OpenGL 2D纹理单元和纹理翻转解决策略

编译链接自定义的着色器（shader）流程

参考: juejin.cn/post/685560…

ios纹理翻转策略

1. 旋转矩阵翻转图形,不翻转纹理

• 顶点着色器程序

attribute vec4 position;
attribute vec2 textCoordinate;
uniform mat4 rotateMatrix;
uniform mat4 scaleMatrix;
varying lowp vec2 varyTextCoord;

void main(){
    varyTextCoord = textCoordinate;
    vec4 vPos = position;
    gl_Position = vPos * rotateMatrix * scaleMatrix;
}

• 设置旋转矩阵

//glLinkProgram之后获取shaderv.vsh中的uniform属性：rotateMatrix
	GLuint rotate = glGetUniformLocation(self.myPrograme, "rotateMatrix");
    GLuint scale = glGetUniformLocation(self.myPrograme, "scaleMatrix");
    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
    };
    GLfloat scaleMartix[16] = {
        -1, 0, 0, 0,
        0, 1, 0, 0,
        0, 0, 1, 0,
        0, 0, 0, 1,
    };
    
   glUniformMatrix4fv(rotate, 1, GL_FALSE, (GLfloat *)&zRotation[0]);
   glUniformMatrix4fv(scale, 1, GL_FALSE, (GLfloat *)&scaleMartix[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);
//沿着x、y平移
CGContextTranslateCTM(spriteContext, rect.origin.x, rect.origin.y);
//沿着y平移
CGContextTranslateCTM(spriteContext, 0, rect.size.height);
//y轴翻转
CGContextScaleCTM(spriteContext, 1.0, -1.0);
//沿着x、y平移
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. 直接从纹理坐标数据映射修改

• 原顶点数据数组

GLfloat attrArr[] =
    {
        0.5f, -0.5f, -1.0f,     1.0f, 0.0f,
        -0.5f, 0.5f, -1.0f,     0.0f, 1.0f,
        -0.5f, -0.5f, -1.0f,    0.0f, 0.0f,

        0.5f, 0.5f, -1.0f,      1.0f, 1.0f,
        -0.5f, 0.5f, -1.0f,     0.0f, 1.0f,
        0.5f, -0.5f, -1.0f,     1.0f, 0.0f,
    };

• 修改后的顶点数组

 GLfloat attrArr[] =
    {
        0.5f, -0.5f, -1.0f,     1.0f, 1.0f,
        -0.5f, 0.5f, -1.0f,     0.0f, 0.0f,
        -0.5f, -0.5f, -1.0f,    0.0f, 1.0f,
        
        0.5f, 0.5f, -1.0f,      1.0f, 0.0f,
        -0.5f, 0.5f, -1.0f,     0.0f, 0.0f,
        0.5f, -0.5f, -1.0f,     1.0f, 1.0f,
    };

6. 直接翻转顶点着色器中的顶点

在翻转顶点时，就不是直接对Y值用1去减，因为顶点的取值范围是[-1 1] ,所以我们直接加上负号做翻转即可

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

void main(){
    varyTextCoord = textCoordinate;
    gl_Position = vec4(position.x, -position.y, position.z, 1);
}