c - OpenGL工作着色器？

Question

有两个着色器，顶点和片段

const char *vShaderstr = 
"#version 300 es\n"
"layout(location = 0) in vec4 a_color;\n"
"layout(location = 1) in vec2 a_position;\n"
"out vec4 v_color;\n"
"void main()\n"
"{\n"
" v_color = a_color;\n"
" gl_Position.xy = a_position;\n"
" gl_Position.z = 0.0;\n"
" gl_Position.w = 1.0;\n"
"}";
const char *fShaderstr =
    "#version 300 es\n"
    "precision lowp float;\n"
    "in vec4 v_color;\n"
    "out vec4 o_fragColor;\n"
    "void main()\n"
    "{\n"
    " o_fragColor = v_color;\n"
    "}";

问题是我没有 float 的颜色，而是无符号的字节。并且位置为 int，而不是 float。当我在漂浮物中沉思时，我就开始思考。为了将数据发送到管道，我使用这样的命令。

glVertexAttribPointer ( 0, 4, GL_UNSIGNED_BYTE, GL_FALSE, 0, pixels );
glVertexAttribPointer ( 1, 2, GL_INT, GL_FALSE, 0, vertices );
glEnableVertexAttribArray ( 0 );
glEnableVertexAttribArray ( 1 );
glDrawArrays ( GL_POINTS, 0, max_draw );

字节中的像素。 int 的顶点。

Answer 1

请参阅 Khronos 组引用页 OpneGL ES glVertexAttribPointer :

For glVertexAttribPointer, if normalized is set to GL_TRUE, it indicates that values stored in an integer format are to be mapped to the range [-1,1] (for signed values) or [0,1] (for unsigned values) when they are accessed and converted to floating point. Otherwise, values will be converted to floats directly without normalization.

For glVertexAttribIPointer, only the integer types GL_BYTE, GL_UNSIGNED_BYTE, GL_SHORT, GL_UNSIGNED_SHORT, GL_INT, GL_UNSIGNED_INT are accepted. Values are always left as integer values.

您必须将 RGB 颜色的颜色 channel 从 [0, 255] 范围转换为 [0.0, 1.0]。所以你必须使用:

// normalized = GL_TRUE: [0, 255] -> [0.0, 1.0]
glVertexAttribPointer( 0, 4, GL_UNSIGNED_BYTE, GL_TRUE, 0, pixels );

但是您想按原样使用顶点坐标，作为积分坐标。因此，您必须使用 glVertexAttribIPointer (重点关注 I):

// glVertexAttribIPointer because of integer coordinates
glVertexAttribIPointer( 1, 2, GL_INT, 0, vertices );

顶点着色器中顶点属性的类型必须从 vec2 更改为 ivec2，因为坐标的数据类型为 GL_INT >:

#version 300 es

layout(location = 0) in vec4 a_color;
layout(location = 1) in ivec2 a_position; // <---- ivec2

out vec4 v_color;

uniform mat4 u_projection;

void main()
{
    v_color = a_color;
    vec2 p = vec2(a_position); // <---- convert from int to float
    gl_Position = u_projection * vec4( p.xy, 0.0, 1.0 );
}

进一步注意，gl_Position 是通过除以 w 组件而转换为标准化设备坐标 (NDC) 的剪辑空间坐标。在标准化设备空间中，左下角位于 (-1.0, -1.0)，右上角位于 (1.0, 1.0)。然后，标准化的设备坐标线性映射到窗口坐标(由 glViewport 设置的视口(viewport))。

您必须设置一个投影矩阵，它将顶点坐标转换为剪辑空间(最终标准化设备空间)。

以下示例设置一个正交投影矩阵，用于投影屏幕空间坐标，其中左上角为 (0, 0)，右下角为 (宽度，高度):

GLuint shader_prog = .... ; // shader program object
float  width       = .... ; // viewport width in pixel
float  height      = .... ; // viewport height in pixel

float prj_mat[] = 
{
     2.0f/width,  0.0f,         0.0f,  0.0f,
     0.0f,       -2.0f/height,  0.0f,  0.0f,
     0.0f,        0.0f,        -1.0f,  0.0f,
    -1.0f,        1.0f,         0.0f,  1.0f
};

glLinkProgram( shader_prog );
GLint prj_loc = glGetUniformLocation( shader_prog, "u_projection" );

glUseProgram( shader_prog );
glUniformMatrix4fv( prj_loc, 1, GL_FALSE, prj_mat );