code is for reading i-th pixel. a-value isn't used by MPlayer
the definition of OpenGL values can be found in e.g.:
The OpenGL Graphics System: A Specification (Version 1.3)
glspec13.pdf, around pages 94-100 in their numbering (106-112 in gv)

RGB32:
gl format: GL_RGBA
gl type: GL_UNSIGNED_BYTE
code:
uint8_t *data;
r=data[4*i];
g=data[4*i+1];
b=data[4*i+2];
a=data[4*i+3];

===========================
RGB24:
gl format: GL_RGB
gl type: GL_UNSIGNED_BYTE
code:
uint8_t *data;
r=data[3*i];
g=data[3*i+1];
b=data[3*i+2];

===========================
RGB16:
gl format: GL_RGB
gl type: GL_UNSIGNED_SHORT_5_6_5
code:
uint16_t *data;
r=(data[i] & 0xf800) >> 11;
g=(data[i] & 0x07e0) >> 5;
b=(data[i] & 0x001f);

===========================
RGB15:
gl format: GL_RGBA
gl type: GL_UNSIGNED_SHORT_5_5_5_1
code:
uint16_t *data;
r=(data[i] & 0xf800) >> 11;
g=(data[i] & 0x07c0) >> 6;
b=(data[i] & 0x002e) >> 1;
a=(data[i] & 0x0001);

===========================
RGB8:
gl format: GL_RGB
gl type: GL_UNSIGNED_BYTE_3_3_2
code:
uint8_t *data;
r=(data[i] & 0xe0) >> 5;
g=(data[i] & 0x1c) >> 2;
b=(data[i] & 0x03);

===========================
BGR32:
gl format: GL_BGRA
gl type: GL_UNSIGNED_BYTE
code:
uint8_t *data;
b=data[4*i];
g=data[4*i+1];
r=data[4*i+2];
a=data[4*i+3];

===========================
BGR24:
gl format: GL_BGR
gl type: GL_UNSIGNED_BYTE
code:
uint8_t *data;
r=data[3*i];
g=data[3*i+1];
b=data[3*i+2];

===========================
BGR16:
gl format: GL_RGB
gl type: GL_UNSIGNED_SHORT_5_6_5_REV
code:
uint16_t *data;
b=(data[i] & 0xf800) >> 11;
g=(data[i] & 0x07e0) >> 5;
r=(data[i] & 0x001f);

===========================
BGR15:
gl format: GL_BGRA
gl type: GL_UNSIGNED_SHORT_5_5_5_1
code:
uint16_t *data;
b=(data[i] & 0xf800) >> 11;
g=(data[i] & 0x07c0) >> 6;
r=(data[i] & 0x002e) >> 1;
a=(data[i] & 0x0001);

===========================
BGR8:
gl format: GL_RGB
gl type: GL_UNSIGNED_BYTE_2_3_3_REV
code:
uint8_t *data;
b=(data[i] & 0xc0) >> 6;
g=(data[i] & 0x38) >> 3;
r=(data[i] & 0x07);

----------------------------------------------------
***************************************************
==================================================

currently different in MPlayer (tested only for little endian!):


RGB16, would be BGR16 in OpenGL
RGB15:
gl format: GL_RGBA
gl type: GL_UNSIGNED_SHORT_1_5_5_5_REV
code:
uint16_t *data;
a=(data[i] & 0x8000) >> 15;
b=(data[i] & 0x7c00) >> 10;
g=(data[i] & 0x03e0) >> 5;
r=(data[i] & 0x001f);

RGB8, would be BGR8 in OpenGL, see postproc/rgb2rgb.c line 595
BGR16, would be RGB16 in OpenGL

BGR15:
gl format: GL_BGRA
gl type: GL_UNSIGNED_SHORT_1_5_5_5_REV
code:
uint16_t *data;
a=(data[i] & 0x8000) >> 15;
b=(data[i] & 0x7c00) >> 10;
g=(data[i] & 0x03e0) >> 5;
r=(data[i] & 0x001f);

BGR8, would be RGB8 in OpenGL