I've seen the shader code using these two. But i don't understand what's difference between them, between texture and fragment.
As i know, fragment is pixels, so what's texture?
Some use these code:
vec2 uv = gl_FragCoord.xy / rectSize.xy;
vec4 bkg_color = texture2D(CC_Texture0, uv);
some use:
vec4 bkg_color = texture2D(CC_Texture0, v_texCoord);
with v_texCoord = a_texCoord;
Both works, except the first way displays inverted image.
In your second example 'v_texCoord' looks like a pre-calculated texture coordinate that is passed to the Fragment Shader as a Vertex Attribute, versus the 'uv' coordinate calculated within the Fragment Shader of the first example.
You can base texture coordinates off whatever you like - so long as you give the texture2D sampler normalised coordinates - its all about your use case and what you want to display from a texture.
Perhaps there is such a use-case difference here, which is why they give different visual outputs.
For more information about how texture coordinates work I recommend this question's answer: How do opengl texture coordinates work?
