GLSL 330：羽绒样本纹理

Question

我想将输入纹理从800x600下采样到四分之一（200x150像素）。但如果我这样做，我只能看到一点点的图像。似乎frament着色器不会对整个纹理进行下采样。以下示例是创建景深效果。

顶点着色器：

#version 330

uniform UVertFrameBuffer
{
    mat4  m_ScreenMatrix;
};

uniform UDofInvertedScreenSize
{
    vec2 m_InvertedScreenSize;
};

// -----------------------------------------------------------------------------
// in variables
// -----------------------------------------------------------------------------

layout(location = 0) in vec3 VertexPosition;

// -----------------------------------------------------------------------------
// out variables
// -----------------------------------------------------------------------------

struct SPixelCoords
{
    vec2 tcColor0;
    vec2 tcColor1;
};

out SPixelCoords vs_PixelCoords;

// -----------------------------------------------------------------------------
// Program
// -----------------------------------------------------------------------------

void main()
{
    vec4 Position = vec4(VertexPosition.xy, 0.0f, 1.0f);

    vec4 PositionInClipSpace = m_ScreenMatrix * Position;
    vec2 ScreenCoords        = VertexPosition.xy;

    // -----------------------------------------------------------------------------

    vs_PixelCoords.tcColor0 = ScreenCoords + vec2(-1.0f, -1.0f) * m_InvertedScreenSize;
    vs_PixelCoords.tcColor1 = ScreenCoords + vec2(+1.0f, -1.0f) * m_InvertedScreenSize;

    // -----------------------------------------------------------------------------

    gl_Position = PositionInClipSpace;
}

片段着色器：

#version 330

uniform sampler2D g_ColorTex;
uniform sampler2D g_DepthTex;

uniform UDofDownBuffer
{
    vec2  m_DofNear;
    vec2  m_DofRowDelta;
};

// -----------------------------------------------------------------------------
// Inputs per vertice
// -----------------------------------------------------------------------------

struct SPixelCoords
{
    vec2 tcColor0;
    vec2 tcColor1;
};

in SPixelCoords vs_PixelCoords;

// -----------------------------------------------------------------------------
// Output to graphic card
// -----------------------------------------------------------------------------

layout (location = 0) out vec4 FragColor;

// -----------------------------------------------------------------------------
// Program
// -----------------------------------------------------------------------------

void main() 
{
    // Initialize variables
    vec3  Color;
    float MaxCoc;
    vec4  Depth;
    vec4  CurCoc;
    vec4  Coc;
    vec2  RowOfs[4];

    // Calculate row offset
    RowOfs[0] = vec2(0.0f);
    RowOfs[1] = m_DofRowDelta.xy;
    RowOfs[2] = m_DofRowDelta.xy * 2.0f;
    RowOfs[3] = m_DofRowDelta.xy * 3.0f;

    // Bilinear filtering to average 4 color samples
    Color  = vec3(0.0f);
    Color += texture(g_ColorTex, vs_PixelCoords.tcColor0.xy + RowOfs[0]).rgb;
    Color += texture(g_ColorTex, vs_PixelCoords.tcColor1.xy + RowOfs[0]).rgb;
    Color += texture(g_ColorTex, vs_PixelCoords.tcColor0.xy + RowOfs[2]).rgb;
    Color += texture(g_ColorTex, vs_PixelCoords.tcColor1.xy + RowOfs[2]).rgb;
    Color /= 4.0f;

    // Calculate CoC
    ...

    // Calculate fragment color
    FragColor = vec4(Color, MaxCoc);
}

输入纹理为800x600，输出纹理为200x150像素。作为m_InvertedScreenSize，我使用1/800和1/600像素。是吗？

我上传了两个代表OpenGL屏幕坐标的三角形。

QuadVertices[][1] = { 
{ 0.0f, 1.0f, 0.0f, }, 
{ 1.0f, 1.0f, 0.0f, }, 
{ 1.0f, 0.0f, 0.0f, }, 
{ 0.0f, 0.0f, 0.0f, }, }; 

QuadIndices[][2] = { 
{ 0, 1, 2, }, 
{ 0, 2, 3, }, }; 

我的屏幕矩阵通过正交矩阵将这些顶点转换为剪裁空间。

Position(0.0f, 0.0f, 1.0f);
Target(0.0f, 0.0f, 0.0f);
Up(0.0f, 1.0f, 0.0f);

LookAt(Position, Target, Up);

SetOrthographic(0.0f, 1.0f, 0.0f, 1.0f, -1.0f, 1.0f);

以下图像显示输入纹理和结果。第一个是没有下采样的原始图像。其次是实际的下采样纹理。第三个是下采样纹理，计算ScreenCoords * = 4.0f;。