Scenekit：使用着色器为漫反射纹理重新着色

我想编写一些着色器（表面/碎片/ ...），以便以某种新颜色重新着色漫反射纹理。当前，我具有此版本的着色器（我正在尝试实时重新着色纹理）：

//sm_surf

uniform vec4 colorTarget; (used in full version)
uniform vec4 colorTint; (used in full version)

vec4 colorTexture = texture2D(u_diffuseTexture, _surface.diffuseTexcoord);
//vec4 colorTexture = _sample.diffuse;//the same result

vec4 tinted = colorTexture;
tinted.r = max(0.0, colorTexture.r - 0.2);
_surface.diffuse = tinted;

这是OpenCV中的代码（我只是预先给纹理重新着色，然后将其用作新的漫反射纹理）：

image = cv::imread([path UTF8String], cv::IMREAD_UNCHANGED);
for (int i = 0; i < image.rows; i++) {
    for (int j = 0; j < image.cols; j++) {
        cv::Vec4b pixel = image.at<cv::Vec4b>(i,j);
        pixel[2] = fmin(255, fmax(0, pixel[2] - 50));
        image.at<cv::Vec4b>(i,j) = pixel;
    }
}

cv::imwrite([newPath UTF8String], image);

对于此测试，我只想减少颜色的R分量。结果：

OpenCV（正确）

SceneKit（不正确）

漫反射纹理包含Alpha通道。

（由小工具解决）而且，好像在重新着色着色器Alpha通道后。使用此着色器：

tinted.r = 1;
tinted.g = 0;
tinted.b = 0;

我的结果是：

代替：

原始纹理：

How can I just recolor diffuse texture like in openCV?

---

UPDATE: This are result for SceneKit shader and OpenCV (I have removed all transparent pixels from image):

OpenCV:

SceneKit:

shader:

vec4 colorTexture = _surface.diffuse;
vec3 tinted = colorTexture.a > 0.0 ? colorTexture.rgb / colorTexture.a : colorTexture.rgb;
if (colorTexture.a == 1) {
    tinted.r = max(0.0, colorTexture.r - 0.2);
} else {
    colorTexture.a = 0;
}
_surface.diffuse = vec4(tinted, 1.0) * colorTexture.a;

and OpenCV code:

pixel[2] = fmax(0, pixel[2] - 50);//2 because it's bgr in OpenCV
if (pixel[3] != 255) {
    pixel[3] = 0;
}

---

Some more strange things: I have changed my OpenCV code to this to generate new texture

pixel[0] = 255 - (j % 4) * 30;//b
pixel[1] = 0;//g
pixel[2] = 0;//r
pixel[3] = 255;

If I change this texture like this:

if (pixel[0] == 255) {
    pixel[0] = 255;pixel[1] = 255;pixel[2] = 255;
} else {
    pixel[0] = 0;pixel[1] = 0;pixel[2] = 0;
}

I receive smth like this:

With this SceneKit shader it should be the same:

vec4 colorTexture = _surface.diffuse;
vec3 tinted = colorTexture.rgb; // colorTexture.a == 1
if (tinted.b > 0.99) {
    tinted = vec3(0,0,0);
} else {
    tinted = vec3(1,1,1);
}
_surface.diffuse = vec4(tinted, 1.0) * colorTexture.a;

But I receive this:

There are some white stripes, but way too thin.

I can increase them by changing condition to tinted.b > 0.85, but it's already an error, because color in _surface.diffuse not the same as in texture. Seems like SceneKit interpolates the texture or smth like that.

---

UPDATE2:

I have added source code (1.5Mb) with this problem. There are 3 spheres:

1 Top) With original texture

2 Left) With texture recolored by shader (newR = r - 0.2) (float)

3 Right) With texture recolored by OpenCV (newR = r - 51) (uint8)

and they are different! Scene doesn't contain any light/env/... just 3 spheres.