -> (10,0)
(125,0) = [0.98039222, 0.00392157] -> (125,0)
(126,0) = [0.98823535, 0.00392157] -> (126,0)
(127,0) = [0.99607849, 0.00392157] -> (127,0)
(0,1) = [0.00392157, 0.01176471] -> (0,1)
(1,1) = [0.01176471, 0.01176471] -> (1,1)
(2,1) = [0.01960784, 0.01176471] -> (2,1)
(124,127) = [0.97254908, 0.99607849] -> (124,127)
(125,127) = [0.98039222, 0.99607849] -> (125,127)
(126,127) = [0.98823535, 0.99607849] -> (126,127)
(127,127) = [0.99607849, 0.99607849] -> (127,127)

现在来回答问题。我试图理解这些生成的坐标。

---

魔法值

0.00392157

是

（0.5*1/127.5）

。我理解0.5因子是一个预加的舍入值，但为什么是127.5<代码>（0.5*1/128.0）会更有意义吗？有人能解释这些坐标吗？我只想从纹理坐标生成整数网格坐标（在OpenGL ES中没有

sampler2Drect

。

我一直在努力解决这个问题，所以让我们看看是否可以解释它

想象你的网格中有一行。在下图中，我对该行中的每个片段（0-127）进行了编号。我也在像素下面放了一些纹理坐标。请注意，最左边的边为0.0，最右边的边为1.0

 +-----------+-----------+-----------+-----------+---    ---+-----------+
 |           |           |           |           |          |           |
 |           |           |           |           |          |           |
 |     0     |     1     |     2     |     3     |   . . .  |    127    |
 |           |           |           |           |          |           |
 |           |           |           |           |          |           |
 +-----------+-----------+-----------+-----------+---    ---+-----------+
 ^           ^           ^           ^                      ^           ^
 |           |           |           |                      |           |
 |           |           |           |                      |           |
0/128       1/128       2/128       3/128                127/128     128/128

当渲染器想要对片段进行纹理处理时，它使用片段中心的纹理坐标，其中的数字为。请注意，片段0的中心位于0/128（aka 0）和1/128（aka.0078125）之间的中间。这是1/256（又名00390625）

因此，我认为这个公式最好表述为：

coordinate = (pixelId + 0.5) / 128

下面是一些python，它们得到了与您类似的答案：

for i in range(128):
   print (0.5 + i) / 128


0.00390625
0.01171875
0.01953125
0.02734375
0.03515625
0.04296875
...
0.97265625
0.98046875
0.98828125
0.99609375

我怀疑我的结果与您的结果之间的差异与以下事实有关：您的值被压缩到一个[8位]颜色通道中，以从着色器返回它们

---

我希望这会有帮助。

你能把这句话弄清楚吗，“想象一下你的128行网格。”？

for i in range(128):
   print (0.5 + i) / 128


0.00390625
0.01171875
0.01953125
0.02734375
0.03515625
0.04296875
...
0.97265625
0.98046875
0.98828125
0.99609375