OpenGL subimages using pixel coordinates

Question

I've worked through a couple of tutorials in the breakout series on learnopengl.com, so I have a very simple 2D renderer. I want to add a subimage feature to it, though, where I can specify a vec4 for a kind of "source rectangle", so if the vec4 was (10, 10, 32, 32), it would only render a rectangle at 10, 10 with a width and height of 32, kind of like how the SDL renderer works.

The way the renderer is set up is there is a quad VAO which all the sprites use, which contains the texture coordinates. Initially, I though I could use an array of VAO's for each sprite, each with different texture coordinates, but I'd like to be able to change the source rectangle before the sprite gets drawn, to make things like animation easier... My second idea was to have a seperate uniform vec4 passed into the fragment shader for the source rectangle, but how do I only render that section in pixel coordinates?

Answer 1

Use the Primitiv type GL_TRIANGLE_STRIP or GL_TRIANGLE_FAN to render a quad. Use integral one-dimensional vertex coordinates instead of floating-point vertex coordinates. The vertex coordinates are the indices of the quad corners. For a GL_TRIANGLE_FAN they are:

vertex 1: 0
vertex 2: 1
vertex 3: 2
vertex 4: 3

Set the rectangle definition (10, 10, 32, 32) in the vertex shader uisng a Uniform variable of type vec4. With this information, you can calculate the vertex coordinate in the vertex shader:

in int cornerIndex;
uniform vec4 rectangle;

void main()
{
    vec2 vertexArray[4] = 
        vec2[4](rectangle.xy, rectangle.zy, rectangle.zw, rectangle.xw);
    vec2 vertex = vertexArray[cornerIndex];

    // [...]
}

The Vertex Shader provides the built-in input gl_VertexID, which specifies the index of the vertex currently being processed. This variable could be used instead of cornerIndex in this case. Note that it is not necessary for the vertex shader to have any explicit input.

Answer 2

I ended up doing this in the vertex shader. I passed in the vec4 as a uniform to the vertex shader, as well as the size of the image, and used the below calculation:

// convert pixel coordinates to vertex coordinates
float widthPixel = 1.0f / u_imageSize.x;
float heightPixel = 1.0f / u_imageSize.y;

float startX = u_sourceRect.x, startY = u_sourceRect.y, width = u_sourceRect.z, height = u_sourceRect.w;
v_texCoords = vec2(widthPixel * startX + width * widthPixel * texPos.x, heightPixel * startY + height * heightPixel * texPos.y);

v_texCoords is a varying that the fragment shader uses to map the texture.