CODEWITHSUNDEEP
c++opengl-3
Keles
Keles

Reputation: 379

OpenGL glReadPixels returns 0

When Im rendering to on-screen buffer everything goes fine, but when reading pixels from FrameBuffer with glReadPixels it always returns 0.

The pseudocode is the following:

Bind texture to FrameBuffer:

GLuint texture;
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
int width, height;
width = 2;
height = 2;
float texture_data[] = {
    1.0f, 0.0f, 0.0f,   1.0f, 1.0f, 1.0f,
    1.0f, 1.0f, 1.0f,   1.0f, 0.0f, 0.0f
};
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_FLOAT, texture_data);
GLuint framebuffer;
glGenFramebuffers(1, &framebuffer); 
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer); 
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0); 
GLint texture_coord_attribute = glGetAttribLocation(program, "texture_coord");
glEnableVertexAttribArray(texture_coord_attribute);
glVertexAttribPointer(texture_coord_attribute, 2, GL_FLOAT, GL_FALSE,
    sizeof(vertices[0]), (void*)(sizeof(float) * 5));

Fragment/Vertex shaders:

static const char* vertex_shader_text =
"#version 330\n""
"attribute vec2 vPos;\n"
"attribute vec2 texture_coord;\n"
"varying vec2 Texcoord;\n"
"void main()\n"
"{\n"
"    gl_Position = vec4(vPos, 0.0, 1.0);\n"
"    Texcoord = texture_coord;\n"
"}\n";

static const char* fragment_shader_text =
"#version 330\n"
"varying vec2 Texcoord;\n"
"uniform sampler2D tex;\n"
"void main()\n"
"{\n"
"    gl_FragColor = texture(tex, Texcoord);\n"
"}\n";

Read Pixels in Main loop:

glViewport(0, 0, 720, 480);
glUseProgram(program);
glDrawArrays(GL_TRIANGLES, 0, 6);

GLubyte pixels[3] = {0};
glReadBuffer(GL_COLOR_ATTACHMENT0);
glReadPixels(360, 240, 1, 1, GL_RGB, GL_FLOAT, pixels);
// Any value returns 0 not only 360 and 240
printf("|%f||%f||%f|\n", pixels[0], pixels[1], pixels[2]);

glfwSwapBuffers(window);
glfwPollEvents();

This is the pipeline I follow. What is wrong here? Thanks.

Upvotes: 3

Views: 2413

Answers (1)

Rabbid76
Rabbid76

Reputation: 210878

The 5th and 6th parameter (format and type) of glReadPixels specifies the format and data type of the target pixel data.

Since you want to read to a buffer with the element data type GLubyte, the type has to be GL_BYTE.

Change your code like this:

glReadPixels(360, 240, 1, 1, GL_RGB, GL_BYTE, pixels);

Or read the data to a buffer of type GLfloat:

GLfloat pixels[3];
glReadPixels(360, 240, 1, 1, GL_RGB, GL_FLOAT, pixels);


Note, what you do, is to read 12 bytes (sizeof(float)*3) to a buffer with a size of 3 bytes (GLubyte pixels[3]). This means a part of the floating point value which represents the red color channel is stored to the buffer. The rest overwrites some memory with bad access.

Upvotes: 2

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