c++ - 当我在 glTexImage2D 中选择与在着色器中采样不同的内部格式时，情况会怎样？

Question

例如，我在代码中这样写:

glTexImage2D(GL_TEXTURE_2D, i, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, data);

在着色器中:

glFragColor = texture2D(sampler, uv).rgba

会发生什么，官方对这种情况有定义吗？

Answer 1

结果将是相同的，就像您执行以下操作一样:

glFragColor = vec4( texture2D(sampler, uv).rgb, 1.0 );

因为Image Format Khronos 集团的规范说:

Image formats do not have to store each component. When the shader samples such a texture, it will still resolve to a 4-value RGBA vector. The components not stored by the image format are filled in automatically. Zeros are used if R, G, or B is missing, while a missing Alpha always resolves to 1.

进一步查看 OpenGL 4.6 API Core Profile Specification; 15.2. SHADER EXECUTION; page 487 ]:

When a texture lookup is performed in a fragment shader, the GL computes the filtered texture value ... and converts it to a texture base color Cb as shown in table 15.1, ...

Texture Base Texture base color Internal Format    Cb              Ab
RED                                                (Rt, 0, 0)      1
RG                                                 (Rt, Gt, 0)     1
RGB                                                (Rt, Gt, Bt)    1
RGBA                                               (Rt, Gt, Bt)    At

Table 15.1: Correspondence of filtered texture components to texture base components.