什么是 HYDRA（像素弯曲器）sampleLinear 的 OpenGL ES 2 着色器语言模拟？

Question

所以我查看了 OpenGL ES 着色器规范，但是没有看到这样的...

例如 - 我创建了简单的“捏缩放”和“旋转转身”和“移动到中心”HYDRA 像素弯曲过滤器。它可以在闪存中执行。它基于默认的像素弯曲器旋转示例和 this ：

<languageVersion: 1.0;> 

kernel zoomandrotate
<   namespace : "Pixel Bender Samples";
    vendor : "Kabumbus";
    version : 3;
    description : "rotate and zoom an image around"; >
{
    // define PI for the degrees to radians calculation
    const float PI = 3.14159265;

    // An input parameter to specify the center of the twirl effect.
    // As above, we're using metadata to indicate the minimum,
    // maximum, and default values, so that the tools can set the values 
    // in the correctly in the UI for the filter.
    parameter float2 center
    <
        minValue:float2(0.0, 0.0);
        maxValue:float2(2048.0, 2048.0);
        defaultValue:float2(256.0, 256.0);
    >;

    // An input parameter to specify the angle that we would like to twirl.
    // For this parameter, we're using metadata to indicate the minimum,
    // maximum, and default values, so that the tools can set the values 
    // in the correctly in the UI for the filter.
    parameter float twirlAngle
    <
        minValue:float(0.0);
        maxValue:float(360.0);
        defaultValue:float(90.0);
    >;

     parameter float zoomAmount
    <
        minValue:float(0.01);
        maxValue:float(10.0);
        defaultValue:float(1);
    >;

    // An input parameter that indicates how we want to vary the twirling
    // within the radius.  We've added support to modulate by one of two 
    // functions, a gaussian or a sinc function.  Since Flash does not support
    // bool parameters, we instead are using this as an int with two possible
    // values. Setting this parameter to be 1 will
    // cause the gaussian function to be used, unchecking it will cause 
    // the sinc function to be used.
    parameter int gaussOrSinc
    <
        minValue:int(0);
        maxValue:int(1);
        defaultValue:int(0);
    >;

    input image4 oImage;
    output float4 outputColor;

    // evaluatePixel(): The function of the filter that actually does the 
    //                  processing of the image.  This function is called once 
    //                  for each pixel of the output image.
    void
    evaluatePixel()
    {
        // convert the angle to radians
        float twirlAngleRadians = radians(twirlAngle);

        // calculate where we are relative to the center of the twirl
        float2 relativePos = outCoord() - center;

        // calculate the absolute distance from the center normalized 
        // by the twirl radius.
        float distFromCenter = length( relativePos );
        distFromCenter = 1.0;
        // modulate the angle based on either a gaussian or a sync.
        float adjustedRadians;

        // precalculate either the gaussian or the sinc weight
        float sincWeight = sin( distFromCenter ) * twirlAngleRadians / ( distFromCenter );
        float gaussWeight = exp( -1.0 * distFromCenter * distFromCenter ) * twirlAngleRadians;

        // protect the algorithm from a 1 / 0 error
        adjustedRadians = (distFromCenter == 0.0) ? twirlAngleRadians : sincWeight;

        // switch between a gaussian falloff or a sinc fallof
        adjustedRadians = (gaussOrSinc == 1) ? adjustedRadians : gaussWeight;

        // rotate the pixel sample location.
        float cosAngle = cos( adjustedRadians );
        float sinAngle = sin( adjustedRadians );

        float2x2 rotationMat = float2x2(
            cosAngle,   sinAngle,
            -sinAngle,  cosAngle
        );

        relativePos = rotationMat * relativePos; 
        float scale =  zoomAmount;
        // sample and set as the output color.  since relativePos
        // is related to the center location, we need to add it back in.
        // We use linear sampling to smooth out some of the pixelation.
        outputColor = sampleLinear( oImage, relativePos/scale + center );
    }
}

所以现在我想将它移植到OpenGL ES着色器中。数学和参数可以转换为OpenGL ES着色器语言，但是如何使用sampleLinear呢？ openGL ES 着色器语言中它的模拟是什么？

更新：

所以我创建了类似于我的 HYDRA 滤镜的东西...与 webGL 和 OpenGL ES 着色器兼容...

#ifdef GL_ES
precision highp float;
#endif

uniform vec2 resolution;
uniform float time;
uniform sampler2D tex0;


void main(void)
{
    vec2 p = -1.0 + 2.0 * gl_FragCoord.xy / resolution.xy;
    // a rotozoom
    vec2 cst = vec2( cos(.5*time), sin(.5*time) );
    mat2 rot = 0.5*cst.x*mat2(cst.x,-cst.y,cst.y,cst.x);
    vec3 col = texture2D(tex0,0.5*rot*p+sin(0.1*time)).xyz;

    gl_FragColor = vec4(col,1.0);
}

要了解它是如何工作的，请使用现代浏览器，导航到shadertoy 为其提供一种纹理 ( http://www.iquilezles.org例如 /apps/shadertoy/presets/tex4.jpg），将我的代码粘贴到可编辑文本 aeria 中并点击

Answer 1

根据 Adob​​e 的 像素混合器参考，sampleLinear “通过对相邻像素值执行双线性插值来处理不在像素中心的坐标。”

在OpenGL中实现这一点的正确方法是使用texture2D，就像你已经做的那样，但是要通过 glTexParameter。

您可以使用阶跃函数并乘以其结果以获得越界像素的黑色，或者为纹理提供单个像素黑色边框并切换到夹紧而不是重复，也可以通过 glTexParameter 实现。

如果您想在代码中执行此操作，请尝试：

#ifdef GL_ES
precision highp float;
#endif

uniform vec2 resolution;
uniform float time;
uniform sampler2D tex0;


void main(void)
{
    vec2 p = -1.0 + 2.0 * gl_FragCoord.xy / resolution.xy;
    // a rotozoom
    vec2 cst = vec2( cos(.5*time), sin(.5*time) );
    mat2 rot = 0.5*cst.x*mat2(cst.x,-cst.y,cst.y,cst.x);
    vec2 samplePos = 0.5*rot*p+sin(0.1*time);
    float mask = step(samplePos.x, 0.0) * step(samplePos.y, 0.0) * (1.0 - step(samplePos.x, 1.0)) * (1.0 - step(samplePos.y, 1.0));
    vec3 col = texture2D(tex0,samplePos).xyz;

    gl_FragColor = vec4(col*mask,1.0);
}

这会将颜色限制为从 (0, 0) 到 (1, 1) 的框，但看起来着色器会前往一些明显倾斜的地方，所以我不确定你到底想要什么。