OpenGL ES--纹理翻转

OpenGL要求y轴0.0坐标是在图片的底部的，但是图片的y轴0.0坐标通常在顶部，所以要进行翻转。

1.旋转矩阵翻转图形,不翻转纹理

让图形顶点坐标旋转180°. 而纹理保持原状.

GLuint rotate = glGetUniformLocation(self.myPrograme, "rotateMatrix");
    float radians = 180 * 3.14159f / 180.0f;
    float s = sin(radians);
    float c = cos(radians);
    
GLfloat zRotation[16] = {
        c, -s, 0, 0,
        s, c, 0, 0,
        0, 0, 1.0, 0,
        0.0, 0, 0, 1.0
    };
    
 glUniformMatrix4fv(rotate, 1, GL_FALSE, (GLfloat *)&zRotation[0]);

2.解压图片时,将图片源文件翻转

CGImageRef spriteImage = [UIImage imageNamed:fileName].CGImage;

size_t width = CGImageGetWidth(spriteImage);
size_t height = CGImageGetHeight(spriteImage);
GLubyte * spriteData = (GLubyte *) calloc(width * height * 4, sizeof(GLubyte));

CGContextRef spriteContext = CGBitmapContextCreate(spriteData, width, height, 8, width*4,CGImageGetColorSpace(spriteImage), kCGImageAlphaPremultipliedLast);
  
CGRect rect = CGRectMake(0, 0, width, height);
CGContextDrawImage(spriteContext, CGRectMake(0, 0, width, height), spriteImage);

CGContextTranslateCTM(spriteContext, rect.origin.x, rect.origin.y);
CGContextTranslateCTM(spriteContext, 0, rect.size.height);
CGContextScaleCTM(spriteContext, 1.0, -1.0);
CGContextTranslateCTM(spriteContext, -rect.origin.x, -rect.origin.y);
CGContextDrawImage(spriteContext, rect, spriteImage); 

CGContextRelease(spriteContext);
glBindTexture(GL_TEXTURE_2D, 0);

3. 修改片元着色器,纹理坐标

varying lowp vec2 varyTextCoord;
uniform sampler2D colorMap;
void main()
{
    gl_FragColor = texture2D(colorMap, vec2(varyTextCoord.x,1.0-varyTextCoord.y));
}

4. 修改顶点着色器,纹理坐标

attribute vec4 position;
attribute vec2 textCoordinate;
varying lowp vec2 varyTextCoord;

void main()
{
    varyTextCoord = vec2(textCoordinate.x,1.0-textCoordinate.y);
    gl_Position = position;
}

5.直接从源纹理坐标数据修改

GLfloat attrArr[] =
     {
     0.5f, -0.5f, 0.0f,        1.0f, 1.0f, //右下
     -0.5f, 0.5f, 0.0f,        0.0f, 0.0f, // 左上
     -0.5f, -0.5f, 0.0f,       0.0f, 1.0f, // 左下
     0.5f, 0.5f, 0.0f,         1.0f, 0.0f, // 右上
     -0.5f, 0.5f, 0.0f,        0.0f, 0.0f, // 左上
     0.5f, -0.5f, 0.0f,        1.0f, 1.0f, // 右下
     };
     */