openGL es 2.0 TextureCoordinates parameter ios - c++

Question

I have to crop a video.
So i modify the TextureCoordinates.
The code is as below.
const GLfloat squareTextureCoordinates[] = {
0.22f , 1.0f,
0.22f, 0.0f,
0.78f, 1.0f,
0.78f, 0.0f,
};
glVertexAttribPointer(positionAttribute, 2, GL_FLOAT, 0, 0, squareVertices);
glVertexAttribPointer(inputTextureCoordinateAttribute, 2, GL_FLOAT, 0, 0, squareTextureCoordinates);
This code is work.
However,i need to support many size of iphone .(example:iphone6 have different size to iphone5)
So i use variable rather than const in TextureCoordinates.
And the new code is as below;
const float cropX=(float)(SCREEN_HEIGHT-SCREEN_WIDTH)/(float)(SCREEN_HEIGHT*2);
const float cropXR=1- (float)(SCREEN_HEIGHT-SCREEN_WIDTH)/(float)(SCREEN_HEIGHT*2);
const GLfloat squareTextureCoordinates[] = {
cropX , 1.0f,
cropX, 0.0f,
cropXR, 1.0f,
cropXR, 0.0f,
};
glVertexAttribPointer(positionAttribute, 2, GL_FLOAT, 0, 0, squareVertices);
glVertexAttribPointer(inputTextureCoordinateAttribute, 2, GL_FLOAT, 0, 0, squareTextureCoordinates);
However, it doesn't work.
The screen is display nothing.
i cant figure out why i can't use const variable for TextureCoordinates.

Answer 1

I think you could test the second part of your code by changing the const values to the constants you used in the first part:
const float cropX = 0.22f;
const float cropXR = 0.78f;
const GLfloat squareTextureCoordinates[] = {
cropX , 1.0f,
cropX, 0.0f,
cropXR, 1.0f,
cropXR, 0.0f,
};
glVertexAttribPointer(positionAttribute, 2, GL_FLOAT, 0, 0, squareVertices);
glVertexAttribPointer(inputTextureCoordinateAttribute, 2, GL_FLOAT, 0, 0, squareTextureCoordinates);
If this works (it should), you will see you can use const variables as a texture coordinate. However, I reckon the wrong part of the code is this:
const float cropX=(float)(SCREEN_HEIGHT-SCREEN_WIDTH)/(float)(SCREEN_HEIGHT*2);
const float cropXR=1- (float)(SCREEN_HEIGHT-SCREEN_WIDTH)/(float)(SCREEN_HEIGHT*2);
In texture coordinates, correct values are between 0 and 1. If width is bigger than height, this code won't work. In the specific case of an iPhone 5 and 6, which have the same proportion, same texture coordinates should work if the view that renders OpenGL code was resized (if you use autolayout for that view, you will probably solve the problem automatically).