how to warp texture in openGL? (Perspective correction?) - c++

Question

I wanna make the OpenGL program can present images and warp the images presented.
Although I achieved image rendering using OpenGL, I don't know how to warp an image.
An warpable example I want is (Reference):
But a picture I got is:
As I know, this problem is related to perspective correction mapping.
But I don't know about that well.
Here is my source code.
void imageRender(Shader initShader, Shader imgShader, char *path){
glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_FASTEST);
float positions = { 0.5f, 1.0f, 0.0f,
1.0f, -1.0f, 0.0f,
-1.0f, -1.0f, 0.0f,
-1.0f, 1.0f, 0.0f };
float vertices[] = {
// positions // colors // texture coords
position[0], position[1], position[2], 1.0f, 0.0f, 0.0f, 1.0f, 1.0f, // top right
position[3], position[4], position[5], 0.0f, 1.0f, 0.0f, 1.0f, 0.0f, // bottom right
position[6], position[7], position[8], 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, // bottom left
position[9], position[10],position[11], 1.0f, 1.0f, 0.0f, 0.0f, 1.0f // top left
};
unsigned int indices[] = {
0, 1, 3,
1, 2, 3
};
unsigned int VAO, VBO, EBO;
glGenVertexArrays(1, &VAO);
glBindVertexArray(VAO);
glGenBuffers(1, &VBO);
glGenBuffers(1, &EBO);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
// position attribute
//glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
glEnableVertexAttribArray(1);
// color attribute
glVertexAttribPointer(2, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(3 * sizeof(float)));
glEnableVertexAttribArray(2);
//texture attribute
glVertexAttribPointer(3, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(6 * sizeof(float)));
glEnableVertexAttribArray(3);
FREE_IMAGE_FORMAT format = FreeImage_GetFileType(path, 0);
if (format == -1){
cerr << BOLDRED << "[ERROR] IMAGE_NOT_FOUND" << RESET << endl;
exit(1);
}
if (format == FIF_UNKNOWN){
cerr << BOLDRED << "[ERROR] UNKNOWN_IMAGE_FORMAT" << RESET << endl;
format = FreeImage_GetFIFFromFilename(path);
if (!FreeImage_FIFSupportsReading(format)){
cerr << BOLDRED << "[ERROR] IMAGE_FORMAT_NOT_READABLE" << RESET << endl;
exit(1);
}
}
FIBITMAP *bitmap = FreeImage_Load(format, path);
FIBITMAP *bitmap32;
int bitsPerPixel = FreeImage_GetBPP(bitmap);
bitmap32 = FreeImage_ConvertTo32Bits(bitmap);
int imageWidth = FreeImage_GetWidth(bitmap32);
int imageHeight = FreeImage_GetHeight(bitmap32);
GLubyte *textureData = FreeImage_GetBits(bitmap32);
GLuint texture1;
glGenTextures(1, &texture1);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture1);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, imageWidth, imageHeight, 0, GL_BGRA, GL_UNSIGNED_BYTE, textureData);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT); // set texture wrapping to GL_REPEAT (default wrapping method)
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
initShader.use();
glBindVertexArray(VAO);
int initalTime = time(NULL);
while(1){
glBindVertexArray(VAO);
int timecal = time(NULL);
//glDrawElements(GL_TRIANGLE_STRIP, 6, GL_UNSIGNED_INT, 0);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
glfwSwapBuffers(window);
glfwPollEvents();
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
if ((timecal - initalTime) > imageTime) // imageTime value is 10
break;
}
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
glDeleteTextures(1, &texture1);// image
glDisable(GL_TEXTURE_2D);//image
FreeImage_Unload(bitmap32);
FreeImage_Unload(bitmap);
glDeleteVertexArrays(1, &VAO);
glDeleteBuffers(1, &VBO);
glDeleteBuffers(1, &EBO);
}
}
Shader code is like that
//shader.vs
#version 330 core
layout(location = 1) in vec3 position;
layout(location = 2) in vec3 color;
layout(location = 3) in vec2 texcoord;
out vec3 Color;
out vec2 Texcoord;
void main()
{
gl_Position = vec4(position, 1.0);
Texcoord = texcoord;
}
//shader.fs
#version 330 core
in vec3 Color;
in vec2 Texcoord;
out vec4 outColor;
uniform sampler2D texture5;
void main()
{
outColor = texture2D(texture5, Texcoord);
}
How can I warp texture?
And then Is it correct to use position value to warp texture image?

Answer 1

The issue has nothing to do with perspective projection. You draw a polygon with 4 vertices parallel to the XY plane of the view, but the polygon is not a quad! Change the x coordinate of the 1st vertex (0.5f -> 1.0f). Perspective projection works with Homogeneous coordinates.
In generalperspective projection is achieved by a Perspective projection matrix. Of course you can define homogeneous vertices to inspect the behavior:
Define an attribute tuple with homogenous vertices (4 components):
float vertices[] = {
// positions // colors // texture coords
1.0f, 1.0f, 0.5f, 2.0f, 1.0f, 0.0f, 0.0f, 1.0f, 1.0f, // top right
1.0f, -1.0f, -0.5f, 1.0f, 0.0f, 1.0f, 0.0f, 1.0f, 0.0f, // bottom right
-1.0f, -1.0f, -0.5f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, // bottom left
-1.0f, 1.0f, 0.5f, 2.0f, 1.0f, 1.0f, 0.0f, 0.0f, 1.0f // top left
};
Adapt the vertex specification and the vertex shader:
// position attribute
glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 9 * sizeof(float), (void*)0);
glEnableVertexAttribArray(1);
// color attribute
glVertexAttribPointer(2, 3, GL_FLOAT, GL_FALSE, 9 * sizeof(float), (void*)(4 * sizeof(float)));
glEnableVertexAttribArray(2);
//texture attribute
glVertexAttribPointer(3, 2, GL_FLOAT, GL_FALSE, 9 * sizeof(float), (void*)(7 * sizeof(float)));
glEnableVertexAttribArray(3);
#version 330 core
layout(location = 1) in vec4 position;
layout(location = 2) in vec3 color;
layout(location = 3) in vec2 texcoord;
out vec3 Color;
out vec2 Texcoord;
void main()
{
gl_Position = position;
Texcoord = texcoord;
}
Another option to achieve the effect is, to a an Z component to the geometry. e.g:
float positions = { 1.0f, 1.0f, 0.5f,
1.0f, -1.0f, 0.0f,
-1.0f, -1.0f, -0.5f,
-1.0f, 1.0f, 0.0f };
and to compute the w component dependent on z in the vertex shader (e.g. w = z + 2.5:
#version 330 core
layout(location = 1) in vec3 position;
layout(location = 2) in vec3 color;
layout(location = 3) in vec2 texcoord;
out vec3 Color;
out vec2 Texcoord;
void main()
{
gl_Position = vec4(position, position.z + 2.5);
Texcoord = texcoord;
}