javascript - 如何改进这个 WebGL / GLSL 图像下采样着色器

Question

我正在使用 WebGL 在我正在开发的应用程序中快速调整图像客户端的大小。我编写了一个 GLSL 着色器，它对我正在缩小的图像执行简单的双线性过滤。

它在大多数情况下都可以正常工作，但在许多情况下调整大小非常大，例如从 2048x2048 图像缩小到 110x110 以生成缩略图。在这些情况下，质量很差而且太模糊。

我当前的 GLSL 着色器如下：

uniform float textureSizeWidth;\
uniform float textureSizeHeight;\
uniform float texelSizeX;\
uniform float texelSizeY;\
varying mediump vec2 texCoord;\
uniform sampler2D texture;\
\
vec4 tex2DBiLinear( sampler2D textureSampler_i, vec2 texCoord_i )\
{\
    vec4 p0q0 = texture2D(textureSampler_i, texCoord_i);\
    vec4 p1q0 = texture2D(textureSampler_i, texCoord_i + vec2(texelSizeX, 0));\
\
    vec4 p0q1 = texture2D(textureSampler_i, texCoord_i + vec2(0, texelSizeY));\
    vec4 p1q1 = texture2D(textureSampler_i, texCoord_i + vec2(texelSizeX , texelSizeY));\
\
    float a = fract( texCoord_i.x * textureSizeWidth );\
\
    vec4 pInterp_q0 = mix( p0q0, p1q0, a );\
    vec4 pInterp_q1 = mix( p0q1, p1q1, a );\
\
    float b = fract( texCoord_i.y * textureSizeHeight );\
    return mix( pInterp_q0, pInterp_q1, b );\
}\
void main() { \
\
    gl_FragColor = tex2DBiLinear(texture,texCoord);\
}');

TexelsizeX 和 TexelsizeY 分别只是（1.0 / 纹理宽度）和高度......

我想实现更高质量的过滤技术，理想情况下是 [Lancosz][1] 过滤器，它应该会产生更好的结果，但我似乎无法理解如何使用 GLSL 实现算法，因为我对 WebGL 非常陌生，并且GLSL 一般。

有人能指出我正确的方向吗？

提前致谢。

Answer 1

如果您正在寻找 Lanczos 重采样，以下是我在开源 GPUImage 库中使用的着色器程序：

顶点着色器：

 attribute vec4 position;
 attribute vec2 inputTextureCoordinate;

 uniform float texelWidthOffset;
 uniform float texelHeightOffset;

 varying vec2 centerTextureCoordinate;
 varying vec2 oneStepLeftTextureCoordinate;
 varying vec2 twoStepsLeftTextureCoordinate;
 varying vec2 threeStepsLeftTextureCoordinate;
 varying vec2 fourStepsLeftTextureCoordinate;
 varying vec2 oneStepRightTextureCoordinate;
 varying vec2 twoStepsRightTextureCoordinate;
 varying vec2 threeStepsRightTextureCoordinate;
 varying vec2 fourStepsRightTextureCoordinate;

 void main()
 {
     gl_Position = position;

     vec2 firstOffset = vec2(texelWidthOffset, texelHeightOffset);
     vec2 secondOffset = vec2(2.0 * texelWidthOffset, 2.0 * texelHeightOffset);
     vec2 thirdOffset = vec2(3.0 * texelWidthOffset, 3.0 * texelHeightOffset);
     vec2 fourthOffset = vec2(4.0 * texelWidthOffset, 4.0 * texelHeightOffset);

     centerTextureCoordinate = inputTextureCoordinate;
     oneStepLeftTextureCoordinate = inputTextureCoordinate - firstOffset;
     twoStepsLeftTextureCoordinate = inputTextureCoordinate - secondOffset;
     threeStepsLeftTextureCoordinate = inputTextureCoordinate - thirdOffset;
     fourStepsLeftTextureCoordinate = inputTextureCoordinate - fourthOffset;
     oneStepRightTextureCoordinate = inputTextureCoordinate + firstOffset;
     twoStepsRightTextureCoordinate = inputTextureCoordinate + secondOffset;
     threeStepsRightTextureCoordinate = inputTextureCoordinate + thirdOffset;
     fourStepsRightTextureCoordinate = inputTextureCoordinate + fourthOffset;
 }

片段着色器：

 precision highp float;

 uniform sampler2D inputImageTexture;

 varying vec2 centerTextureCoordinate;
 varying vec2 oneStepLeftTextureCoordinate;
 varying vec2 twoStepsLeftTextureCoordinate;
 varying vec2 threeStepsLeftTextureCoordinate;
 varying vec2 fourStepsLeftTextureCoordinate;
 varying vec2 oneStepRightTextureCoordinate;
 varying vec2 twoStepsRightTextureCoordinate;
 varying vec2 threeStepsRightTextureCoordinate;
 varying vec2 fourStepsRightTextureCoordinate;

 // sinc(x) * sinc(x/a) = (a * sin(pi * x) * sin(pi * x / a)) / (pi^2 * x^2)
 // Assuming a Lanczos constant of 2.0, and scaling values to max out at x = +/- 1.5

 void main()
 {
     lowp vec4 fragmentColor = texture2D(inputImageTexture, centerTextureCoordinate) * 0.38026;

     fragmentColor += texture2D(inputImageTexture, oneStepLeftTextureCoordinate) * 0.27667;
     fragmentColor += texture2D(inputImageTexture, oneStepRightTextureCoordinate) * 0.27667;

     fragmentColor += texture2D(inputImageTexture, twoStepsLeftTextureCoordinate) * 0.08074;
     fragmentColor += texture2D(inputImageTexture, twoStepsRightTextureCoordinate) * 0.08074;

     fragmentColor += texture2D(inputImageTexture, threeStepsLeftTextureCoordinate) * -0.02612;
     fragmentColor += texture2D(inputImageTexture, threeStepsRightTextureCoordinate) * -0.02612;

     fragmentColor += texture2D(inputImageTexture, fourStepsLeftTextureCoordinate) * -0.02143;
     fragmentColor += texture2D(inputImageTexture, fourStepsRightTextureCoordinate) * -0.02143;

     gl_FragColor = fragmentColor;
 }

这分两次应用，第一次执行水平下采样，第二次执行垂直下采样。和制服交替设置为 0.0 和图像中单个像素的宽度分数或高度分数texelWidthOffset。texelHeightOffset

我在顶点着色器中硬计算了纹素偏移，因为这避免了我所针对的移动设备上的依赖纹理读取，从而显着提高了性能。不过，这有点冗长。

此 Lanczos 重采样的结果：

正常双线性下采样：

最近邻下采样：