WebGL中如何实现亮度+对比度？

Question

我正在尝试应用类似于 this example 的 window 级别，将鼠标拖到图像上以查看亮度 + 对比度效果。

但我想使用 WebGL 实现相同的效果，因为 GPU 处理它们的速度比 CPU 快。

这是我所做的：

顶点着色器：

attribute vec3 attrVertexPos;
attribute vec2 attrTextureCoord;

varying highp vec2 vTextureCoord;

void main(void) {
    gl_Position = vec4(attrVertexPos, 0.81);
    vTextureCoord = attrTextureCoord;
}

片段着色器：

#ifdef GL_ES
    precision highp float;
#endif

varying highp vec2 vTextureCoord;

uniform sampler2D uImage;
uniform float brightnessFactor;
uniform float contrastFactor;

void main(void) {
    gl_FragColor = texture2D(uImage, vTextureCoord) *(brightnessFactor/contrastFactor);
}

但是根据上面给出的link，它并没有像预期的那样工作。

这是 Javascript 代码：

var isMouseDown = false;

document.getElementById('canvas').onmousedown = function() { isMouseDown = true  };
document.getElementById('canvas').onmouseup   = function() { isMouseDown = false };
document.getElementById('canvas').onmousemove = function(e) {
    if (isMouseDown) {
        var intWidth = $('canvas').innerWidth();
        var intHeight = $('canvas').innerHeight();
        var x = (e.clientX/intWidth)*100;
        var y = (e.clientY/intHeight)*100;

        console.log(x/10 + ' :: ' + y/10);

        brightnessVal = x/10;
        gl.uniform1f(gl.getUniformLocation(program, "contrastFactor"), brightnessVal);
        contrastVal = y/10;
        gl.uniform1f(gl.getUniformLocation(program, "brightnessFactor"), contrastVal);
    }
};

Answer 1

区别在于像素操作。您只是乘以一个系数（另外，您使用的系数名称不正确），在链接示例中，发生了一些更复杂的事情。源图像的某些颜色范围（由其中心和宽度描述）扩展到完整的 [0,1] 范围：

newColor = (oldColor - rangeCenter) / rangeWidth + 0.5

我不知道为什么要这样做（该页面是医学影像库的示例，我对此一无所知）。不过，我已经设法将公式移植到您的代码中。一、片段着色器变化：

#ifdef GL_FRAGMENT_PRECISION_HIGH
    precision highp float;
#else
    precision mediump float;
#endif

varying highp vec2 vTextureCoord;
uniform sampler2D uImage;

// New uniforms
uniform float rangeCenter;
uniform float rangeWidth;

void main(void) {
    vec3 c = texture2D(uImage, vTextureCoord).rgb;
    gl_FragColor = vec4(
        // The above formula, clamped to [0, 1]
        clamp((c - windowCenter) / windowWidth + 0.5, 0.0, 1.0),
        // Also, let's not screw alpha
        1
    );
}

至于 JavaScript，我冒昧地让它更接近链接示例：

var isMouseDown = false,
    // Initially we set "equality" colour mapping
    rangeCenter = 0.5,
    ragneWidth = 1,
    lastX, lastY;

document.getElementById('canvas').onmousedown = function(e) {
    lastX = e.clientX;
    lastY = e.clientY;
    isMouseDown = true
};
document.getElementById('canvas').onmouseup = function() {
    isMouseDown = false
};
document.getElementById('canvas').onmousemove = function(e) {
    if (isMouseDown) {
        var intWidth = $('canvas').innerWidth();
        var intHeight = $('canvas').innerHeight();

        // Change params according to cursor coords delta
        rangeWidth += (e.clientX - lastX) / intWidth;
        rangeCenter += (e.clientY - lastY) / intHeight;

        gl.uniform1f(
            gl.getUniformLocation(program, "rangeWidth"),
            rangeWidth
        );

        gl.uniform1f(
            gl.getUniformLocation(program, "rangeCenter"),
            rangeCenter
        );

        lastX = e.clientX;
        lastY = e.clientY;
    }
};

旧答案：

The problem is that, according to your code, you don't redraw the picture after mousemove event. Just insert a draw call (or several draw calls) with appropriate parameters after setting uniforms. For example, it may look like this:
document.getElementById('canvas').onmousemove = function(e) {
    if (isMouseDown) {
        // your code

        gl.drawArrays(/* params, e.g gl.TRIANGLES, 0 and vertex count */); 
    }
};
A better way would be using requestAnimationFrame callback to redraw. To do that, define a draw function, which will make the draw call for you and use it a requestAnimationFrame callback:
function draw () {
    /* your draw calls here */
}

document.getElementById('canvas').onmousemove = function(e) {
    if (isMouseDown) {
        // your code

        requestAnimationFrame(draw);
    }
};

P.S。另外，我很感兴趣，这个 #ifdef GL_ES 东西是从哪里来的？根据GL_ES宏设置highp精度是不正确。硬件不必根据标准支持它。正确的方法是：

#ifdef GL_FRAGMENT_PRECISION_HIGH
    precision highp float;
#else
    precision mediump float;
#endif

WebGL中如何实现亮度+对比度？

How to achieve brightness + contrast in WebGL?

opengl-es

webgl