从 openGL 控件的顶部和底部切割特定区域时投影不起作用

Question

在 link 的帮助下，我可以在纹理上应用投影。现在我想 cut/remove glcontrol 顶部和底部的面积相等，然后需要在剩余区域上应用相同的投影。我试过如下。但如图所示，投影中缺少顶部和底部曲线。我怎样才能把它带回剩余区域？

precision highp float;
uniform sampler2D sTexture;
varying vec2 vTexCoord;
void main()
{
    float img_h_px  = 432.0; // height of the image in pixel
    float area_h_px = 39.0;  // area height in pixel

    float w = area_h_px/img_h_px;
    if (vTexCoord.y < w || vTexCoord.y > (1.0-w)){
        gl_FragColor= vec4(1.0,0.0,1.0,1.0);
    }
    else
    {
        vec2  pos     = vTexCoord.xy * 2.0 - 1.0;
        float b       = 0.5;
        float v_scale = (1.0 + b) / (1.0 + b * sqrt(1.0 - pos.x*pos.x));

        float u = asin( pos.x ) / 3.1415 + 0.5;
        float v = (pos.y * v_scale) * 0.5 + 0.5;
        if ( v < 0.0 || v > 1.0 )
            discard;

        vec3 texColor = texture2D( u_texture, vec2(u, v) ).rgb;
        gl_FragColor  = vec4( texColor.rgb, 1.0 );
    }    
}

Answer 1

底部和顶部区域的大小（底部和顶部区域的总和），相对于控件的大小是2.0*area_h_px/img_h_px = 2.0*w。控件大小与"visible"面积的比例（h_ratio）为：

float w = area_h_px/img_h_px;
float h_ratio = 1.0 - 2.0*w;

您必须通过 "visible" 区域和控件大小的比率来缩放纹理查找的 y 坐标，这是 h_ratio (1.0/h_ratio) 的倒数：

float v = (pos.y * v_scale / h_ratio) * 0.5 + 0.5;

最终着色器：

precision highp float;
uniform sampler2D sTexture;
varying vec2 vTexCoord;

void main()
{
    float img_h_px  = 432.0; // height of the image in pixel
    float area_h_px = 39.0;  // area height in pixel
    float w = area_h_px/img_h_px;
    float h_ratio = 1.0 - 2.0*w;

    vec2  pos     = vTexCoord.xy * 2.0 - 1.0;
    float b       = 0.5;
    float v_scale = (1.0 + b) / (1.0 + b * sqrt(1.0 - pos.x*pos.x));

    float u = asin(pos.x) / 3.1415 + 0.5;
    float v = (pos.y * v_scale / h_ratio) * 0.5 + 0.5;

    vec3 texColor = texture2D(sTexture, vec2(u, v)).rgb;

    vec4 color = vec4(texColor.rgb, 1.0);
    if (vTexCoord.y < w || vTexCoord.y > (1.0-w))
        color = vec4(1.0, 0.0, 1.0, 1.0);
    else if (v < 0.0 || v > 1.0)
        discard;

    gl_FragColor = color; 
}

如果要将整个区域染成紫色，则必须设置 color，而不是 discarding 片段：

if (v < 0.0 || v > 1.0)
    color = vec4(1.0, 0.0, 1.0, 1.0);

从 openGL 控件的顶部和底部切割特定区域时投影不起作用

Projection is not working when cutting particular area from top and bottom of openGL control

c#

opengl

glsl

opentk

coordinate-transformation