如果你正在寻找Lanczos重采样,下面是我在开源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;
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;
}
这个过程分两步进行,第一步进行水平降采样,第二步进行垂直降采样。 texelWidthOffset
和 texelHeightOffset
uniform 交替设置为0.0和图像中单个像素的宽度或高度分数。
我在顶点着色器中硬计算纹理偏移量,因为这避免了我针对移动设备进行优化时出现依赖纹理读取,从而提高了性能。虽然有些冗长。
Lanczos重采样的结果:
![Lanczos](https://istack.dev59.com/KK5tu.webp)
普通双线性降采样:
![Bilinear](https://istack.dev59.com/VkitJ.webp)
最近邻降采样:
![Nearest-neighbor](https://istack.dev59.com/qRXjf.webp)