顶点着色器中的纹理查找在iPad设备与iPad模拟器 - OpenGL ES 2.0上表现不同 [英] Texture lookup in vertex shader behaves differently on iPad device vs iPad simulator - OpenGL ES 2.0

问题描述

我有一个顶点着色器，我在其中进行纹理查找以确定gl_Position。我使用它作为GPU粒子模拟系统的一部分，其中粒子位置存储在纹理中。

I have a vertex shader in which I do a texture lookup to determine gl_Position. I am using this as part of a GPU particle simulation system, where particle positions are stored in a texture.

似乎： vec4 textureValue = texture2D（dataTexture，vec2（1.0,1.0））; 在模拟器上的行为与iPad设备不同。在模拟器上，纹理查找成功（该位置的值为0.5,0.5），我的粒子出现在那里。但是，在iPad本身，纹理查找不断返回0.0,0.0。

It seems that: vec4 textureValue = texture2D(dataTexture, vec2(1.0, 1.0)); behaves differently on the simulator than the iPad device. On the simulator, the texture lookup succeeds (the value at that location is 0.5, 0.5) and my particle appears there. However, on the iPad itself the texture lookup is constantly returning 0.0, 0.0.

我已经尝试了格式为GL_FLOAT和GL_UNSIGNED_BYTE的两种纹理。

I have tried both textures of the format GL_FLOAT and GL_UNSIGNED_BYTE.

还有其他人经历过这个吗？ GLSL ES规范说可以在顶点和片段着色器中完成纹理查找，所以我看不出问题所在。

Has anyone else experienced this? The GLSL ES spec says that texture lookups can be done in both the vertex and fragment shaders, so I don't see what the problem is.

我正在使用最新的iOS SDK 4.2的GM Beta

I am using the latest GM Beta of iOS SDK 4.2

推荐答案

我刚试过这个。使用iOS 4.3，可以在设备和模拟器上的顶点着色器上进行纹理查找。虽然有点奇怪（这也许是为什么它不像szu所说的那样官方）。在实际设备上（我在iPad 2上测试过），你必须在片段着色器和顶点着色器上进行查找。也就是说，如果您实际上没有在片段着色器上使用它，您仍然必须以某种方式引用它。这是一个简单的例子，我在传递纹理并使用红色像素重新定位顶点的y值：

I just tested this as well. Using iOS 4.3, you can do a texture lookup on the vertex shader on both the device and the simulator. There is a bit of strangeness though (which is maybe why it's not "official" as szu mentioned). On the actual device (I tested on the iPad 2) you have to do a lookup on the fragment shader as well as on the vertex shader. That is, if you are not actually using it on the fragment shader, you'll still have to reference it in some way. Here's a trivial example where I'm passing in a texture and using the red pixel to reposition the y value of the vertex by a little bit:

/////fragment shader
uniform sampler2D tex; //necessary even though not actually used

void main() {
  vec4 notUsed = texture2D(tex, vec2(0.0,0.0)); //necessary even though not actually used
  gl_FragColor = vec4(1.0,1.0,1.0,1.0);
}

/////vertex shader
attribute vec4 position;
attribute vec2 texCoord;   
uniform sampler2D tex; 

void main() {
  float offset = texture2D(tex, texCoord).x;
  offset = (offset - 0.5) * 2.0; //map 0->1 to -1 to +1
  float posx = position.x;
  float posy = position.y + offset/8.0;

  gl_Position = vec4(posx, posy, 0.0, 1.0);  
}

我在 http://www.mat.ucsb.edu/a.forbes/blog/?p= 453

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